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

> Paying $300/month extra on a $300k 30-year mortgage at 6% saves $119,701 in interest and shortens the loan by exactly 9 years.

- **Answer:** Save ~$119,701 in interest
- **Category:** Finance & Money
- **Interactive calculator:** https://youcalc.com/en/finance-money/mortgage-payoff/extra-300-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=300&c=USD`

| Metric | Value |
| --- | --- |
| Interest saved | $119,701 |
| Time saved | 9 yr |
| New payoff term | 21 yr |
| Monthly payment | $2,099 |
| Original term | 30 yr |


## Overview

An extra $300/month on a $300,000 mortgage at 6% removes 9 full years from the schedule and saves $119,701 in interest — that's more than a third of the original loan balance returned to you instead of paid in interest. This is also a useful "what if I refinanced to a 21-year term?" comparison: you get the same payoff date but with the flexibility of a 30-year minimum payment if cash flow tightens.

## 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-300-per-month/ · From YouCalc — https://youcalc.com