To find how much you can borrow from a monthly payment, run the loan formula in reverse: maximum loan = payment x [1 - (1 + r)^-n] / r, where r is the monthly interest rate and n is the number of payments. Pick a payment you can comfortably afford, lock in a realistic rate and term, and the math hands you the largest loan that payment supports. A $1,500 monthly payment at 6.5% over 30 years works backward to about $237,316 in principal.
This is the opposite of a normal loan calculator. A forward calculator asks "here is the loan, what is the payment?" An affordability calculator flips it: "here is the payment I can live with, how big a loan does that buy?" That single reversal is why it is the right tool when you are budgeting before you shop, not after. Run your own numbers in the Loan Affordability Calculator and use this guide to read the result correctly.
The reverse formula, in plain English
Borrowing power is just the present value of your future payments. Every dollar you will pay in the future is worth a little less than a dollar today, because money today could earn interest. Discount your full stream of payments back to today and you get the maximum loan a lender can hand you now.
The full formula is:
Max loan = M x [1 - (1 + r)-n] / r
- M = the monthly payment you can afford (principal and interest only).
- r = the monthly rate = annual rate / 12. At 6.5%, r = 0.065 / 12 = 0.0054167.
- n = total number of payments = years x 12. A 30-year loan is 360 payments.
Plug in M = $1,500, r = 0.0054167, n = 360 and you get roughly $237,316. This is the same amortization math a forward calculator uses, only solved for the loan amount instead of the payment. If you want to see the standard direction first, read how loan interest works.
How interest rate changes your borrowing power
A higher rate shrinks the loan a fixed payment can buy, because more of each payment goes to interest and less to principal. Keep the payment at $1,500 and the term at 30 years, and watch the maximum loan move as only the rate changes:
| Interest rate | Max loan on $1,500/mo, 30 years |
|---|---|
| 5.0% | $279,422 |
| 6.0% | $250,187 |
| 6.5% | $237,316 |
| 7.0% | $225,461 |
| 8.0% | $204,425 |
The gap is large. Going from 5% to 8% on the same $1,500 payment cuts your borrowing power by about $74,997 - roughly a quarter - without you spending one extra dollar a month. A single point from 6% to 7% costs about $24,726, or close to 10% of the loan. This is why a small rate move matters far more than people expect, and why locking a rate is a real decision, not a formality.
How loan term changes your borrowing power
A longer term buys a bigger loan for the same payment, but you pay far more interest to get there. Hold the payment at $1,500 and the rate at 6.5%, and stretch only the term:
| Term | Max loan on $1,500/mo at 6.5% | Total paid over the term |
|---|---|---|
| 15 years | $172,195 | $270,000 |
| 20 years | $201,188 | $360,000 |
| 30 years | $237,316 | $540,000 |
Stretching from 15 to 30 years lets you borrow about $65,121 more on the same payment. But total payments jump from $270,000 to $540,000 - an extra $270,000 out of pocket, almost all of it interest. The longer term does not make the loan cheaper; it makes the monthly payment smaller and the lifetime cost much larger. That trade-off is the same one behind a 15-year vs 30-year mortgage.
A full borrowing-power grid
For a quick read on any payment, scale a $1,000 grid up or down - borrowing power is exactly proportional to the payment. If $1,000 buys $166,792, then $2,000 buys exactly double, $1,500 buys 1.5x, and so on. Here is the maximum loan a $1,000 monthly payment supports across common rates and terms:
| Term | 5% | 6% | 7% | 8% |
|---|---|---|---|---|
| 15 years | $126,455 | $118,504 | $111,256 | $104,641 |
| 20 years | $151,525 | $139,581 | $128,983 | $119,554 |
| 30 years | $186,282 | $166,792 | $150,308 | $136,283 |
To use it: find your cell, then multiply by your real payment in thousands. Affording $1,800 a month at 7% over 30 years? That is 1.8 x $150,308 = about $270,554. The proportionality trick only works when the rate and term are identical, so keep those fixed when you scale.
