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Amortization Schedule Calculator

Builds the amortization schedule of a fixed-rate loan: the level monthly payment, a year-by-year summary of principal, interest and balance, the first 12 monthly rows, and the interest and time saved by a fixed extra monthly payment.

When to use

You need to see how each payment splits into principal and interest over the life of a mortgage or loan, the balance at the end of each year, or the effect of paying extra every month.

Do not use when: You only need the payment amount (use loan-payment or mortgage-payment), or you know the payment and want the payoff time of a revolving balance (use debt-payoff).

Formula

i = annual_rate_percent / 1200; n = 12·term_years + term_months; monthly_payment = principal × i / (1 − (1 + i)^−n) (principal / n if i = 0). Each month: interest = balance × i; principal_paid = monthly_payment + extra_monthly_payment − interest (capped at the balance); balance −= principal_paid, until the balance is 0.

Standard monthly amortization with end-of-month payments and the extra amount applied to principal from the first month; the final payment is reduced to what remains. Lenders round payments to cents, so real schedules can differ by a few cents.

Inputs

Parameter	Type	Unit	Required	Description
`principal`	number		yes	Amount borrowed. Range: > 0, ≤ 1000000000000
`annual_rate_percent`	number	%	yes	Fixed nominal annual rate in percent; the monthly rate is this / 12. Range: ≥ 0, ≤ 100
`term_years`	number	years	no	Loan term in years; added to term_months when both are given. Total term is limited to 40 years. Range: ≥ 0, ≤ 40
`term_months`	integer	months	default 0	Additional months of term (e.g. term_months=60 alone for a five-year loan). Range: ≥ 0, ≤ 480
`extra_monthly_payment`	number		default 0	Optional fixed amount paid on top of the scheduled payment every month, applied to principal. Range: ≥ 0, ≤ 1000000000000
`start_date`	date		no	Optional loan origination date (YYYY-MM-DD); the first payment is due one month later and payoff_date is reported.

Outputs

Output	Type	Unit	Description
`monthly_payment`	number		Level payment from the annuity formula (before any extra payment).
`number_of_payments`	integer		12 × term_years + term_months.
`payoff_months`	integer	months	Actual number of payments including the extra payment (equals number_of_payments without one).
`payoff_time`	string		payoff_months in years and months.
`payoff_date`	date		start_date plus payoff_months (only when start_date is given).
`total_interest`	number		Interest paid over the actual schedule (with extra payments).
`total_paid`	number		principal + total_interest.
`scheduled_total_interest`	number		monthly_payment × number_of_payments − principal.
`interest_saved`	number		scheduled_total_interest − total_interest.
`months_saved`	integer	months	number_of_payments − payoff_months.
`first_month_interest`	number		principal × annual_rate_percent / 1200.
`first_month_principal`	number		monthly_payment + extra_monthly_payment − first_month_interest.
`yearly_summary`	list		One row per loan year: year, principal_paid, interest_paid, ending_balance (at most 40 rows).
`first_12_months`	list		Rows for months 1–12: month, payment, principal, interest, balance.

Example

200,000 at 6 % for 30 years: {"principal":200000,"annual_rate_percent":6,"term_years":30} → {"monthly_payment":1199.1,"number_of_payments":360,"payoff_months":360,"payoff_time":"30 years","total_interest":231676.38,"total_paid":431676.38,"scheduled_total_interest":231676.38,"interest_saved":0,"months_saved":0,"first_month_interest":1000,"first_month_principal":199.1}

300,000 at 5 % for 15 years with 300 extra per month from 2026-01-01: {"principal":300000,"annual_rate_percent":5,"term_years":15,"extra_monthly_payment":300,"start_date":"2026-01-01"} → {"monthly_payment":2372.38,"number_of_payments":180,"payoff_months":152,"payoff_time":"12 years 8 months","payoff_date":"2038-09-01","total_interest":105316.26,"scheduled_total_interest":127028.56,"interest_saved":21712.3,"months_saved":28}

GET https://tttkmbb.com/api/v1/calculate/amortization-schedule?principal=200000&annual_rate_percent=6&term_years=30

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/amortization-schedule (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/amortization-schedule · Markdown: https://tttkmbb.com/finance/amortization-schedule.md · JSON definition: https://tttkmbb.com/finance/amortization-schedule.json
MCP: server https://tttkmbb.com/mcp, tool calculate_amortization_schedule
OpenAPI operationId: calculate_amortization_schedule
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Amortization calculator (annuity payment formula) (reference)
Investopedia – Amortization Schedule: Definition, Formula and Example (reference)
CFPB – Owning a Home: loan costs and monthly payment components (government)

FAQ

Why is the first payment mostly interest?

Interest is charged on the outstanding balance, which is largest at the start: 200,000 × 6 % / 12 = 1,000 of the first 1,199.10 payment is interest and only 199.10 reduces the balance. The split reverses as the balance falls.

How is the extra payment applied?

It is added to every scheduled payment and goes entirely to principal, which shortens the term and lowers total interest; the scheduled payment itself does not change.

Why only a yearly summary and 12 months?

A 30-year loan has 360 rows; the yearly totals plus the first year give the shape of the schedule. Any single month's balance follows from balance = P(1+i)^k − PMT((1+i)^k − 1)/i.

Related calculators

Loan Payment Calculator — Payment and totals without the schedule.
Mortgage Payment Calculator — Full housing payment including taxes and insurance.
Debt Payoff Calculator — Payoff time for a chosen payment on a revolving balance.