PayoffTable

Mortgage Calculator with Extra Payments

If you are shopping for a home or refinance, the honest question is not "what is my payment?" but "what is my payment, and what happens if I build an extra $100 of principal into it from the first month?" This calculator answers both at once.

Reviewed by the PayoffTable editorial team · Updated 2026-10-24

Your numbers

Total purchase price
Cash you pay at closing
Annual rate on the loan
15, 20, or 30 years are most common
Principal only, added every month

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The standard advice — "buy the home, then figure out prepayment later" — costs real money. An extra payment started in month 1 of a $320,000 loan at 6.5% saves tens of thousands more than the same extra started in month 60, purely because of compounding time. If you already know you will prepay, the shopping comparison should include it.

This calculator is the purchase-side companion to our existing-loan tools. Enter the home price and down payment you are actually considering, the rate you have been quoted, and the term — then add the monthly extra you intend to sustain. The result is your real amortization schedule, not the contractual minimum you were never going to follow anyway.

Use it to compare loan options on equal footing: a 30-year loan with a $200/month extra versus a 15-year loan at a lower rate often land within striking distance of each other on total interest, with very different monthly obligations and risk profiles. The 15 vs 30 year guide works through that comparison in detail.

How the payment plus extra is calculated

The base payment is the standard amortization formula on the loan amount (home price minus down payment) at your quoted rate over the term. The extra is applied to principal immediately after each scheduled payment, starting with payment 1.

Two numbers deserve attention in the output: the "payoff date" — which will be years before the contractual end — and "total interest," which is the real cost of the loan as you will actually run it. Lenders quote the contractual numbers; this tool shows your numbers.

M = P · r(1+r)ⁿ / ((1+r)ⁿ − 1), P = price − down then: balance_m+1 = balance_m − (M − balance_m × r) − extra

Note what is deliberately absent: property tax, homeowner's insurance, and mortgage insurance. Those ride in escrow on top of the payment and are not part of amortization. If you need a full housing-cost estimate, add your annual tax and insurance to the payment mentally — they do not change any payoff math.

Worked example. Home $400,000, down $80,000 → loan $320,000 at 6.5%, 30 years. Contractual: payment $2,022.62, interest $408,142.36. With $100/month extra from payment one: effective payment $2,122.62, payoff Dec 2052, interest $346,443.89 — $61,698.47 saved and 46 months sooner.

Terms & definitions

Amortization
The process of paying off a loan with scheduled payments that cover both interest and principal. Early payments are mostly interest; later payments are mostly principal.
Principal
The amount you borrowed and still owe. Extra payments reduce principal directly, which shrinks every future interest charge.
APR
Annual Percentage Rate — the yearly cost of the loan including fees. Your monthly interest charge uses the note rate, which is usually a touch lower.
Term
The number of years the loan is scheduled to last. A 30-year term has lower payments but roughly double the total interest of a 15-year term at the same rate.

Frequently asked questions

Should I get a 30-year and pay it like a 15-year?
Mathematically, paying a 30-year loan on a 15-year pace (via extras) nearly matches a real 15-year loan's interest savings, at a slightly higher rate than the 15-year would offer. The advantage is flexibility: your contractual minimum stays low if income drops. The disadvantage is discipline: nothing forces the extra. Our 15 vs 30 year guide quantifies the tradeoff.
Does this calculator include PMI?
No — private mortgage insurance is a cost of borrowing with less than 20% down, not part of amortization. It typically cancels automatically at 78% loan-to-value (or 80% on request), so aggressive prepayment with less than 20% equity can eliminate PMI earlier — a bonus this calculator does not model.
Can I change the extra amount later?
Yes — that is the flexibility argument. The schedule shown assumes a constant extra for illustration; in practice you can raise, lower, or pause extras any month with zero penalty. Recalculate any time with your then-current balance.
What extra amount should I budget as a new buyer?
Only what survives a realistic stress test: cover the emergency fund first (3–6 months of expenses, new roofs and furnaces do not wait), capture any employer retirement match, then fix an extra that still works in your worst month, not your best. Consistency beats size — the extra payment ladder shows what each level buys.

References

  1. Consumer Financial Protection Bureau — Loan Estimate explainer (how principal, interest, and amortization appear on your closing documents)
  2. CFPB — What is an amortization schedule?
  3. Federal Reserve — Mortgage origination resources (APR vs note rate)
Educational content only — not financial, tax, or legal advice. Your loan terms govern; confirm details with your servicer.

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