loan calculator

Amortization Calculator

The Amortization Calculator estimates a fixed-rate monthly schedule summary. It solves the scheduled monthly payment from loan amount, term years and months, and annual interest rate, then simulates month-by-month interest, principal reduction, optional extra principal, payoff timing, first-month split, first-year split, total payments, total interest, and interest saved.

Result

Monthly payment: $1,687.71
Total payments: $303,788.46
Total interest: $103,788.46
Payoff months: 180
Payoff years: 15
Interest saved by extra payments: $0.00
First month interest: $1,000.00
First month principal: $687.71
First month ending balance: $199,312.29
First year interest: $11,769.23
First year principal: $8,483.33
What this means: Fixed-rate amortization estimate with optional extra principal payments.

Scenario nameNotes

Save this result, then use saved scenarios to switch between assumptions.

Before you calculate

Use this calculator when you want to see how a fixed principal-and-interest payment reduces a balance over time or how extra principal could change payoff timing in the app's month-offset model. Use Loan Payment for the simpler payment-only estimate, Payment when solving either payment amount or payoff time, APR when selected fees need a cash-flow-rate estimate, Mortgage Payment when taxes, insurance, PMI, or escrow matter, and lender or servicer documents for real payoff statements, payment allocation, legal disclosures, product terms, or jurisdiction-specific questions.

Formula summary

The app rounds the payment count with termYears * 12 + termMonths and converts the entered annual rate with annualRate / 100 / 12. The scheduled payment is PMT = P * m / (1 - (1 + m)^-n), where P is loan amount, m is monthly rate, and n is payment count; at 0 percent, PMT = P / n. The schedule helper starts with the loan balance, computes monthly interest as balance * m, sets scheduled principal to max(0, PMT - interest), adds extraMonthlyPay every month, adds extraYearlyPay on every 12th payoff month, adds extraOneTimePay when the 1-based payoff month equals extraOneTimeMonth, caps principal at the remaining balance, and repeats until the balance is effectively zero or the 12000-month safety cap is reached. Interest saved is the no-extra schedule's total interest minus the selected schedule's total interest.

Worked example

Example: no extra payments and extra principal

With the default inputs, a $200,000 loan amount, 15 years, 0 months, and a 6 percent annual rate become 180 monthly payments. The calculator reports a $1,687.71 scheduled monthly payment, $303,788.46 total payments, $103,788.46 total interest, 180 payoff months, 15 payoff years, $0.00 interest saved, $1,000.00 first-month interest, $687.71 first-month principal, a $199,312.29 first-month ending balance, $11,769.23 first-year interest, and $8,483.33 first-year principal. If the same loan adds $100 monthly, $1,000 every 12th payoff month, and $5,000 in month 24, the fixture reports 149 payoff months, 12.42 payoff years, $282,879.98 total payments, $82,879.98 total interest, and $20,908.48 interest saved.

The example shows why earlier principal reduction can lower later interest under a fixed-rate model. It is still a mechanical estimate from entered assumptions, not a lender payoff quote, servicer instruction, legal disclosure, tax answer, or personal borrowing advice.

Assumptions

The calculator models one fixed-rate monthly principal-and-interest schedule. It does not model variable rates, daily accrual, custom compounding, interest-only periods, negative amortization, balloon payments, skipped payments, refinancing, payment holidays, late fees, taxes, insurance, escrow, mortgage insurance, lender credits, or product-specific contract rules.
Extra monthly payments are modeled as principal additions every month, extra yearly payments are modeled on every 12th payoff month, and the one-time payment is modeled by 1-based payoff month rather than calendar date. Real lenders and servicers may apply payments first to fees, past-due amounts, current interest, escrow, or future-payment credits before principal.
Scoped independent comparator checks pass 4 of 4 scoped no-extra fixtures for overlapping fixed-rate schedule outputs. Extra-payment validation is limited because external schedule tools often model extra payments by calendar date while this app uses month offsets. CFPB mortgage amortization and principal-and-interest materials are needed for mortgage payment-split context; CFPB prepayment, servicer, and student-loan materials plus lender documents are needed for extra-payment application claims; CFPB APR guidance and Regulation Z sections 1026.18 and 1026.22 are needed for legal APR, finance-charge, total-of-payments, payment-schedule, or Truth in Lending claims. Results are educational estimates, not financial, tax, legal, lending, mortgage, student-loan, credit, regulatory, consumer-rights, or personalised advice.

Common mistakes to avoid

Reading monthly payment as a full mortgage payment. This calculator models principal and interest only; taxes, homeowners insurance, PMI, HOA dues, escrow, and lender fees need separate sources or calculators.
Assuming every extra payment immediately reduces principal in the same way. The app treats extras as principal additions in its schedule, but real payment allocation and paid-ahead rules depend on lender, servicer, loan type, and jurisdiction.
Using a zero-month term, legal APR question, lender payoff amount, payoff statement, disclosure schedule, student-loan repayment-plan rule, or borrowing recommendation as if this calculator produced an official answer.

Methodology and sources

This calculator is an original implementation based on documented formulas, app-specific assumptions, deterministic fixtures, edge cases, rounding policy tests, and internal validation. It is not copied from a single source.

Outputs are checked with deterministic fixtures, edge cases, rounding policy tests, and internal independent comparator checks where overlapping outputs are available. The result remains an educational estimate, not a quote, approval, tax answer, or personalised advice.

Formula version 2026.05.22-generic-amortization-extra-payments. The version marks the calculation logic and validation fixture set used for this estimate.

CFPB mortgage amortization and principal-and-interest materials should be used when copy explains mortgage principal, interest allocation, fixed-rate amortization, or total monthly mortgage-payment boundaries.
CFPB prepayment penalty, mortgage-servicer extra-payment, student-loan extra-payment, and lender or servicer materials should be used when copy discusses extra-payment application, principal-only payments, paid-ahead status, payoff timing, or prepayment penalties.
CFPB APR versus interest-rate guidance and Regulation Z sections 1026.18 and 1026.22 should be used when copy discusses APR, payment schedules, total of payments, finance charges, disclosures, or legal lending terminology.
Lender disclosures and relevant local regulator sources should be used before copy makes product-specific, jurisdiction-specific, variable-rate, escrow, tax, insurance, mortgage insurance, negative-amortization, payoff statement, or consumer-credit legal claims.

Estimate only

The result is educational and is not financial, tax, legal, lending, investment, or regulated advice. Real provider terms, fees, rates, taxes, and personal circumstances can change the final answer.

Formula and help: read the full amortization calculator methodology notes.