How the math works

DebtSnowball runs a month-by-month simulation of your debts — the same arithmetic you could do by hand with a statement and a calculator, just faster. This page explains every rule the simulation uses, so you never have to wonder where a number came from.

Each month, in order

• Every debt accrues one month of interest: your balance times your APR, divided by 12, rounded to the cent.
• Every debt receives its minimum payment (never more than what you owe).
• Your extra monthly payment — plus every minimum freed up by debts you've already knocked out — goes to your target debt. That rollover is the “snowball” effect, and both methods use it.
• If the target debt is finished with money left over, the remainder cascades to the next debt in line that same month. Your money never idles.

How the target is chosen

The snowball method targets the smallest balance first — quick wins that keep you going. The stacking method (also known as the debt avalanche) targets the highest APR first — the mathematically optimal order that minimizes total interest. Ties break deterministically (by the other measure, then by name), so the same debts always produce the same plan.

We always compute both

Whichever method you pick, the calculator quietly runs the other one on identical inputs and tells you the honest difference — what stacking saves in interest, and how much sooner snowball clears your first debt. No thumb on the scale.

When a plan can't work

If a minimum payment can't even cover a debt's monthly interest, that balance grows instead of shrinking. Rather than showing you a fantasy 100-year projection, the calculator stops and tells you which debt is stuck and what it would take to unstick it.

What the simulation doesn't model

Real lenders compound daily, charge fees, and change variable rates. Your statements are the source of truth; this plan is an honest, consistent estimate for choosing a strategy and staying motivated — not a penny-perfect amortization contract. See the disclaimer.

Your numbers stay yours

Everything above happens in your browser. Your debts are never uploaded, and there's no account to create. Read the privacy policy for the details, or go build your plan.