Mortgage Amortization Schedule: Formula and Guide
Learn how mortgage payments split between principal and interest, read an amortization schedule, and test loan terms with DTC calculators.
A fixed mortgage payment can stay the same while the amount reducing the loan changes every month. Early in the schedule, interest is calculated on a large balance, so less of the payment reaches principal. Later, the balance is smaller, interest declines, and more of the same payment reduces principal. An amortization schedule makes that change visible row by row.
Quick Answer: How Mortgage Amortization Works
- Amortization spreads repayment of principal and interest across a defined number of monthly payments.
- For a standard fixed-rate loan, the scheduled principal-and-interest payment is level, but its split changes.
- Monthly interest equals the remaining balance multiplied by the monthly interest rate.
- The DTC Amortization Calculator models principal and interest only; the Mortgage Calculator adds estimated taxes, insurance, PMI, and HOA dues.
- APR is broader than the note interest rate because it may reflect points, broker fees, and other charges.
- Extra-payment effects depend on lender rules and are not part of the current DTC amortization input model.
What a Mortgage Amortization Schedule Shows
An amortization schedule is a chronological table of scheduled payments. Each row normally reports the payment number, total principal-and-interest payment, interest charged for that period, principal repaid, and remaining balance. The schedule should end near zero after the final scheduled payment; small final-payment differences can occur when a lender rounds each monthly amount to cents.
Swipe sideways to compare columns.
| Column | Meaning | Reason to review it |
|---|---|---|
| Payment | Scheduled principal plus interest | Shows the recurring loan obligation |
| Interest | Balance x monthly rate | Shows borrowing cost for the period |
| Principal | Payment minus interest | Shows how much the balance falls |
| Remaining balance | Prior balance minus principal | Shows payoff progress and equity from repayment |
| Cumulative interest | Sum of interest through a date | Helps evaluate total cost or refinance timing |
Why the Payment Split Matters
The principal portion reduces what you owe. The interest portion is the cost of borrowing for that month. A borrower who sells or refinances early may discover that the balance has fallen less than expected because early payments contained more interest. That result follows directly from calculating interest on the outstanding balance; it is not a separate fee or an optional allocation chosen each month.
Mortgage Payment and Schedule Formulas
How one amortization row becomes the next
The ending balance from one month becomes the starting balance for the following month.
Start with balance
Use the original principal for month one, then the prior ending balance.
Calculate interest
Multiply that balance by the monthly rate.
Find principal
Subtract monthly interest from the fixed principal-and-interest payment.
Reduce balance
Subtract principal and repeat until the loan is paid off.
Taxes, insurance, PMI, and HOA dues are outside this principal-and-interest loop.
Worked Example: $200,000 at 6.5% for 30 Years
The live DTC amortization logic converts 6.5% to a monthly rate of 0.065 / 12 and uses 360 payments. The calculated monthly principal-and-interest payment is about $1,264.14. In month one, interest is about $1,083.33 and principal is about $180.80, leaving a balance near $199,819.20. Across the modeled term, total interest is approximately $255,088.98 and total payments are approximately $455,088.98.
Swipe sideways to compare columns.
| Payment | Payment amount | Principal | Interest | Balance after payment |
|---|---|---|---|---|
| 1 | $1,264.14 | $180.80 | $1,083.33 | $199,819.20 |
| 2 | $1,264.14 | $181.78 | $1,082.35 | $199,637.42 |
| 3 | $1,264.14 | $182.77 | $1,081.37 | $199,454.65 |
| 360 | Adjusted near $1,264.14 | Remaining balance | About $6.81 | $0 |
The payment stays level while the split changes
A simplified comparison of the first and last scheduled payments in the example.
Payment 1
Interest is high because the balance is close to $200,000.
- $1,083.33 interest
- $180.80 principal
- $199,819.20 remaining
Payment 360
Interest is low because only the final balance remains.
- About $6.81 interest
- Most of the payment is principal
- Balance reaches zero
Amounts are rounded. Actual lender schedules can differ because of payment dates and cent-level rounding.
Interest Rate vs APR
The note interest rate drives the principal-and-interest amortization calculation. APR is a broader disclosure measure that reflects the interest rate plus certain points, broker fees, and other loan charges. CFPB guidance notes that APR is usually higher than the interest rate for that reason. Do not place APR into a basic payment formula unless the tool specifically requests it; compare both rate and APR when reviewing loan offers.
