Amortization with extra payments
An amortization schedule shows how each payment splits into interest and principal. When you pay extra toward principal, the balance drops sooner, so future interest is lower and the payoff date can move earlier.
What changes in the table
- Principal is higher the month you add extra.
- Next month's interest is lower because the balance is smaller.
- The final payment happens earlier (often fewer total payments).
What doesn't change (usually)
- Your required monthly payment stays the same unless you recast.
- Escrow (tax/insurance) is separate; extra principal doesn't reduce escrow.
- Your statement may round differently than a calculator.
How to read the columns
A standard amortization row answers one question: "After this payment, what is the new balance?" Once you understand that, the rest of the table is much easier to interpret.
- Starting balance: what you owe at the beginning of the month.
- Interest: starting balance * monthly rate.
- Principal: your scheduled P&I payment minus interest.
- Extra principal: any additional amount you send that month (principal-only).
- Ending balance: starting balance - principal - extra principal.
The next month's interest is computed from the ending balance. That's why extra principal has a compounding effect on interest saved over time.
One important nuance: your mortgage servicer may show "total payment" including escrow, but an amortization schedule is usually about principal and interest. Escrow payments can change over time due to tax/insurance changes, while extra principal targets the loan balance itself.
If you're making extra payments, look for a line item or notation that confirms the extra was applied as principal-only. If a payment is recorded as "paid ahead" without reducing principal, you won't get the interest savings you expect.
Timing matters
The earlier you reduce principal, the more interest you can avoid. A smaller monthly extra can beat a larger lump sum that happens much later.
To model timing, compare scenarios in the Extra Payment Mortgage Calculator. If you only need the baseline principal-and-interest table, use the Amortization Schedule Calculator.
Amortization with extra payments inputs
- Current balance and remaining term.
- Note rate (not APR).
- Monthly extra amount or one-time lump sum timing.
- Principal-only posting rules.
Worked example: what changes month to month
Example: $300,000 at 6.50% APR for 360 months. Compare baseline to adding $300/month extra principal.
| Scenario | Month 1 interest | Month 1 principal | Month 1 extra | Month 1 ending balance | Month 2 interest |
|---|---|---|---|---|---|
| Baseline | $1,625.00 | $271.20 | $0.00 | $299,728.80 | $1,623.53 |
| With extra | $1,625.00 | $271.20 | $300.00 | $299,428.80 | $1,621.91 |
Because the ending balance is lower after extra principal, the next month's interest is lower. Over time, that can save substantial interest and shorten the payoff timeline.
In this example, the baseline finishes in 360 months with about $382,633 of interest. With extra principal it finishes in 250 months with about $247,518 of interest.
Common mistakes
- Assuming an extra payment lowers your required payment (it usually shortens the term instead).
- Not confirming the lender applies extra payments as principal-only.
- Comparing scenarios without keeping the baseline consistent (rate, term, escrow assumptions).
Checklist: to read an amortization table correctly
- Look at ending balance: that's what drives next month's interest.
- Separate P&I vs escrow: extra principal affects loan balance, not taxes/insurance.
- Confirm principal-only: make sure extra is not just advancing the due date.
- Keep assumptions consistent: rate, term, start month, and extra timing must match.
- Check rounding: lenders may round slightly differently than a calculator.
FAQ
Do extra payments reduce total interest?
Usually yes, because you reduce principal sooner, which reduces the balance used to compute interest in future months.
Does extra principal change the monthly payment?
Typically no. Most mortgages keep the required payment the same and shorten the term, unless you recast or refinance.
Is it better to pay extra early or later?
Earlier tends to save more interest because it reduces the balance sooner. But cash flow and emergency savings matter too.
References
- CFPB: Mortgages resources
Next steps
Questions or corrections? Email admin@practicalfinancetools.com.
Last updated: 2026-02-08