Simple interest is calculated only on the principal: Interest = P × r × t. Useful for short-term loans and notes.
How the Simple Interest Calculator works
Simple interest is calculated on your original principal only, using one formula: Interest = P x r x t. No interest is ever charged or earned on interest already accrued, which is exactly what separates it from compounding.
The three variables are: P = the principal (the starting amount you borrowed or invested, in dollars), r = the annual interest rate written as a decimal (so 8% = 0.08), and t = the time in years (a 6-month note is 0.5, a 90-day note is roughly 0.247). Multiply all three and you get the total interest in dollars; add it back to the principal for the payoff or maturity amount.
Here is exactly what the calculator does internally:
- Reads your principal, rate, and time, and converts the rate from a percent to a decimal by dividing by 100.
- Normalizes time to years. If you enter months, it divides by 12; if you enter days, it divides by 365 (the convention most US consumer loans use).
- Computes interest with I = P x r x t.
- Computes the total balance as P + I.
- Derives the per-day and per-month interest (P x r / 365 or P x r / 12) so you can see daily accrual on a payoff quote.
Edge cases the tool handles: a 0% rate returns zero interest and a total equal to principal; fractional time (days or months) is supported because simple interest scales linearly, so half the time means exactly half the interest; and very short terms (a 30-day payday or title loan) work without rounding artifacts. Because the relationship is perfectly linear, doubling the principal doubles the interest and doubling the time doubles the interest. Want growth that snowballs instead? Use the compound interest calculator for the opposite behavior.
Example calculation
Here are three real US scenarios, each recomputed with Interest = P x r x t.
Example 1 - a 3-year personal loan. You borrow $5,000 at 8% for 3 years. Interest = 5,000 x 0.08 x 3 = $1,200. You repay $6,200 total, just $400 of interest per year, every year, because it never compounds. The same loan under annual compounding would cost about $1,299, so simple interest saves the borrower roughly $99 here.
Example 2 - a 6-month car title loan. You borrow $2,000 at a 25% annual rate for 0.5 years. Interest = 2,000 x 0.25 x 0.5 = $250, so you owe $2,250 at payoff. Title and payday lenders often quote a monthly rate instead; 25% per month would be a 300% APR, so always confirm whether the rate is monthly or annual.
Example 3 - a 5-year Treasury-style coupon. You hold a $10,000 note paying a 4.5% annual coupon for 5 years. Simple coupon interest = 10,000 x 0.045 x 5 = $2,250 paid out over the term, returning the $10,000 face value at maturity. Coupons are classic simple interest because each payment is based on the unchanging face value, not a growing balance.
| Scenario | Principal (P) | Rate (r) | Time (t) | Interest (I) | Total |
|---|---|---|---|---|---|
| Personal loan | $5,000.00 | 8% | 3 yr | $1,200.00 | $6,200.00 |
| Title loan | $2,000.00 | 25% | 0.5 yr | $250.00 | $2,250.00 |
| Treasury coupon | $10,000.00 | 4.5% | 5 yr | $2,250.00 | $12,250.00 |
The pattern is identical across all three: interest rises in a straight line with time. If Example 1 ran for 6 years instead of 3, the interest would be exactly double ($2,400), never more. That linearity is the whole point of simple interest, and it is why a longer simple-interest loan is far gentler than a longer compounding debt.
Tips for using the Simple Interest Calculator
- On most US auto loans, interest is simple but accrues daily on the outstanding balance, so paying a few days early genuinely lowers your interest. At 7% on an $18,000 balance, that is about $3.45 per day ($18,000 x 0.07 / 365).
- Because auto-loan interest is daily-simple, sending your payment a week early or making an extra mid-cycle payment cuts the days that accrue interest. Model the payoff with the <a href="/auto-loan-calculator/">auto loan calculator</a> first.
- Always confirm whether a payday or title lender quoted a monthly or annual rate. A 25% rate sounds simple, but 25% per month is a 300% APR (25% x 12), not 25% a year.
- For terms under a year, enter time as a fraction: 90 days is 0.247 years (90 / 365), 6 months is 0.5. Simple interest scales exactly, so half the time is half the dollars.
