How your monthly payment is calculated
Fixed-rate loans are amortized: each payment covers the month's interest first, and whatever remains reduces your principal. Because the balance shrinks each month, the interest portion falls over time while the principal portion grows — that's why the bar above looks so uneven at the start.
Where P is the amount borrowed, i = APR ÷ 12 (the monthly rate) and n is the total number of monthly payments. If the rate is 0%, the payment is simply P ÷ n.
What moves your payment the most
- Term — the biggest lever. Same $300,000 at 6% APR: a 30-year loan costs $1,798.65/mo with $347,514 total interest, while a 15-year costs $2,531.57/mo with only $155,683. Paying $733 more per month saves about $191,800.
- Rate — each percentage point matters. On that same 30-year loan, 7% instead of 6% adds roughly $200 to every monthly payment.
- Amount — obviously proportional, but remember a larger down payment also often unlocks better rates.
Does this work for my loan type?
| Loan type | Works with this calculator? |
|---|---|
| Fixed-rate mortgage | ✅ Yes — exactly this formula |
| Auto loan | ✅ Yes (enter term in months, e.g. 60) |
| Personal loan | ✅ Yes |
| Federal student loan (fixed) | ✅ Yes |
| Adjustable-rate mortgage (ARM) | ⚠️ Only for the initial fixed period |
| Interest-only / credit cards | ❌ No — different mechanics |
Frequently asked questions
How is a monthly loan payment calculated?
With the amortization formula M = P × i × (1+i)ⁿ ÷ ((1+i)ⁿ − 1), where P is the principal, i the monthly rate (APR ÷ 12) and n the number of payments. The calculator applies it instantly, including the 0% edge case.
Does a shorter term really save money?
Yes — see the 15 vs 30-year example above: about $191,800 saved on a $300,000 loan at 6%, for $733 more per month.
Does making extra payments help?
Yes. Extra payments hit the principal directly, so every future month accrues interest on a smaller balance. One extra payment per year on a 30-year mortgage typically shortens it by 4–6 years.
Is APR the same as the interest rate?
Not quite. The rate is the cost of borrowing; the APR also rolls in certain fees, making it the better number for comparing offers. This calculator assumes the rate you enter covers the whole cost — check your lender's APR.