# $500/Month Extra on a $300k Mortgage — Mortgage Payoff Calculator

> Adding $500/month extra to a $300k mortgage at 6% saves $160,295 in interest and cuts the 30-year loan down to just 17 years 8 months.

- **Answer:** Save ~$160,295 in interest
- **Category:** Finance & Money
- **Interactive calculator:** https://youcalc.com/en/finance-money/mortgage-payoff/extra-500-per-month/
- **Price:** Free, no sign-up required
- **Full calculator:** [Mortgage Payoff Calculator](https://youcalc.com/en/finance-money/mortgage-payoff/)
- **Pre-filled inputs:** `m=known-term&p=300000&r=6&t=30&ry=30&rm=0&pk=extra-monthly&ea=500&c=USD`

| Metric | Value |
| --- | --- |
| Interest saved | $160,295 |
| Time saved | 12 yr 4 mo |
| New payoff term | 17 yr 8 mo |
| Monthly payment | $2,299 |
| Original term | 30 yr |


## Overview

An extra $500/month halves the remaining loan life — a 30-year mortgage becomes a 17-year 8-month loan — and saves $160,295 in interest, nearly half the original purchase price. Notice how diminishing returns set in: the jump from +$0 to +$100 saves $53,347, but the jump from +$400 to +$500 saves proportionally less. This is because each dollar of extra payment becomes less powerful as the remaining balance shrinks.

## Method

The calculation uses standard mortgage amortisation. The scheduled monthly payment is derived from the level-payment formula: P × r(1+r)^n / ((1+r)^n − 1), where P is the original principal, r the monthly interest rate (annual rate divided by 12), and n the total number of payments. From there, the calculator runs a month-by-month simulation: each period it applies the monthly rate to the current balance to find the interest charge, subtracts that from the payment (base plus any extra) to find the principal reduction, and carries the lower balance into the next period. Extra payments are added to the principal portion only, directly reducing the outstanding balance. The simulation stops when the balance reaches zero, and the total months elapsed becomes the new payoff term. Savings in interest and months are the difference between the no-extra simulation and the with-extra simulation, measured from the current period forward. The finance utilities supply the level-payment and balance-after-k-payments helpers used in the preliminary setup step.

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## Sources

- https://www.investopedia.com/mortgage-calculator-5096931
- https://www.calculator.net/mortgage-payoff-calculator.html

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Interactive version: https://youcalc.com/en/finance-money/mortgage-payoff/extra-500-per-month/ · From YouCalc — https://youcalc.com