Methodology & formulas

A schedule generator is only as trustworthy as its willingness to show the math. This page documents every formula, convention and rounding rule the calculator uses — enough to reproduce any number it outputs by hand or in a spreadsheet. The worked example below is rendered by the same engine that powers the calculator, so the two cannot disagree. Found an error? Tell us — corrections outrank all other mail.

The payment formula

For a loan of principal P, periodic interest rate i, and n scheduled payments, the level payment is the standard annuity formula:

Payment = P × i × (1 + i)ⁿ ÷ ((1 + i)ⁿ − 1)

For monthly payments, i is the annual note rate divided by 12 and n is the term in months. The result is rounded to the cent. A zero-rate loan divides the principal evenly instead.

How each period is computed

Interest = current balance × periodic rate, rounded to the cent.
Principal = payment − interest.
Extra amounts are applied as additional principal in the same period, after the scheduled principal.
Balance = previous balance − principal − extra.
The final payment absorbs rounding. Because the payment is rounded, a few dollars of residue accumulate over the term; the last payment pays exactly the remaining balance plus its interest, so it is almost never identical to the others — exactly like a real servicer’s schedule.

Worked example: $200,000 at 6% for 30 years

Monthly rate = 6% ÷ 12 = 0.5%. Payment = 200,000 × 0.005 × 1.005³⁶⁰ ÷ (1.005³⁶⁰ − 1) = $1,199.10.

Payment	Interest	Principal	Balance after
#1 (Sep 2026)	$1,000.00 (= 200,000 × 0.5%)	$199.10	$199,800.90
#2 (Oct 2026)	$999.00	$200.10	$199,600.80
… #360 (Aug 2056)	total interest over the loan: $231,677	$0.00

Conventions & deliberate simplifications

Monthly-style accrual. Interest is charged per period on the scheduled balance — the standard convention for US fixed-rate mortgages. Daily-simple-interest loans (many auto and student loans) will differ by a few dollars depending on exactly when payments post; on-time payers land very close to this schedule.
Biweekly mode models the DIY approach: half the monthly payment every 14 days, 26 payments a year, with the periodic rate = annual rate ÷ 26. Lender “biweekly programs” vary — some hold half-payments until month-end, which produces a different (worse) result than shown here.
The note rate, not APR. Schedules run on the interest rate in your note. APR mixes in fees for shopping comparisons and does not drive payments.
Principal and interest only. Escrow items — property taxes, homeowners insurance, PMI — ride on top of real mortgage bills and are not amortized here.
Fixed rates only. Variable-rate loans, HELOCs and income-driven student plans have no knowable schedule; we don’t pretend otherwise.
Extra payments are assumed principal-only. Real servicers sometimes apply unmarked extras as early next payments — how to make sure yours don’t.
Dates. Monthly payments are dated to the first of each month from your chosen first-payment month; biweekly payments step exactly 14 days. Servicer due dates vary; the sequence and math don’t.

Sources

The annuity payment formula is standard fixed-rate loan mathematics (any finance text; it matches calculator.net, Bankrate and lender schedules). Accrual-convention notes (mortgage monthly accrual vs. auto-loan daily simple interest) were compiled July 2026 from the American Financial Services Association’s simple-interest fact sheet and lender documentation. Prepayment-penalty facts cited on tool pages come from the CFPB’s Ability-to-Repay/Qualified Mortgage rule (effective January 10, 2014). This tool computes schedules; it does not give financial advice.