- Use the per-day figure when you request a 10-day payoff quote from a lender; the quote should equal your remaining balance plus principal x rate / 365, times the number of days outstanding.
- Simple interest favors you as a borrower but works against you as a saver, because you never earn interest on interest. Compare any simple-interest CD or note against a compounding one in the <a href="/cd-calculator/">CD calculator</a> before you commit.
- On a promissory note between family or business partners, write down the rate, the day-count basis (365 vs 360), and that interest is simple. Ambiguity on these three points causes most note disputes.
- Bond and Treasury coupons are simple interest on face value, but if you reinvest each coupon you manufacture compounding yourself. Track that growth separately, not in this tool.
- Watch for simple-interest add-on auto contracts: the lender computes total interest upfront as P x r x t, adds it to principal, then splits the sum into equal payments. Your effective APR is higher than the stated rate because you keep paying interest on money you have already repaid.
- If a note uses a 360-day year (common in commercial lending), daily interest is slightly higher than on a 365-day basis. Ask which convention applies before signing.
Simple interest vs compound interest: the gap grows every year
Simple interest is charged only on the original principal, while compound interest is charged on principal plus all previously accrued interest, so the gap between them widens every year. Over one year they are identical; over decades they diverge dramatically. Simple interest is the linear case, the exact opposite of compounding.
Here is the same $10,000 at 6% under both methods, with no payments made and annual compounding, every figure recomputed:
| Years | Simple interest | Compound interest (annual) | Extra from compounding |
|---|---|---|---|
| 1 | $600.00 | $600.00 | $0.00 |
| 5 | $3,000.00 | $3,382.26 | $382.26 |
| 10 | $6,000.00 | $7,908.48 | $1,908.48 |
| 20 | $12,000.00 | $22,071.35 | $10,071.35 |
| 30 | $18,000.00 | $47,434.91 | $29,434.91 |
For a borrower, simple interest is the friendlier deal because the cost grows in a straight line and never accelerates. For a saver or investor, that same linearity is a drawback. Run the same numbers in the compound interest calculator to see the snowball, or model a long savings horizon with the investment calculator.
Where simple interest is actually used in the US
Most US auto loans, short-term personal and payday loans, car title loans, promissory notes, and bond or Treasury coupons all run on simple interest. Knowing this tells you exactly when to reach for this calculator instead of a compounding one.
- Auto loans: nearly all are simple-interest loans where interest accrues daily on the outstanding balance, which is why early or extra payments save you money. The federal regulator's CFPB auto loan basics explains the structure.
- Payday and short-term personal loans: quoted as a flat fee or simple rate over a few weeks; the danger is the APR, not compounding. A $15-per-$100 fee on a 14-day loan is about a 391% APR.
- Car title loans: short, high-rate, simple-interest, and secured by your vehicle title, often quoted monthly to look small.
- Promissory notes: family loans and seller financing usually specify simple interest in plain language and a fixed day-count basis.
- Bonds and Treasuries: coupons are simple interest on the unchanging face value, paid out rather than reinvested.
For the full amortized picture of an installment loan, pair this with the loan calculator.
Common simple-interest mistakes to avoid
The biggest simple-interest errors come from mixing up rate and time units, not from the formula itself. Avoid these six:
- Leaving the rate as a percent: you must use 0.08, not 8, in P x r x t. Entering 8 makes the answer 100 times too large.
- Using months as if they were years: a 9-month note is t = 0.75, not 9. The calculator converts for you, but hand math does not.
- Confusing monthly and annual rates: a 2% monthly rate is a 24% annual rate. Lenders sometimes quote the smaller number on purpose.
- Assuming auto-loan interest compounds: it does not; it accrues daily-simple on the falling balance, which is precisely why paying early helps.
- Treating an add-on loan as plain simple interest: add-on contracts front-load all interest, so your true APR is higher than the quoted rate. Check the APR, not just the rate.
- Ignoring the day-count basis: 360-day-year notes accrue slightly faster than 365-day notes for the same stated rate.
How to calculate simple interest by hand or in Excel
By hand, multiply principal x rate x time; in Excel, the simplest formula is =P*r*t with the rate as a decimal. No special function is needed because simple interest is just multiplication.
