How we verify every calculator
Every number on this site is recomputed independently before it ships. Not reviewed — recomputed, from scratch, in a different language, by a separate pass. Here is exactly what that means, what it doesn't mean, and how you can check our work.
Independent recomputation
Every calculator on this site runs on JavaScript in your browser. Before any calculator goes live, its formulas are re-derived and re-checked in Python — a completely separate implementation, written independently from the page's code. The two must agree to the dollar (or the rupee, pound, or cent) on every test case before the page ships.
We test against cases with known answers. The classic: $10,000 at 7% for 30 years compounds to $76,123 — no contributions, no tricks, just the formula. If our JavaScript and our Python disagree on that, nothing ships until we find out why.
Published test cases
A sample of the checks every release must pass. Each is recomputed independently in Python; the page's JavaScript must match exactly.
| Calculator | Test case | Expected result |
|---|---|---|
| Compound Interest | $10,000 lump sum, 7%, 30 years | $76,123 |
| Compound Interest | $10,000 + $500/mo, 7%, 30 years | $691,150 |
| Compound Interest | Goal mode: $1M target, $10k start, 7%, 30y → required monthly | $753.16/mo (forward check ≈ $1,000,000) |
| Compound Interest | Cost of waiting 5 years on the above | $228,860 |
| Compound Interest | $691,150 in today's dollars at 3% inflation, 30y | $284,745 |
| UK Stocks & Shares ISA | £500/mo, 7%, 20 years | £260,463 |
| Canada TFSA | C$500/mo, 7%, 20 years | C$260,463 |
| Canada RRSP | C$10,000/yr + 30% refund reinvested, 6%, 25 years | C$713,239 |
| UK Pension | £45k salary, 5% employee + 3% employer, 6%, 30y | £348,441 pot |
| FIRE | Goal mode: $50k saved, $60k/yr spend, 7% → retire in 20y | $2,491.83/mo (forward check ≈ $1.5M FI number) |
| Drawdown Lab | $500k portfolio, $40k/yr spend, 6%, 3% inflation → 4% rule | Runs dry in year 16 |
Current tax rules and limits
Tax figures go stale fast, and a stale tax number is worse than no number at all. Before publishing, we check every contribution limit, tax bracket, and allowance against the official source for the current tax year — IRS notices and revenue procedures for the US, HMRC for the UK, the CRA for Canada, and Budget documents for India.
The 2026 figures currently used across our calculators include: IRA contributions capped at $7,500; the UK Stocks & Shares ISA allowance at £20,000 for the 2026/27 tax year; and the Canadian TFSA annual limit at C$7,000. When a limit changes, the calculator changes with it — the number on the page always comes from the verified figure, never from memory.
What "verified" does not mean
Verification means the math is right. It doesn't mean the future will cooperate. Every calculator on this site makes simplifying assumptions: constant returns (real markets bounce around), no fees or taxes unless stated, and inflation handled only where we say so. A projection is a way to think, not a prediction.
And the standing rule for this whole site: this is educational content, not financial advice. We show you the math; the decisions are yours — ideally with a licensed professional for the big ones.
Review cadence
Tax figures are re-checked every January, when the new year's limits and brackets land. Formulas are re-verified whenever a calculator changes. This page was last reviewed in October 2026, and each calculator page states the assumptions it is built on.
Found an error?
We would genuinely like to know. The fastest way to reach us is to reply to any Compound Toolbox newsletter email — every reply lands in a real inbox and gets read. Tell us which calculator you used, the inputs you entered, and what looked wrong, and we will re-run the verification.
Common questions
Do you store my numbers?
No. Every calculation runs 100% in your browser. We never see your inputs, and there is nothing to leak, sell, or subpoena.
Why trust an independent site over my bank's calculator?
A bank's calculator is built to sell you the bank's products. Ours has no products. The formulas are published on each page, the test cases are described above, and the incentives point in exactly one direction: correct math.
How can I check the math myself?
Every calculator page shows its formula and a worked example. Plug the example inputs into any spreadsheet — if you don't get our answer, we want to hear about it (see above).
What happens when tax rules change?
We re-check every January and update the affected calculators, then note the new figures on this page. If you ever spot a stale number between reviews, the newsletter reply is the fastest way to flag it.