Debt Payoff Timeline Calculator Formula

Understand the math behind the debt payoff timeline calculator. Each variable explained with a worked example.

Use the Debt Payoff Timeline Calculator

Formulas Used

Months to Pay Off

payoff_months = monthly_rate > 0 ? (monthly_payment > balance * monthly_rate ? ceil(-log(1 - balance * monthly_rate / monthly_payment) / log(1 + monthly_rate)) : 999) : ceil(balance / monthly_payment)

Total Interest

total_interest = monthly_rate > 0 ? (monthly_payment > balance * monthly_rate ? monthly_payment * ceil(-log(1 - balance * monthly_rate / monthly_payment) / log(1 + monthly_rate)) - balance : balance * 10) : 0

Total Cost

total_cost = monthly_rate > 0 ? (monthly_payment > balance * monthly_rate ? monthly_payment * ceil(-log(1 - balance * monthly_rate / monthly_payment) / log(1 + monthly_rate)) : balance * 10) : balance

First Month Interest

first_month_interest = balance * monthly_rate

First Month to Principal

first_month_principal = monthly_payment - balance * monthly_rate

Variables

VariableDescriptionDefault
balanceStarting Balance(USD)20000
annual_rateAnnual Interest Rate(%)15
monthly_paymentFixed Monthly Payment(USD)600
monthly_rateDerived value= annual_rate / 12 / 100calculated

How It Works

Debt Payoff Timeline

A payoff timeline shows how each payment is split between interest and principal.

Early vs Late Payments

In the early months, a large portion goes to interest. As the balance shrinks, more of each payment attacks the principal. This is why extra payments early on are so powerful.

Month 1 Breakdown

  • Interest = Balance x Monthly Rate
  • Principal = Payment - Interest
  • New Balance = Old Balance - Principal

    • This process repeats each month until the balance reaches zero.

    Worked Example

    $20,000 at 15% with $600/month payments.

    balance = 20000annual_rate = 15monthly_payment = 600
    1. 01Month 1 interest = $20,000 x 1.25% = $250
    2. 02Month 1 principal = $600 - $250 = $350
    3. 03New balance = $20,000 - $350 = $19,650
    4. 04Total payoff = 42 months (3.5 years)
    5. 05Total interest = $5,200

    Ready to run the numbers?

    Open Debt Payoff Timeline Calculator