For $5,000 at 8% for 3 years, type into a cell:
- Interest: =5000*0.08*3 returns 1200.
- Total payoff: =5000*(1+0.08*3) returns 6200.
To work backward, rearrange the formula: principal = I / (r x t), rate = I / (P x t), and time = I / (P x r). In a spreadsheet with P in A2, the annual rate in B2, and years in C2, the interest cell is =A2*B2*C2 and the maturity cell is =A2*(1+B2*C2). For a daily auto-loan payoff quote, compute the per-day amount with =A2*B2/365 and multiply by the number of days outstanding. Avoid Excel's =FV() function here, because FV assumes compounding and will overstate a simple-interest result.
Is your simple-interest rate good or bad?
For US borrowers in 2026, simple-interest rates roughly track loan type and credit: well-qualified auto loans tend to run in the high single digits to low teens, while title and payday products often carry triple-digit APRs. Use these benchmarks to judge an offer:
| Loan type | Typical simple-interest range | Read |
|---|---|---|
| New-car loan, strong credit | ~6% - 9% | Competitive |
| Used-car loan, fair credit | ~11% - 18% | Acceptable, shop around |
| Short-term personal loan | ~10% - 36% | 36% is a common cap; above it, walk away |
| Title / payday loan | 100%+ APR | Avoid if at all possible |
| Treasury / high-grade bond coupon | ~4% - 5% | Reasonable for a saver |
These are general ranges, not any single lender's current rate; always compare written offers. To check whether a monthly payment fits your budget, run it through the loan affordability calculator.
Advanced simple-interest uses and variations
Once you understand P x r x t, you can handle most short-term lending math and several common variations that lenders rely on.
- Daily-simple auto interest: compute daily accrual as P x r / 365, then multiply by days since your last payment to verify a payoff quote.
- Prorated interest: for any partial period, use the exact fraction of a year; simple interest is the only model where this is perfectly linear.
- Add-on installment loans: total interest is figured upfront as P x r x t, added to principal, then divided into equal payments, which raises the effective APR above the stated rate.
- Discount notes: some short-term notes deduct the interest at the start, so you receive less than face value but repay the full face amount.
- Investor coupons: if you reinvest each bond coupon, you manufacture compounding; track that growth with the future value calculator rather than here.
For a quick mental check of growth, remember the Rule of 72 applies to compounding only; under simple interest, the time to double your money is simply 1 divided by the rate (so 1 / 0.06 = about 16.7 years at 6%).
Tax and regional notes for simple interest
Simple interest you earn is usually taxable income, and simple interest you pay is sometimes deductible, but the rules depend on the loan type and your state. A few practical points for US users:
- Interest earned on notes, CDs, and bonds is generally reported as taxable interest income for the year it is paid or credited.
- US Treasury coupon interest is exempt from state and local income tax, though still subject to federal tax.
- Auto and personal loan interest is typically not tax-deductible for personal use, unlike qualifying mortgage interest.
- State usury caps limit the maximum legal rate on many loans, which is why some high-rate lenders structure products as fees rather than interest.
- Family promissory notes should charge at least the IRS applicable federal rate to avoid the loan being treated as a gift.
This is general information, not tax advice; confirm specifics for your situation with a qualified professional.
Simple interest quick-reference: interest earned or owed by amount and rate (3-year term)
Simple interest is always principal x rate x time, so for any 3-year loan or deposit you multiply the amount by the rate, then by 3. The table below shows the exact simple interest for common amounts and rates over 3 years, with every figure recomputed. Add the interest to the principal to get the total owed or earned. Remember: this is the linear, non-compounding case used by most US auto loans, short-term notes, and bond coupons, so the cost rises in a straight line rather than accelerating.
| Principal | 4% (3 yr) | 6% (3 yr) | 8% (3 yr) | 10% (3 yr) |
|---|---|---|---|---|
| $1,000 | $120 | $180 | $240 | $300 |
| $5,000 | $600 | $900 | $1,200 | $1,500 |
| $10,000 | $1,200 | $1,800 | $2,400 | $3,000 |
| $25,000 | $3,000 | $4,500 | $6,000 | $7,500 |
For example, $10,000 at 8% for 3 years earns or costs $2,400 in simple interest ($10,000 x 0.08 x 3), for a $12,400 total. To grow savings instead, use the compound interest calculator, where interest earns interest.
