Mortgage Calculator with Extra Payments

Why a Small Extra Payment Does So Much

A fixed-rate mortgage is front-loaded with interest. Every month the lender charges interest on whatever balance remains, and only the rest of your payment reduces the balance — so in the early years almost nothing you pay builds equity. In the first twelve months of the example loan above, $19,401 goes to interest and just $3,353 to principal. Interest is 85.3% of every payment you make that year.

That is precisely why extra payments punch above their weight. An extra dollar is not just a dollar of debt repaid; it is a dollar of balance that stops accruing interest for the remaining life of the loan. Ten years in, the annual interest on this loan has only fallen to $16,745 — the balance is still large, and there are still twenty years of compounding left to short-circuit.

The Example: $200 a Month on a $300,000 Loan

A $300,000 loan at 6.5% over 30 years costs $1,896 a month and $382,633 in total interest. Add $200 a month and the loan is paid off in 277 payments instead of 360 — that is 23.1 years instead of 30, and 83 payments cut from the schedule.

The saving is $103,449 — about 27% of the interest you would otherwise pay — in exchange for $55,400 of extra principal spread over those 277 months. Nothing else about the loan changes: same house, same rate, same lender.

Extra Payment Amounts Compared

All figures below use the same $300,000 loan at 6.5% over 30 years, with the extra amount applied every month from the first payment.

Notice that the returns are not linear in the way people expect. Going from $50 to $100 saves $27,413 more; going from $500 to $1,000 — ten times the first step — saves $61,403 more. The bigger the extra payment, the more compounding you short-circuit, but the sooner the loan ends the less time there is left to save.

Biweekly Payments vs. Monthly

A biweekly plan has you pay half your monthly amount every two weeks. Because there are 26 two-week periods in a year, you make 26 half-payments — 13 full payments instead of 12. The entire benefit is that one hidden extra payment, which on this loan works out to $158.02 a month.

Modelled that way, biweekly payment cuts the loan to 290 payments — 24.2 years — and saves $87,256 of interest. You can reproduce almost exactly the same result by paying $158.02 extra on your own schedule, with no enrolment and no fee. One caveat on the arithmetic: this model applies the extra amount monthly, whereas a true biweekly plan reduces principal every two weeks, so a real plan shaves off slightly more.

Extra Payments vs. Taking a 15-Year Loan

A 15-year loan at the same 6.5% costs $2,613 a month — 37.8% more than the 30-year payment — but only $170,398 in total interest, saving $212,235. That is the biggest interest saving available, and it comes from a contractual obligation rather than a habit.

The middle path is more interesting. On a 30-year loan, paying $717 extra a month — the difference between the two payments — brings the total interest down to $170,398 in exactly 180 payments, matching the 15-year loan to the dollar. You get identical economics with the option to stop whenever money gets tight. The 15-year loan's real advantage is discipline, and on some loans a slightly lower rate; the extra-payment route's advantage is flexibility.

Prepayment Penalties

Federal rules bar prepayment penalties on the large majority of home loans written today, and any penalty that does exist must be disclosed in your loan documents. Where they still appear — some non-qualified mortgages, certain jumbo and investor loans, and most hard-money or bridge financing — they are usually structured as a percentage of the balance or a set number of months of interest, applying only if you pay off or refinance within the first few years.

Read the prepayment section of your note before you send extra money. If the clause is there, the calculator's savings figure is not the whole story; subtract the penalty to see whether paying ahead still wins.

Frequently Asked Questions

Does an extra payment automatically go to principal?

It should, but you have to make sure. Many servicers apply an unspecified overpayment to the next scheduled payment or hold it in a suspense account instead of reducing the balance, which saves you nothing. Tell your servicer in writing that the extra amount is to be applied to principal, then check the following statement to confirm the balance dropped by the full amount. This calculator assumes every extra dollar goes straight to principal.

Is paying biweekly better than paying monthly?

Only because it hides an extra payment. Paying half your monthly amount every two weeks means 26 half-payments a year, which is 13 full payments instead of 12 — the equivalent of adding $158.02 a month to the example on this page. That cuts the loan to 290 payments and saves $87,256 in interest. You get the same result by paying $158.02 extra every month yourself, without signing up for a program that may charge a fee.

Should I pay extra on my mortgage or invest the money instead?

That depends on what you could earn elsewhere and on how you feel about debt, and this calculator only shows one half of the comparison. What it gives you is the guaranteed side: every dollar of extra principal saves exactly the interest that dollar would have accrued, which on the example here is 6.5% a year. Compare that after-tax figure against a realistic after-tax investment return, and remember that equity in a house is not liquid. This page does not tell you which to choose.

Do mortgages charge a prepayment penalty?

Most do not. Federal rules bar prepayment penalties on the large majority of home loans written today, and any penalty that does exist has to be disclosed in your loan documents. Some non-qualified mortgages, certain jumbo and investor loans, and most hard-money or bridge financing still carry them, typically as a percentage of the balance or a set number of months of interest that applies only if you pay off or refinance within the first few years. Read the prepayment section of your note before you send extra money.

References

• CFPB — What is an amortization schedule?: how each payment splits between interest and principal over the life of the loan.
• CFPB — Can I make extra payments on my mortgage?: what to ask your servicer and how to check that the payment was applied correctly.
• Investopedia — Amortization: a clear explanation of why early payments are mostly interest.

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