How We Test Our Calculators

1. Formula review

Every calculator’s math is reviewed against standard financial formulas before it ships: compound-interest annuities, loan amortization, progressive tax bands, withdrawal models and inflation adjustments. The formulas are public on the Methodology page, so anyone can reproduce a result by hand.

2. Boundary testing

Calculators are tested with edge cases that break naive implementations:

• Zero interest, negative returns and zero contributions.
• Zero principal with contributions only, and vice versa.
• Tax threshold boundaries (e.g. where a personal allowance phases out).
• Division-by-zero guards when the rate is 0%.
• Very long horizons, very small rates, and decimal rates.

3. Cross-verification

Results are independently recomputed where it matters — loan amortization schedules are checked against a standard amortization calculation, tax figures against band-by-band manual math, and fee impact against an independent fee-drag model. A result is only shipped when both paths agree.

4. Internal consistency

• Totals always reconcile: Final = Principal + Contributions + Growth (plus government bonus where applicable).
• Scenario comparisons reuse the same calculation engine as the main calculator — one source of truth, so the “Current” scenario always matches the main result exactly.

5. Automated tests and CI

Caspenda runs 125 unit and integration tests before every deploy. The production build then regenerates the sitemap, search index and blog index, checks that all three agree, verifies the static site, and fails the pipeline if anything drifted.

6. Browser verification

Before release, pages are rendered in a real browser at desktop and mobile widths to confirm: calculators produce results, no console errors, no broken links, no horizontal overflow, and the header, logo and navigation are intact.

7. In-browser calculation

All math runs locally in your browser. No input ever leaves your device — there is no server-side calculation and no data collection.

8. Update policy

Tax-year data and other time-sensitive rules are reviewed when official figures change, and each page records when it was last reviewed. Sources are re-verified on the same schedule. We document our sources on the Data Sources page.

9. Known limitations

Our calculators assume a constant return and do not model market volatility, sequence-of-returns risk, taxes, fees or inflation unless the page explicitly says otherwise. Projections are illustrative scenarios, not forecasts. See the Methodology for every assumption.

Last updated: September 2026.

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