🏠 Mortgage Extra Payment Calculator: See How Much Interest and Time You Save

How to use this extra mortgage payment calculator

1. Enter your current loan balance (or original amount and payments already made), the annual interest rate and the remaining term in months.
2. Check the scheduled payment. The tool computes it from balance, rate and remaining term; if your servicer charges a different amount, type your actual payment over it.
3. Add any combination of three extras: an extra monthly amount, an extra yearly amount with its month, and a one-time lump sum with the month it is paid.
4. Read the headline result (months saved, interest saved, new payoff date), then open the annual summary and the full month-by-month comparison table.
5. Scroll to the method panel to see the formula with your numbers substituted, and to the source list for what was and was not verified.

How much interest will I save with extra mortgage payments?

Interest each month equals the balance multiplied by the annual rate divided by twelve, so a principal reduction today lowers the interest charged in every month that follows. The size of your saving depends on three things: how large the extra is, how early it starts, and how long the balance stays reduced. A payment made in month 1 of a 30-year loan works for 359 more months; the same payment made in year 20 works for far fewer.

The headline card above shows baseline total interest, new total interest and the difference. The tables below show where the money goes, year by year and month by month, so you can verify the arithmetic yourself instead of trusting a single number.

Mortgage payoff calculator: schedule comparison

Pay off mortgage early calculator: two side-by-side comparisons

Comparison table #1 — three hypothetical extra monthly amounts

Same loan, three different extra monthly amounts. These are examples built from your current balance, rate and term; they are not your servicer's figures.

Comparison table #2 — same yearly budget, three patterns

The same total dollars per year, spread three ways, so you can see whether a monthly drip, one lump sum or one yearly payment does more work.

Both tables are labeled hypothetical: they use your inputs but ignore escrow, fees and servicer posting rules.

How the calculation works

Let r be the monthly rate, B the starting balance, n the number of months and S the scheduled payment.

1. Monthly rate: r = annual rate ÷ 100 ÷ 12
2. Scheduled payment: S = B × r ÷ (1 − (1 + r)−n) — and S = B ÷ n when the rate is 0.
3. Each month: interest = round(Balance × r)
4. Payment that month: S + extra monthly + (extra yearly if it is its month) + (lump sum if it is its month), capped at Balance + interest.
5. Principal: payment − interest, then Balance = Balance − principal. The loan ends when the balance reaches zero.

The formula with numbers substituted (your current inputs)

Worked example (hypothetical, fixture-tested)

Hypothetical inputs: loan $300,000.00, rate 6.5%, term 30 years (360 months), scheduled payment $1,896.20, start month November 2026, extra $100.00 per month applied to principal in the same month.

First-month interest on that loan: $300,000.00 × 0.065 ÷ 12 = $1,625.00, identical with or without extra, because the extra reduces principal only. Zero extra reproduces the original schedule exactly, and an extra larger than the balance is capped at balance plus interest so the balance never goes negative. These values test arithmetic only, not law.

Infographic: what extra principal does to total interest

Infographic: before you prepay — checklist

Common mistakes and key conditions

• Assuming the servicer will apply the money to principal. Without written instruction, an extra payment may sit in suspense, pay next month's installment, or go to escrow. Your loan documents and the servicer's practice control.
• Using the original loan amount instead of the current balance. Interest accrues on what you owe today, so the calculator needs the current balance.
• Ignoring a user-entered payment that does not match the computed one. If your actual payment is higher (taxes and insurance in escrow), the principal portion differs; type the principal-and-interest part only.
• Starting the extra payments late. The same amount started a year later saves less, because 12 months of smaller balances never happen.
• Expecting a biweekly or recast result here.
• Treating the number as guaranteed. This is an estimate with limitations: lenders use daily interest accrual, posting dates and their own rounding.

Glossary

Principal

The part of a payment that reduces what you owe. Interest is the cost charged on the balance.

Scheduled payment

The monthly principal-and-interest amount required by the loan, before escrow additions.

Amortization schedule

The month-by-month list showing how each payment splits between interest and principal until the balance hits zero.

Extra principal payment

Money paid beyond the scheduled amount and applied directly to the balance, shortening the term.

Lump sum

A single one-time payment applied to principal in a chosen month.

Recast

Re-amortizing the same balance over a new term to lower the monthly payment; not modeled here.

Prepayment penalty

A fee some loans charge for paying off principal early; check your loan documents, because this page does not determine whether yours has one.

Months saved

Baseline months minus new months, using the same start date for both schedules.

Related calculators

• Mortgage Calculator — build the base monthly payment and full amortization schedule first.
• Amortization calculator with extra payments — schedule-focused view of payments over time.
• Mortgage calculator with taxes, insurance and PMI — the full monthly housing cost, including escrow.
• Home Affordability Calculator — how much house the income supports before you compare loans.
• Property Tax Calculator — an estimate of the tax line that usually rides in escrow.
• Mortgage refinance calculator with break-even — the alternative use of the same cash: refinance instead of prepay.

For the base payment and general mortgage questions, the Mortgage Calculator and the Home Affordability Calculator are the primary tools on this site; this page only answers the extra-payment question.

Official references

These are official starting points a reader should open to verify any rule.

• CFPB — consumer mortgage resources: https://www.consumerfinance.gov/consumer-tools/mortgages/
• CFPB Regulation Z on eCFR — 12 CFR part 1026, including candidate § 1026.43(g) on prepayment penalties: https://www.ecfr.gov/current/title-12/chapter-X/part-1026 (starting point, not opened)
• CFPB Regulation X on eCFR — 12 CFR part 1024, mortgage servicing: https://www.ecfr.gov/current/title-12/chapter-X/part-1024 (starting point, not opened)
• Federal mortgage servicing rules, candidate citations 12 CFR 1026.36 and 12 CFR part 1024: verify on eCFR yourself

Byline: Varnda Editorial Team. Corrections: use the correction channel published on the site's contact or about page.

Limitations, exclusions and assumptions

• Fixed-rate, monthly-payment loans only. ARM resets, interest-only periods and balloon loans are outside this model.
• Extra amounts are assumed to be applied to principal in the same month as the scheduled payment. Actual posting can differ.
• Escrow (property tax, insurance, PMI), servicer fees, rate changes and refinancing are not represented.
• Interest here is rounded monthly to cents; many lenders accrue daily and use their own posting rules, so real schedules can differ by small amounts.
• No tax treatment of mortgage interest and no opportunity cost of the extra cash is modeled. This page gives information, not financial, legal or tax advice.