From Loan Payment to Total Monthly Housing Cost
The DTC Mortgage Calculator accepts home price, down payment, interest rate, term, annual property tax, annual insurance, and monthly HOA dues. It calculates the loan amount, principal-and-interest payment, monthly tax and insurance estimates, HOA dues, and an estimated PMI amount when the down payment is below 20%. The PMI estimate uses a simplified 1% annual rate on the loan balance; actual mortgage-insurance eligibility, price, and cancellation rules depend on the loan and provider.
Swipe sideways to compare columns.
| Question | Use this tool | Included in the model |
|---|---|---|
| How does the balance change? | Amortization Calculator | Principal, rate, term, payment schedule |
| What is the broader monthly estimate? | Mortgage Calculator | P&I, estimated PMI, taxes, insurance, HOA |
| How does a general installment loan work? | Loan Calculator | Principal, rate, term, total interest |
Worked Term Comparison: 15 Years vs 30 Years
Holding a $200,000 principal and 6.5% rate constant isolates the term effect. The 15-year model produces a payment of about $1,742.21 and total interest near $113,599. The 30-year model produces a payment of about $1,264.14 and total interest near $255,089. The shorter term requires about $478 more each month but reduces modeled interest by roughly $141,490.
Swipe sideways to compare columns.
| Measure | 15-year term | 30-year term |
|---|---|---|
| Principal | $200,000 | $200,000 |
| Rate | 6.5% | 6.5% |
| Monthly principal and interest | $1,742.21 | $1,264.14 |
| Modeled total interest | About $113,599 | About $255,089 |
| Number of payments | 180 | 360 |
This is not a statement that a 15-year offer is always better. Actual 15-year and 30-year offers may use different rates, points, fees, and qualification standards. A higher required payment also reduces monthly flexibility. Compare the Loan Estimates, the cash needed at closing, emergency reserves, and the ability to sustain the payment under income or expense shocks.
Comparing Loan Terms Responsibly
A shorter term generally produces a higher required payment and less total interest when the principal and rate are held constant. A longer term generally lowers the required payment but keeps the balance outstanding longer. Real offers may also have different rates, points, fees, or qualification requirements, so compare actual disclosures rather than changing only the term in a hypothetical example.
What Extra Payments Can Change
When a servicer applies an additional amount directly to principal, the next interest calculation begins from a smaller balance. That can shorten the payoff period and reduce interest. The effect depends on timing, loan terms, prepayment provisions, and how the servicer applies the funds. The current DTC amortization page does not accept an extra-payment input, so do not use its base schedule as an extra-payment quote.
- Confirm that the loan permits prepayment without a penalty.
- Tell the servicer how the extra amount should be applied and verify the next statement.
- Keep enough liquidity for taxes, insurance, repairs, and emergencies.
- Compare the guaranteed interest reduction with other uses of the money, considering risk and taxes.
- Request an official payoff statement when closing or refinancing; a calculator balance is not a payoff quote.
Why a Fixed-Rate Borrower's Total Payment Can Change
Fixed rate describes the note rate, not every item collected with the payment. Property tax assessments can change, insurance premiums can change, mortgage insurance may be added or removed under applicable rules, and an escrow review can create a shortage or surplus. HOA dues are normally separate from the mortgage contract and may also change. Keep the loan amortization schedule separate from the household cash-flow forecast.
When reviewing a payment change, compare the principal-and-interest line with the escrow and other lines rather than assuming the interest rate changed. If the loan is adjustable, review the index, margin, adjustment schedule, caps, and required notices. The current DTC tools do not model those adjustable-rate mechanics.
Using Amortization in a Refinance Review
A lower payment can come from a lower rate, a smaller balance, or restarting repayment over a longer term. Compare the new loan's closing costs and total expected cost over the period you expect to keep it, not only the monthly difference. Also compare the new balance with the current official payoff and check whether costs are paid in cash or added to principal.
Common Amortization Mistakes
- Confusing the principal-and-interest payment with the full monthly housing outflow.
- Entering APR where the calculator asks for the note interest rate.
- Assuming every payment reduces principal by the same amount.
- Ignoring taxes, insurance, mortgage insurance, HOA dues, and maintenance when budgeting.
- Treating an estimated PMI amount as a lender quote.