Affordability vs forward loan and down-payment calculators
Use the affordability calculator to set your budget, a forward loan calculator to confirm a specific loan, and a down-payment calculator to bridge price and loan size. Each answers a different question:
- Loan affordability: payment in, maximum loan out. Best before you shop, when you know your budget but not the price.
- Forward loan or mortgage calculator: loan amount in, payment out. Best once you have a specific price in mind. Try the Loan Calculator or Mortgage Calculator.
- Down payment: turns a purchase price and a loan size into the cash you need up front. See the Down Payment Calculator.
A common flow: the affordability tool says a $1,500 payment buys a $237,316 loan, your down payment adds $50,000 of cash, so you shop near a $287,000 price - then a forward mortgage calculator confirms the exact payment with taxes and insurance added.
One number people forget: payment is principal and interest only
The maximum loan only reflects principal and interest, so for a mortgage you must subtract taxes, insurance, and HOA from your housing budget before running the math. If you can spend $1,500 a month total on housing but $350 of that goes to property tax and insurance, only $1,150 is available for principal and interest. At 6.5% over 30 years that supports about $181,942, not $237,316 - a $55,374 difference. Budgeting the full $1,500 into the loan is the single most common way people overshoot. For a personal loan or auto loan with no escrow, the full payment usually applies. Confirm the real number behind your budget with the Take-Home Pay Calculator.
Try it yourself
Enter your comfortable payment, a realistic rate, and a term you actually want into the Loan Affordability Calculator, then sanity-check the loan against a forward Mortgage Calculator. For a neutral primer on how lenders weigh affordability, the CFPB's home-buying guide is a solid reference.
Try it yourself
Run your own numbers in the free Loan Affordability Calculator — instant, private, no sign-up.
Open the Loan Affordability Calculator →Frequently asked questions
- How do I calculate the maximum loan from a monthly payment?
- Use the reverse amortization formula: max loan = payment x [1 - (1 + r)^-n] / r, where r is the annual rate divided by 12 and n is the number of months. A $1,500 payment at 6.5% over 30 years (r = 0.0054167, n = 360) supports about $237,316 in principal.
- How much can I borrow with a $1,000 monthly payment?
- A $1,000 monthly payment supports about $166,792 at 6% over 30 years, $150,308 at 7%, or $118,504 at 6% over just 15 years. Borrowing power is proportional, so a $2,000 payment buys exactly double these amounts at the same rate and term.
- Does a longer loan term let me borrow more?
- Yes, a longer term lets you borrow more on the same payment, but it costs far more interest. At $1,500 a month and 6.5%, a 30-year term buys about $237,316 versus $172,195 over 15 years - about $65,121 more borrowing power, but $270,000 more in total payments over the life of the loan.
- How much does the interest rate change my borrowing power?
- A higher rate sharply reduces what a fixed payment can borrow. On a $1,500 payment over 30 years, 5% supports about $279,422 while 8% supports only $204,425 - a $74,997 drop. Even one point, from 6% to 7%, cuts borrowing power by roughly $24,726.
- Is the maximum loan the same as my purchase price?
- No, the maximum loan is what you can finance; your purchase price is the loan plus your down payment. If the formula says $237,316 and you have $50,000 in cash, you can shop near a $287,000 price. A down-payment calculator bridges the two figures.
- Does the affordability number include property taxes and insurance?
- No, the maximum loan reflects only principal and interest. For a mortgage you must subtract taxes, insurance, and any HOA from your housing budget first. If $350 of a $1,500 housing budget goes to tax and insurance, only $1,150 buys the loan - about $181,942 instead of $237,316.
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How to Calculate a Mortgage Payment, Step by Step · 15-Year vs 30-Year Mortgage: Which Should You Choose? · How Does Loan Interest Work? · How to Calculate a Car Loan Payment