Related on this site
Compound Interest Calculator · Auto Loan Calculator · Loan Calculator · Loan Affordability Calculator · Future Value Calculator · CD Calculator
For a related deep dive, see CFPB auto loan basics.
Simple Interest Calculator — frequently asked questions
- Simple vs compound?
- Simple interest ignores past interest; compound interest earns interest on interest, growing faster.
- Where is it used?
- Car loans, short-term personal loans and some bonds use simple interest.
- Where is simple interest used?
- Short-term personal loans, some car loans and certain bonds and notes.
- Simple vs compound — which grows more?
- Compound always grows faster because it earns interest on accumulated interest.
- How much interest does a $5,000 loan at 6% cost over 4 years with simple interest?
- A $5,000 loan at 6% simple interest costs $1,200 in interest over 4 years, making the total $6,200. The math is principal x rate x time: $5,000 x 0.06 x 4 = $1,200. Because simple interest never charges interest on past interest, the cost stays flat at $300 per year. The same loan under yearly compounding would run about $1,312, so simple interest saves you roughly $112 here. See the <a href="/compound-interest-calculator/">Compound Interest Calculator</a> to compare.
- How much interest will I pay on a $20,000 car loan at 7% for 5 years?
- A $20,000 auto loan at 7% simple interest carries about $7,000 in interest over 5 years if no extra principal is paid, for a $27,000 total. Most US car loans are simple-interest: interest accrues daily on the remaining balance, so paying early shrinks the cost. The $7,000 figure ($20,000 x 0.07 x 5) is the maximum, reached only if you never pay ahead. Estimate payments with the <a href="/auto-loan-calculator/">Auto Loan Calculator</a>.
- What is the dollar gap between simple and compound interest on $10,000 at 5% over 30 years?
- Over 30 years, $10,000 at 5% earns $15,000 of simple interest but about $33,219 with annual compounding, a gap of roughly $18,219. Simple interest adds a flat $500 each year ($10,000 x 0.05), so the balance reaches $25,000. Compounding earns interest on interest, ending near $43,219. The gap is tiny early on but explodes over decades, which is why long-term savers want compounding. Run it in the <a href="/compound-interest-calculator/">Compound Interest Calculator</a>.
- How do I calculate simple interest in Excel?
- In Excel, calculate simple interest with the formula =Principal*Rate*Time entered in one cell. For example, =10000*0.05*3 returns 1500, the interest on $10,000 at 5% for 3 years. To get the total amount owed or earned, use =Principal*(1+Rate*Time), so =10000*(1+0.05*3) returns 11500. Keep the rate as a decimal (5% = 0.05) and time in years. There is no built-in SIMPLE function, so the plain multiplication formula is the standard approach. Avoid =FV(), which assumes compounding.
- How do I calculate simple interest by hand?
- To calculate simple interest by hand, multiply the principal by the annual rate (as a decimal) by the time in years: I = P x r x t. For $8,000 at 9% for 90 days, first convert the days to years (90 / 365 = 0.2466), then $8,000 x 0.09 x 0.2466 = about $177.53 in interest. Add it to the principal for the payoff: $8,177.53. The only tricky steps are turning the rate into a decimal and the term into years.
- How much does a $1,500 payday loan really cost in fees?
- A typical $1,500 payday loan costs roughly $225 in fees for a two-week term, because lenders charge about $15 per $100 borrowed. That $225 fee on a 14-day loan equals a simple-interest APR near 391%, even though it is not always labeled as interest. Payday loans use short flat fees rather than amortized interest, so rolling the loan over multiplies the cost fast. Always compare against a cheaper short-term option before borrowing.
- What APR does a $3,000 car title loan at 25% per month work out to?
