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Mortgage Amortization Calculator: Principal vs Interest Over Time

A mortgage amortization calculator shows how a fixed principal-and-interest payment is divided over time and estimates the balance remaining after a chosen number of months. Taxes, insurance, HOA dues, and variable-rate changes are not included.

Updated 2026-07-09Reviewed by US Finance Hub Editorial TeamCalculations run in your browser

Quick answer

Early mortgage payments usually contain more interest because the outstanding balance is highest. As principal declines, a larger share of the fixed payment goes to principal.

At a glance

What it calculates
Calculate a mortgage payment, total interest, and remaining balance after a selected number of payments.
Who it is for
US homeowners checking payoff progress or the effect of additional principal payments.
Coverage
United States (federal rules; state and local rules vary)
Data and assumptions
A fixed-rate, fully amortising loan with payments applied monthly; escrow, PMI removal and rate resets are excluded.
Cost
Free, no signup, calculations run in your browser
Last reviewed
2026-07-09

Calculator

Enter your numbers

Initial loan amount.
Fixed annual rate.
Original mortgage term.
Number of scheduled monthly payments completed.

How to use this calculator

  1. Enter the original mortgage amount.
  2. Enter the fixed rate and original term.
  3. Enter the number of payments already made.
  4. Review principal paid, interest paid, and balance.

Explanation

What it is

A mortgage amortization calculator shows how a fixed principal-and-interest payment is divided over time and estimates the balance remaining after a chosen number of months. Taxes, insurance, HOA dues, and variable-rate changes are not included.

How it works

The calculator finds the fixed payment, then applies the standard remaining-balance formula after the selected number of payments.

When to use it

Use it when you want to see how quickly principal actually falls, or to test what an extra monthly payment does to the payoff date.

Limitations

  • The schedule assumes a fixed rate and no missed or partial payments.
  • Escrow, PMI removal, recasting and rate resets are excluded.
  • Servicer rounding and payment posting dates can shift interest slightly.

Key terms

Amortization schedule
A payment-by-payment breakdown of principal and interest.
Principal
The amount borrowed that remains unpaid.
Interest
The lender’s charge for use of money.
Remaining balance
Principal still owed after credited payments.

Formula

The calculator finds the fixed payment, then applies the standard remaining-balance formula after the selected number of payments.

M = P × r(1+r)ⁿ ÷ ((1+r)ⁿ−1); remaining balance follows the present value of unpaid payments

Worked example

A $400,000 30-year mortgage at 6.5% has a fixed principal-and-interest payment. After five years, the balance remains relatively high because early payments contain more interest.

FAQ

How much of my mortgage payment goes to principal?

The share changes each month. Early payments generally contain more interest; later payments contain more principal.

Why is my mortgage balance falling slowly?

Interest is calculated on the outstanding balance, which is highest at the beginning of a long-term loan.

Does this include escrow?

No. It calculates principal and interest only. Property tax, insurance, mortgage insurance, and other charges are separate.

Can I use this for an adjustable-rate mortgage?

Only for a period with a fixed assumed rate. Future adjustments require separate calculations.

Will my lender’s payoff equal this balance?

Not exactly. A payoff quote may include daily interest, fees, unapplied funds, and timing differences.

Common mistakes

  • Assuming early payments reduce principal quickly; most of an early payment is interest.
  • Sending extra money without instructing the servicer to apply it to principal.
  • Ignoring escrow, which makes the actual monthly bill higher than the schedule.
  • Comparing loans on payment alone when the terms differ.

Tips

  • Look at the year-by-year balance, not just the payment.
  • Apply extra payments as principal-only and confirm the servicer records them that way.
  • Compare a 15-year and a 30-year schedule on total interest before choosing a term.
  • Re-run the schedule after any recast or lump-sum payment.

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Sources and editorial review

Educational estimates only; not personalized financial, tax, legal, lending, investment, or insurance advice.