Debt Calculator

How to Use Debt Calculator

The Debt Calculator is designed for a single debt with a fixed APR.

1. Enter the current balance.
2. Enter the APR (annual percentage rate).
3. Enter the planned monthly payment.
4. Enter any extra payment you can add each month — even $25 makes a real difference.
5. Read the result — The Debt Calculator returns months to payoff, total interest, total paid, the months and interest saved by extra payments, and the baseline (no-extra) schedule for comparison.

Formula & Theory - Debt Calculator

The Debt Calculator uses month-by-month amortization:

i = APR / 12

Each month:
  interest = balance × i
  payment  = monthlyPayment + extraPayment
  balance  = balance + interest − min(payment, balance + interest)
  totalInterest += interest
  totalPaid += payment

months = number of months until balance ≤ 0

To compute savings, the Debt Calculator runs two simulations: one with extra payment, one without (extraPayment = 0). The difference yields:

monthsSaved   = baselineMonths − months
interestSaved = baselineInterest − totalInterest

Why extra payments matter so much: Interest compounds monthly. A $50 extra payment today reduces every future month’s interest base. The savings grow geometrically over the life of the debt.

Example: $20,000 at 8% APR, $400/month base payment. Adding $100/month extra:

• Without extra: ~67 months, ~$6,700 interest
• With $100 extra: ~46 months, ~$4,300 interest
• Saves 21 months and ~$2,400 in interest for $4,600 of accelerated principal — a roughly 50% return on that capital.

Avalanche vs. snowball: For a single debt the Debt Calculator is sufficient; for multiple debts, use the Debt Avalanche Calculator (highest APR first, mathematically optimal) or a snowball approach (smallest balance first, psychologically motivating).

Use Cases for Debt Calculator

• Personal loan planning — Model any installment loan with a fixed APR.
• Auto loan extra payments — Quantify the savings from adding to each payment.
• Student loan strategy — Decide between standard repayment and aggressive payoff.
• Credit card payoff — Model the most common consumer debt.
• Family loans — Even informal loans benefit from a payoff schedule.
• Debt vs. invest decision — Compare the guaranteed “return” of debt payoff to expected investment yields.

The Debt Calculator turns abstract debt into a concrete countdown, making the case for aggressive payoff in pure dollars-and-months.