- A car title loan charging 25% per month works out to roughly a 300% annual percentage rate. On $3,000 borrowed for one month, that is $3,000 x 0.25 = $750 in interest for 30 days, and 25% x 12 months = 300% APR. Title loans are quoted monthly to look small, but the simple-interest annual cost is severe and the lender can repossess your vehicle. Treat 300% APR as a last resort, not a normal loan.
- How much interest does a $50,000 bond paying a 3% coupon generate in a year?
- A $50,000 bond with a 3% annual coupon pays $1,500 in interest per year ($50,000 x 0.03). Bond coupons are pure simple interest on the face value, so the payment does not grow year to year unless you reinvest it elsewhere. A $100,000 Treasury with a 4% coupon paid semiannually delivers $2,000 every six months, totaling $4,000 a year. To compound those payments, reinvest each coupon and track it in the <a href="/investment-calculator/">Investment Calculator</a>.
- Is simple interest better than compound interest for a borrower?
- Yes, simple interest is almost always better for a borrower because you never pay interest on unpaid interest. On a $15,000 loan at 8% for 5 years, simple interest costs $6,000, while yearly compounding would cost about $7,040, roughly $1,040 more. The longer the term and higher the rate, the bigger your savings under simple interest. That is one reason simple-interest auto loans reward paying ahead. Savers, by contrast, prefer compounding.
- How much interest will $25,000 earn at 4.5% simple interest over 6 years?
- At 4.5% simple interest, $25,000 earns $6,750 over 6 years, bringing the total to $31,750. The yearly interest is a flat $1,125 ($25,000 x 0.045), repeated six times. Under annual compounding the same deposit would earn about $7,557, so you would give up roughly $807 by choosing a simple-interest product for savings. For growing money, a compounding account in the <a href="/savings-calculator/">Savings Calculator</a> is the better fit.
- What is the simple interest on a $12,000 loan at 6.5% for 7 years?
- A $12,000 loan at 6.5% simple interest accrues $5,460 over 7 years, for a $17,460 payoff. Each year adds a flat $780 ($12,000 x 0.065). Because nothing compounds, the cost rises in a straight line rather than accelerating. If this were compounded annually instead, the interest would climb to about $6,648, around $1,188 more. Simple interest keeps a multi-year fixed loan noticeably cheaper for the person paying it back.
- How much would a $40,000 auto loan at 5% cost in interest over 6 years?
- A $40,000 simple-interest auto loan at 5% would cost up to $12,000 in interest over 6 years if you only make scheduled payments, for a $52,000 total. Each year contributes a flat $2,000 ($40,000 x 0.05). In practice, simple-interest auto loans accrue daily on the falling balance, so steady payments cost less than this ceiling. Annual compounding on the same terms would reach about $13,604, roughly $1,604 more.
- What is the simple interest on a $7,500 personal loan at 10% for 3 years?
- A $7,500 personal loan at 10% simple interest costs $2,250 over 3 years, for a $9,750 total. That is $750 of interest each year ($7,500 x 0.10), three times. Short-term personal loans and promissory notes often use this flat structure, which keeps the math predictable. The same loan compounded yearly would cost about $2,483, so simple interest saves the borrower roughly $233. Compare loan structures in the <a href="/loan-calculator/">Loan Calculator</a>.
- How much simple interest does $100,000 earn at 2.5% in 6 months?
- $100,000 at 2.5% simple interest earns $1,250 in 6 months. The calculation is $100,000 x 0.025 x 0.5 = $1,250, because half a year is 0.5 in the time factor. This is how Treasury and bond coupon math works on a face value. If the same money compounded monthly instead, it would earn about $1,257, only about $7 more, because a short period leaves compounding little room to add up.
- Why does the gap between simple and compound interest grow with time?
- The gap grows because compound interest earns interest on interest while simple interest only ever charges the original principal. On $1,000 at 5%, simple interest adds a flat $50 a year, so after 10 years you have $500 of interest. Compounding reaches about $629, a $129 gap, and that difference keeps widening every year as the compounding base gets larger. Over short terms the gap is small; over decades it dominates the outcome.
Guides & articles
- How to Calculate Simple Interest: Formula, By Hand, and in Excel
- Simple Interest vs Compound Interest: How the Dollar Gap Grows Over Time
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