How CalcWize calculates

Every figure on this site is the output of a formula you can check, not a black box. This page explains how the 25 calculators are built, how often we review them, the sources we lean on, and — just as important — what an estimate can and can’t tell you.

Reviewed by Tiaan Fourie · BCom (Hons) Financial Management Sciences — 2026-05

The principles behind every tool

CalcWize is built on three commitments. First, your numbers stay yours: inputs live in your browser, never on a server, so there’s no account to breach and no profile being built from your finances. Second, estimates, not advice: the tools are designed to frame a decision and start a conversation with a qualified professional, never to replace one. Third, show the working: each calculator’s explainer sets out the exact method, so you can sanity-check the result rather than trust it blindly.

The formulas we use

Wherever a standard, well-established formula exists, we use it rather than inventing our own. The aim is that an accountant, lender, or textbook would recognise the method.

Loans, mortgages, and car finance use the standard amortising-payment formula — payment = principal × r(1+r)ⁿ ÷ ((1+r)ⁿ − 1), where r is the monthly rate and n the number of payments — and build the schedule month by month so interest and principal split correctly.
Compound interest, retirement, and savings compound monthly at the annual rate ÷ 12, adding each contribution before the next period’s growth.
Debt payoff simulates every month: interest accrues, minimums are paid, then the extra is funnelled to one debt at a time — smallest balance first for snowball, highest rate first for avalanche — and both are compared.
Tax applies each country’s current official progressive brackets to taxable income to derive the amount owed and the effective versus marginal rate. We re-check each country’s brackets against the official source on a set cadence and show a last-reviewed date.
Health tools use published equations: BMI is weight ÷ height², and the calorie/TDEE tool uses the Mifflin–St Jeor BMR multiplied by a standard activity factor.
Everyday maths — percentages, sales tax/VAT, age and date differences — uses plain arithmetic with real calendar dates, so leap years and varying month lengths are handled rather than approximated.

How we review and update

Each calculator’s explainer and each guide carries a “Last reviewed” month. When we review a tool we re-check the formula against a worked example, confirm the on-screen explanation still matches what the code does, and update the date. Methods like amortisation or compound interest are mathematically stable and rarely change; figures that drift over time — tax bands, typical interest rates, currency rates — are the ones we re-verify against current official sources and date-stamp, so you can see how current they are.

Where our numbers come from

The formulas are standard financial and scientific mathematics. Default values shown in a calculator are realistic starting points, not advice — change them to your own figures. Currency conversions use indicative reference rates suitable for estimates, not live dealing rates, so they should never be used to book a real trade. Tax brackets reflect each country’s current official tables and are re-checked on a set cadence, though they can briefly lag a mid-year change. For anything consequential, treat CalcWize as the first draft and a qualified professional or official source as the final word.

What these tools don’t do

A calculator can’t know your full situation. CalcWize doesn’t account for your specific tax reliefs, fees, penalties, insurance, or the way rates and prices move in the real world. It isn’t personalised financial, investment, tax, legal, or medical advice. The results are a clear, consistent starting point — useful precisely because they’re simple — and they work best as something you take to an expert, not instead of one.

Who builds CalcWize

CalcWize is built and reviewed by Tiaan Fourie, who holds a BCom (Hons) in Financial Management Sciences from the University of Pretoria and has worked with financial instruments for 15+ years across accounting, operations and fintech. It’s an independent project, not affiliated with any bank, lender, broker, or insurer, and it doesn’t sell your data or take referral fees for steering you toward a product. It’s funded by unobtrusive advertising so the calculators can stay free and sign-up-free. Found an error in a formula or an explanation? We want to fix it — email hello@calcwize.app and we’ll review it.