How we calculate

Read this first: our calculators are built to be checked. Every formula is published on its own calculator page, and this page explains the assumptions behind them. Results are estimates for planning. They are not tax, financial, or legal advice.

This page answers the two questions a careful reader always asks: what math is running under the hood, and where do the numbers come from? The short version: standard finance math you can verify by hand, fed with tax law from primary sources, reviewed on a schedule. The long version follows.

FIRE calculators

A Coast FIRE number answers one question: how much do you need saved today so that growth alone, with zero further contributions, reaches a full retirement fund by retirement age? We compute the full FIRE number first, then discount it back to today:

Coast FIRE number = (Annual spending × 25) ÷ (1 + r)^years

The 25× comes from the 4% safe-withdrawal rule, the Trinity-study tradition that says a portfolio can sustain 4% annual withdrawals over a long retirement. r is the expected annual real return (after inflation), and years is the time left until retirement.

Our default real return is 7%: roughly the long-term real return of the US stock market after inflation. Enter spending in today's dollars and use a real return, and both sides of the equation stay in today's dollars, so inflation cancels out. Prefer caution? Run the same inputs at 5% and compare. The calculator also projects your current savings forward at your chosen rate, so you can see the gap in dollars, not just vibes. Because the discount term compounds over decades, small differences in r move the answer a lot, which is exactly why we let you set it and show the formula.

Debt calculators

The snowball and avalanche tools do not use a closed-form formula. They run a true month-by-month simulation of your actual debts:

Each month, interest accrues on every balance at its APR ÷ 12.
All minimum payments go out first, across every debt.
Your extra payment goes to one target debt: the smallest balance for snowball, the highest APR for avalanche.
When a debt hits zero, its minimum payment joins the extra payment for the next target.
The loop repeats until everything is paid, or a 600-month safety cap is reached.

This is why two strategies can share identical inputs and land on different finish dates and interest totals: payment order changes how fast the most expensive interest gets killed. The single-debt payoff calculator uses the same monthly-accrual math, solved both directions: payment in gives you a debt-free date, target date in gives you the required payment. If a payment ever falls below the interest accrued that month, the calculator says so plainly instead of inventing a finish date.

Paycheck calculators

Gross-to-net pay is subtraction with rules: federal income tax, FICA, and state income tax where it applies.

Federal income tax uses the tax brackets and standard deduction published by the IRS for tax year 2026. FICA breaks down as: Social Security at 6.2% up to the Social Security wage base for 2026, Medicare at 1.45% on all wages, plus the additional 0.9% Medicare tax on wages above the statutory thresholds. Self-employment, pre-tax deductions, 401(k) contributions, and local taxes are out of scope for these tools; each calculator page says exactly what is and is not included.

State income tax follows each state's own tax agency rules, effective for 2026. Flat-rate states, graduated brackets, and no-income-tax states are all modeled. When a state publishes its new figures late, the page notes which numbers are still prior-year until the update lands.

Where our numbers come from

Two kinds of inputs sit behind the calculators: math and law.

The math, compounding, amortization, the 25× rule, comes from standard finance textbooks and published research. It does not change year to year, and it is published on every calculator page so anyone can reproduce our results by hand.

The law, tax brackets, the Social Security wage base, state rates, comes from primary sources only: the federal tax tables and FICA rates published by the IRS for tax year 2026, and each state's tax agency for state rules, effective 2026. We do not copy tax figures from blogs or aggregators. If a primary source corrects a figure, we correct ours and note the change.

Primary sources we work from: IRS — Revenue Procedure 2025-32 (2026 federal brackets and standard deduction); Social Security Administration — 2026 wage base ($184,500); Consumer Financial Protection Bureau — consumer credit and mortgage rules.

See the method applied in the Coast FIRE calculator, the snowball vs avalanche simulator, and the paycheck calculator — or read the guided tour in how our calculators work.

How often we update

Tax figures are reviewed every first quarter, before the bulk of filing season, and immediately whenever a law change takes effect mid-year. Formula pages change only when we find a bug or a clearer way to explain the same math. Because every calculator shows its inputs and formula in the open, any change is visible and checkable, not silent.

What the calculators don't do

Simplified models have edges, and honest ones state them:

The FIRE tools assume smooth returns. Real markets arrive in sequence, and a crash early in your plan hurts more than one near the end. No simple calculator models that.
The debt tools model the APRs you enter. They cannot see rate changes, penalty fees, or new charges you add mid-plan. Re-run them when your inputs change.
The paycheck tools estimate take-home pay from tax law. They are not your employer's payroll system and do not know your deductions, bonuses, or local taxes.

None of this is tax or financial advice, and none of it replaces a qualified professional for decisions you cannot easily reverse. If a result looks wrong, tell us. We would rather fix a bug than let a wrong number travel.

Last updated: October 2026.

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