- Assuming an extra payment will be applied to principal without checking servicing instructions.
- Using a calculator balance as an official payoff amount.
- Comparing terms without also comparing rates, points, fees, and cash required at closing.
How to Use the DTC Mortgage Tools
- Use the Amortization Calculator for a principal-and-interest schedule.
- Enter the loan principal rather than the home price in the amortization tool.
- Use the note rate and the full repayment term from the loan scenario.
- Review monthly payment, total interest, first-month split, and schedule rows.
- Use the Mortgage Calculator separately for taxes, insurance, HOA dues, down payment, and estimated PMI.
- Compare calculator outputs with the lender Loan Estimate and closing documents.
Assumptions and Limitations
Sources to Verify or Cite
- Consumer Financial Protection Bureau, How does paying down a mortgage work?: https://www.consumerfinance.gov/ask-cfpb/how-does-paying-down-a-mortgage-work-en-1943/
- Consumer Financial Protection Bureau, Mortgage interest rate vs APR: https://www.consumerfinance.gov/ask-cfpb/what-is-the-difference-between-a-mortgage-interest-rate-and-an-apr-en-135/
- Do The Calculation mortgage-core and finance-native-core implementations, reviewed for calculation alignment on June 30, 2026.
Mortgage Amortization FAQs
What is mortgage amortization?
It is the scheduled reduction of a loan through payments that cover interest and repay principal over a defined term.
Why is so much of the first payment interest?
Monthly interest is calculated on the outstanding balance. The balance is largest at the start, so the interest amount is also largest then.
Does a fixed mortgage payment ever change?
The scheduled principal-and-interest amount is generally fixed for a standard fixed-rate loan, but escrow items, mortgage insurance, HOA dues, and fees can change the total outflow.
Does the amortization calculator include taxes and insurance?
No. It models principal and interest. Use the DTC Mortgage Calculator for monthly tax, insurance, HOA, and simplified PMI estimates.
Should I enter the interest rate or APR?
Enter the note interest rate when the calculator asks for interest rate. APR is a broader comparison measure and is not the rate used by this basic payment formula.
What happens at a 0% interest rate?
The calculator divides principal evenly across the number of monthly payments, so every scheduled dollar repays principal.
Why might my lender schedule differ by a few cents?
Lenders may round each monthly figure, use exact payment dates, or apply contractual conventions that differ from the calculator model.
Will an extra payment always reduce principal?
Not automatically. Confirm the loan terms and servicing instructions, then verify how the amount appears on the next statement.
Can the calculator model an adjustable-rate mortgage?
No. It assumes one fixed annual rate throughout the selected term.
Is the remaining balance the same as a payoff quote?
No. An official payoff may include interest through a specific date, fees, or other adjustments. Request it from the servicer.
Does a 30-year mortgage always cost more than a 15-year mortgage?
Holding principal and rate constant, the longer term produces more interest. Actual offers may use different rates and fees, so compare full disclosures.
Does the Mortgage Calculator use an actual PMI quote?
No. Its current logic applies a simplified annual estimate of 1% of the loan balance when the down payment is below 20%.
Can I use the schedule for tax reporting?
No. Use lender statements and current tax guidance. Deductibility depends on facts and rules outside this calculator.
When does principal become larger than interest?
The crossover depends on principal, rate, and term. Review the schedule rather than relying on a universal year.
Does amortization show home equity?
It shows equity created by principal repayment only. Total home equity also depends on down payment, property value changes, liens, and transaction costs.
Final Summary
A mortgage amortization schedule explains where each principal-and-interest payment goes and how the balance moves toward zero. Use the schedule to understand the loan, then add taxes, insurance, HOA dues, mortgage insurance, fees, and actual disclosures before judging affordability or comparing offers. The DTC tools separate those two jobs so the assumptions remain visible.
Written by
Do The Calculation Team
Do The Calculation Editorial Board
The Do The Calculation Editorial Board is comprised of software engineers, finance analysts, and technical contributors focused on building clean, accurate, and easy-to-use calculator tools.
Reviewed & Verified By
Dr. Elizabeth Vance, PhD
Senior Editorial Board Member (Finance)
Former investment bank strategist and university lecturer with 15+ years of research in compound growth modeling, asset allocation, and annuity projections. Dr. Vance reviews all core investment and retirement tools to ensure absolute alignment with actuarial standards.