Calculator Documentation
Early Payoff Calculator
See how extra payments shorten your mortgage and how much interest they save
Schema version 1.0.0 · Category: mortgage
Inputs
Current Loan
Your existing mortgage
| Field | Type | Default | Notes |
|---|---|---|---|
| Current Loan Balance | currency | $300,000 | Remaining principal on your mortgage |
| Interest Rate | percentage | 6.00% | Your current annual rate |
| Years Remaining | number | 30 | Years left on the loan at the regular payment |
Extra Payments
What you plan to add
| Field | Type | Default | Notes |
|---|---|---|---|
| Extra Monthly Payment | currency | $200 | Additional principal paid every month |
| One-Time Lump Sum | currency | $0 | A single extra principal payment made now |
Calculations
Every figure this calculator produces, in evaluation order. Formulas reference input ids, parameters, and earlier calculations.
monthlyRateMonthly rate
interestRate / 12
baseTermMonthsRemaining term in months
remainingYears * 12
basePaymentRegular monthly P&I
(loanBalance * monthlyRate * pow(1 + monthlyRate, baseTermMonths)) / (pow(1 + monthlyRate, baseTermMonths) - 1)
effectiveBalanceBalance after the lump sum (floored at $1 to keep the math defined)
max(loanBalance - oneTimeLump, 1)
totalPaymentNew monthly payment including the extra
basePayment + extraMonthly
payoffMonthsMonths to payoff with extra payments
ln(totalPayment / max(totalPayment - effectiveBalance * monthlyRate, 0.000001)) / ln(1 + monthlyRate)
monthsSavedHow many months earlier the loan is paid off
max(baseTermMonths - payoffMonths, 0)
baseTotalInterestInterest paid on the regular schedule
basePayment * baseTermMonths - loanBalance
newTotalInterestInterest paid with extra payments
totalPayment * payoffMonths + oneTimeLump - loanBalance
interestSavedTotal interest saved by paying extra
max(baseTotalInterest - newTotalInterest, 0)
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