calcfi-debt-payoff

Snowball vs avalanche debt payoff strategies. Compute months and total interest. MIT.

CalcFi Debt Payoff Calculator — a single-purpose, MIT-licensed helper for Compute payoff months and total interest for snowball (smallest balance first) and avalanche (highest APR first) debt-payoff strategies. Compares total cost across methods.

What this does

Compute payoff months and total interest for snowball (smallest balance first) and avalanche (highest APR first) debt-payoff strategies. Compares total cost across methods.

Formula — Snowball vs avalanche allocation:

Apply minimums to all; extra payment goes to (snowball: smallest balance | avalanche: highest APR)

Primary source: Standard amortization with extra-principal allocation

Install

npm install calcfi-debt-payoff

Usage

import { compare } from "calcfi-debt-payoff";

const debts = [
  { balance: 5000, apr: 0.24, minimum: 100 },
  { balance: 8000, apr: 0.15, minimum: 200 },
];
compare(debts, 200);

Live calculator

Try the interactive web calculator with full UI at https://calcfi.app/calculators/debt-payoff — no signup required.

Why this package exists

Most personal-finance calculators are buried in advertising-heavy lead-generation sites. The math is freely published by federal agencies and textbooks, but the implementations are wrapped in proprietary opaque code. CalcFi publishes these utility libraries to make every formula:

1. Reproducible — show the math
2. Cited — link to primary sources
3. Free — MIT license, no paid tier
4. Auditable — every line of code is open source

The math here is undergraduate-level. The contribution is the citation discipline and the open distribution.

Part of the CalcFi ecosystem

This package is one of 8 single-purpose helper utilities. Cross-mesh:

Sibling Python packages

• calcfi-mortgage-payment — Standard amortizing-loan monthly payment formula. Free, cited, MIT.
• calcfi-compound-interest — Compound interest with periodic contributions. Future value, present value, time-to-target. Free, MIT.
• calcfi-rule-of-72 — Rule of 72 — approximate years to double an investment at a given rate. Plus exact ln(2) version. MIT.
• calcfi-apr-apy — Convert between APR (simple annual rate) and APY (effective annual yield including compounding). MIT.
• calcfi-real-return — Inflation-adjusted (real) return from nominal return. Formula: real = (1+nominal)/(1+inflation) − 1. MIT.
• calcfi-amortization — Generate a full amortization schedule for a fixed-rate loan: payment, principal, interest, balance per period. MIT.
• calcfi-tax-bracket — US federal income tax bracket math — marginal rate, effective rate, total tax owed. 2024-2026 brackets per IRS Rev. Proc. MIT.

Sibling npm packages

• calcfi-mortgage-payment — Standard amortizing-loan monthly payment formula. Free, cited, MIT.
• calcfi-compound-interest — Compound interest with periodic contributions. Future value, present value, time-to-target. Free, MIT.
• calcfi-rule-of-72 — Rule of 72 — approximate years to double an investment at a given rate. Plus exact ln(2) version. MIT.
• calcfi-apr-apy — Convert between APR (simple annual rate) and APY (effective annual yield including compounding). MIT.
• calcfi-real-return — Inflation-adjusted (real) return from nominal return. Formula: real = (1+nominal)/(1+inflation) − 1. MIT.
• calcfi-amortization — Generate a full amortization schedule for a fixed-rate loan: payment, principal, interest, balance per period. MIT.
• calcfi-tax-bracket — US federal income tax bracket math — marginal rate, effective rate, total tax owed. 2024-2026 brackets per IRS Rev. Proc. MIT.

Companion data

The mortgage rate, inflation, Treasury yield, and macro data used to validate these calculators are published as CalcFi Open Data — 34 free CC BY 4.0 time series, 117,956 observations, 5 permanent DOIs.

• CalcFi Open Data on Hugging Face
• Python calcfidata SDK
• npm calcfidata SDK
• Live SQL endpoint (Datasette)
• DOI 10.6084/m9.figshare.32332290

Calculators on CalcFi (free, no signup)

Every formula in these packages has a live web calculator at calcfi.app:

• Mortgage Payment
• Compound Interest
• Tax Bracket
• Amortization Schedule
• APR vs APY
• Rule of 72
• Inflation-Adjusted Return
• Debt Payoff

License

MIT. Author: Jere Salmisto (ORCID 0009-0000-0916-8684), founder of CalcFi. Based in Helsinki, Finland.

Citation

If you cite this package in research:

Salmisto, J. (2026). calcfi-debt-payoff: CalcFi Debt Payoff Calculator.
DOI: 10.6084/m9.figshare.32332290 (parent CalcFi Open Data dataset).
Available: https://calcfi.app/calculators/debt-payoff

Or use the CITATION.cff on the parent dataset for the BibTeX entry.