# $100/Month Extra on a $300k Mortgage — Mortgage Payoff Calculator

> Adding $100/month to a $300k 30-year mortgage at 6% saves $53,347 in interest and pays it off almost 4 years early — for just $100 more each month.

- **Answer:** Save ~$53,347 in interest
- **Category:** Finance & Money
- **Interactive calculator:** https://youcalc.com/en/finance-money/mortgage-payoff/extra-100-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=100&c=USD`

| Metric | Value |
| --- | --- |
| Interest saved | $53,347 |
| Time saved | 3 yr 11 mo |
| New payoff term | 26 yr 1 mo |
| Monthly payment | $1,899 |
| Original term | 30 yr |


## Overview

A standard $300,000 30-year mortgage at 6% carries $347,515 in lifetime interest. Paying an extra $100 a month from day one cuts that by $53,347 and pays the loan off in 26 years 1 month instead of 30 — nearly four years saved for the price of one restaurant meal a week. The extra $100 goes almost entirely to principal in the early years, when your loan balance is highest and interest is most expensive.

## 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-100-per-month/ · From YouCalc — https://youcalc.com