Methodology & data sources

Last reviewed: 26 May 2026

Every figure on this site comes from a published formula or an official government source. This page documents which formula or source each calculator uses, how rounding works, and what's deliberately excluded. If you find a discrepancy with an official figure, tell us.

Why we publish methodology

Financial information is a YMYL (your money, your life) topic — Google's quality raters apply higher accuracy and trust standards to it, and so do users. We document the maths and sources behind every result so you can verify our numbers and so search engines can trust our content. Nothing here is proprietary; you could rebuild any calculator from the formulas and sources cited below.

Calculation formulas

Mortgage & loan amortization

The monthly principal-and-interest payment uses the standard amortization formula:

M = P · r · (1 + r)ⁿ / ((1 + r)ⁿ − 1)

where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments (years × 12). The same formula powers the mortgage, loan repayment, and refinance calculators.

Per-payment splits between principal and interest are computed by amortising the schedule month by month: interest for the period is the outstanding balance × r, and principal is the payment minus that interest. The yearly schedule shown on each page aggregates 12 months at a time.

Compound interest with contributions

The compound-interest projection combines a lump-sum future value with an annuity for ongoing contributions:

FV = P · (1 + r)ⁿ + PMT · ((1 + r)ⁿ − 1) / r

where P is the starting balance, PMT is the regular contribution, r is the period rate (annual rate ÷ contribution frequency), and n is the total number of contribution periods. This drives the compound interest, retirement, and FIRE calculators.

Income tax (progressive brackets)

Income tax is computed bracket-by-bracket. Each slice of taxable income is taxed at its bracket's rate; the total is the sum of those slice-by-slice amounts. Taxable income is gross income minus the applicable standard deduction, personal allowance, or basic personal amount for the chosen market. Marginal rate is the rate on the next dollar earned (the top bracket reached); effective rate is total tax ÷ gross income.

Capital gains

Capital gain = sale price − cost basis − selling expenses. The taxable portion of the gain depends on the market and holding period:

• US: short-term (held ≤ 1 year) taxed as ordinary income; long-term (held > 1 year) taxed at 0% / 15% / 20% by income bracket.
• UK: annual exempt amount subtracted first; remainder taxed at 10% / 18% (basic rate) or 20% / 24% (higher rate), depending on asset type.
• Canada: 50% inclusion rate — half the gain is added to taxable income at marginal rates.
• Australia: 50% CGT discount for assets held over 12 months by individuals; remainder taxed at marginal rates.

Safe withdrawal rate (retirement & FIRE)

The 4% rule originates from the Trinity Study (1998) and updates by Bengen and others. The retirement calculator uses 4% as the default for a 30-year horizon; the FIRE calculator lets you adjust to 3.0–3.5% for 40+ year horizons. Recent research (Big ERN's Safe Withdrawal Rate series, Morningstar) suggests slightly lower rates may be appropriate for very long retirements or high stock-market valuations.

Tax bracket & rate sources

All tax brackets and social-insurance rates come from the relevant government tax authority. Each market's source file in our codebase carries the authoritative URL in its header comment.

United States — Tax Year 2025

• Federal brackets, standard deduction: IRS Rev. Proc. 2024-40
• Social Security & Medicare (FICA): published annually by the SSA
• Long-term capital gains thresholds: IRS Topic 409
• State income tax: not yet included (planned)

United Kingdom — Tax Year 2025/26

• Income tax bands & personal allowance: gov.uk/income-tax-rates (England, Wales, NI rates; Scottish bands differ)
• National Insurance: gov.uk/national-insurance-rates-letters
• Student loan plans: gov.uk/repaying-your-student-loan
• Capital Gains Tax: gov.uk/capital-gains-tax

Canada — Tax Year 2025

• Federal tax brackets: canada.ca / CRA
• CPP contributions: CRA CPP rates
• EI premiums: CRA EI premiums
• Provincial tax: not yet included (planned)

Australia — Tax Year 2025/26

• Resident income tax rates: ato.gov.au/rates
• Medicare levy: ATO Medicare levy
• HECS-HELP repayment thresholds: ATO study loan thresholds

Currency conversion

• Frankfurter API — mirrors the European Central Bank's daily reference rates.

Rounding & precision

Internal calculations are performed in double-precision floating point and only rounded for display. Monthly payments and tax owed are typically displayed to the nearest cent; percentages to one or two decimal places. Because each calculator rounds only at the final output, intermediate-step totals you might compute by hand from displayed values can disagree with our totals by a few cents — that's expected rounding behaviour, not an error.

What's deliberately not included

• Mortgage: property tax, homeowners insurance, HOA/strata fees, PMI, and closing costs are excluded from the monthly payment figure.
• Income tax: US state, Canadian provincial, and UK Scottish bands are not yet implemented. The calculator covers federal/UK rest-of-country only.
• Paycheck: employer-sponsored health insurance, voluntary retirement contributions, and post-tax deductions (e.g. union dues, garnishments) are not modelled.
• Capital gains: state/provincial CGT, wash-sale rules, and section 1250 unrecaptured gains are not modelled.
• Crypto: staking-reward income, mining income, DeFi events (LP positions, lending interest), and HMRC's "same-day" / "30-day" matching rules are not modelled.
• Retirement/FIRE: tax on withdrawals, sequence-of-returns risk, and inflation variance are simplified to a single average return assumption.

Update cadence

Tax brackets are refreshed within two weeks of each market's official annual publication (US: late October by the IRS; UK: at each Budget; Canada: November/December by the CRA; Australia: at each Federal Budget). Each tax-bearing calculator page shows the tax year it currently uses. If you spot a stale bracket after the official figures have been published, let us know — corrections are usually applied within 24 hours.

Reporting an error

The fastest way to flag a discrepancy is the contact form. Include the calculator name, the inputs you used, the result we showed, and what you expected (with a source if possible). We read every message and answer most within one business day.

Disclaimer

Every calculator on this site provides estimates for educational and planning purposes only. Results are not financial, tax, legal, or accounting advice. Edge cases (multi-state residency, complex deductions, alternative minimum tax, non-resident status, trusts, marriage allowance, child benefit clawback, etc.) are not modelled. Always verify significant decisions with a qualified professional or the relevant tax authority. See our full disclaimer for details.