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Calculator

Sources and methodology

Calculator publishes figures that people use for payroll, tax and household decisions. This page is the record behind them: which authority each figure comes from, how it is kept current, and when it was last checked. It is generated from the same register the site reads its dates from, so it cannot drift from the pages it describes.

What “last checked” means

A date on a page means somebody — or something — read the source that the figure comes from on that date and the figure still matched. It is a statement about the check, not a promise that the rate is still current today. Tax bands are rewritten by Finance Acts, tariffs change without notice, and exchange rates move every day, so the date is the only honest thing we can publish alongside a number.

No date on this site is written by a clock. A date moves when a check actually happens: an automated check that reads the source, or a manual read of a document no machine can parse. Nothing here moves simply because the site was rebuilt.

Where a manual read is the basis, every row says which of three things it was. Most were read from the authority’s own document. Four were confirmed against a published secondary source that states the same figure, because the authority’s own document could not be read — the PSSSF contribution split, where the Act is published only as a PDF we could not open, and the mobile-money withdrawal levy, where the gazette notice is behind a blocked endpoint. And five are marked “recorded from the source at the time; no document retained”: the figures were recorded from the authority’s own charges, no copy was kept, and none can be produced now. Those rows say so rather than implying a document exists that a reader could be shown, and they are the ones to upgrade first — reading the current charge and recording where it came from moves a row onto a firmer footing.

auto-weekly
Re-read from the source every week by an automated check. If the value the source publishes changes, the site is updated only when every guard passes — otherwise it is held for a person.
human-monthly
No machine-readable source exists, so this is re-read by hand and appears on a monthly review list. The automated job never moves this date.
archival-annual
A stable published fact (an Act, a notice, a dataset). Re-read once a year, and re-dated only if the source itself changes.
definitional
A definition rather than a measurement. It is not re-checked on a schedule.

The figure register

Every figure the site dates appears below — 38 of them today — with the authority it comes from, how it is checked, when it was last checked, and the pages that show it. Where the authority publishes a page or a PDF, the source is a link; where the figure comes from an Act, a notice or another document with no stable fetchable page, the authority is named and the entry says so — including, where that is the case, that no copy is held.

Rate and formula changes

No published figure has changed since this log began. When a rate, band or formula does change, the value is updated in the shared engine and recorded here with the date it changed and the source that changed it — so the history of a number is readable rather than being overwritten.

Corrections

This log began on 26 September 2026. A correction is recorded here when a published answer was wrong — a figure, a formula that did not follow the rule it cited, or an explanation that described something that is not the case. Routine re-checking where nothing changed is not listed; that is what the dates above record.

  • 26 September 2026 — formula on /tools/body-fat-calculator

    Before: The circumference measurements were converted to inches and then used in the form of the US Navy equation that is fitted in centimetres. Body fat was understated as a result: a 175 cm man with a 90 cm waist and 38 cm neck read 14.2% instead of 20.7%, and a 165 cm woman with a 75 cm waist, 95 cm hips and 32 cm neck read 4.5% — reported as "essential fat" — instead of 27.4%.

    Now: Measurements are used in the unit they are entered in (centimetres), which is the unit that form of the equation was fitted in. The same two cases now read 20.7% and 27.4%.

    Settled by: Hodgdon & Beckett, US Navy circumference equations. The two published parameterizations — 86.010 x log10(waist - neck) - 70.041 x log10(height) + 36.76 in inches, and the 495/() - 450 form fitted in centimetres — agree within 0.3 percentage points once each is given its own units; lib/calculators.test.ts now checks the engine against the inch-based form on six cases.

How to report an error

Email calculatorcotz@proton.me with the page, the figure you believe is wrong, what you think it should be, and the source you are checking against. A correction runs through the shared calculation engine, every worked example and dependent page is regenerated from it, and the full test suite has to pass before the fix is published — so a wrong figure cannot survive in one place while being fixed in another. Corrections come first in our inbox; full details are on the contact page.

Who maintains this, and what is not claimed

Calculator is published by Delta Tech Solutions in Dar es Salaam, Tanzania. Its figures are maintained by David M. Every tool, guide and category page carries the date its figures were last checked and the date the next check is due — 6 months later. A page publishing a rate that no readable source confirms says so in that line rather than printing a date, which is why a few of them read “rates not yet dated”.

No page on this site has been reviewed by an external qualified professional. Nobody with a tax, accounting, legal or medical qualification has checked the calculations or the wording, and the site does not claim otherwise. The calculators apply published rules to the numbers you enter; they are not advice, and they cannot see your circumstances. Where a decision matters, confirm the figure with the authority that sets it — the links in the register above are there for that.