1. Every figure carries its source and the date we checked it
Not a bibliography at the bottom. Each real-world number — a tax bracket, a contribution ceiling, a platform fee, a benchmark — is traced to the primary source that publishes it, and the page records the date somebody last read that source. That date is at the foot of every calculator under “Formula last verified”, and it means what it says: a human opened the source on that day and confirmed the figure still matched.
We use the publisher, not a summary of the publisher. Canadian tax figures come from the Canada Revenue Agency’s own payroll formulas and the provincial tables, not from an article about them. Where a source could not be reached directly, the page says so rather than quietly citing something weaker.
2. The math is re-derived independently before it ships
Writing a formula and checking a formula are the same act performed twice by the same mind, which is to say they are one act. So every calculator is re-derived from the published rules by someone who did not write it, working from the source rather than from the code, and the two results are compared to the cent.
This is where most defects are actually caught. The capital-gains calculator understated tax by up to $1,200 because the gain was being run through a payroll routine that granted it employment credits it should never have received. Nothing on the page looked wrong. It was found by someone rebuilding the calculation from scratch and getting a different answer.
3. The arithmetic is pinned by tests you could run yourself
Every calculator family has a golden test: a plain script that reproduces the page’s math and asserts the results against figures derived by hand from the cited sources. There are 18 suites holding roughly 349 assertions across about 2,700 lines, and they run with nothing but Node — no framework, no fixtures, no dependencies at all.
They are ordinary files in the repository, not a claim on a marketing page. One of them asserts that the Canadian tax pages carry no private copy of the rate tables, because three pages once held their own duplicates and keeping them identical by hand was a defect waiting to happen. There is now one shared engine, and the test fails if a page grows its own.
4. The system that catches a number which stopped being true
Here is the uncomfortable thing about tests, and the reason this fourth system exists.
A test proves the page still computes the number we wrote down. It cannot prove that number is still correct. When the Canada Revenue Agency re-indexes a bracket in January, every test above stays green. The page keeps returning a confident answer. And the dated citation — the thing at the bottom that exists to earn your trust — quietly becomes the best available evidence that the page is out of date. The failure is invisible from the inside until somebody files a return on our number.
So each page also records when its sources can next change and why: the payroll formulas are reissued every January, the annual savings limit is announced each November, provincial sales-tax rates move with provincial budgets. When a page passes that date, the build fails. It will not deploy until a person has re-read the source and either corrected the page or confirmed nothing moved.
It is deliberately impossible to satisfy by bumping a date. Changing the date without re-reading the source is the one action that would turn all of this into decoration, and it is the only safeguard here that depends entirely on us being honest rather than on a machine.
What you can check without taking our word for it
- The numbers themselves. The full Canadian tax parameter table is published as open data under CC BY 4.0, generated from the same engine the site computes with, so the published file cannot disagree with what the calculator does.
- The sources. Every calculator’s verification block lists the exact pages we read, with dates.
- The formula. It is written out in plain language on the page, next to the tool that uses it.
- The worked example. Every page computes one all the way through with real figures, so you can follow it by hand.
What this does not promise
These are calculators, not advice, and they model the ordinary case. Every page names what it leaves out — and those omissions are listed on the page itself, not buried here. Tax in particular has more special cases than any tool can carry: if your situation is unusual, the honest answer is that a professional who can see your whole picture will beat any calculator, including this one.
We also make mistakes. When we do, the fix and the corrected date go up the same day, and the source that was misread gets recorded. If you think a number here is wrong, please tell us at hello@countworthy.com — being told is worth more to us than being right, and a calculator nobody checks is just a guess with buttons.