Methodology & sources
Last reviewed:
This page explains how we build, check and update the UK calculators on Kalkulo.eu so you can judge how much weight to put on a result — and reproduce it yourself if you want to.
Which tax year we use
Unless a page says otherwise, UK calculators apply the 2026/27 tax year (6 April 2026 to 5 April 2027). Each calculator states the tax year it uses and the date it was last updated. Where a figure is set annually (for example the Personal Allowance or NI thresholds), we update it when HMRC confirms the new value, not before.
How a result is derived
Each calculator implements the official formula directly: bands and thresholds are applied in order, allowances are tapered where the rules require it (for example the Personal Allowance taper above £100,000), and we show the breakdown — not just the final number — so you can see where each deduction comes from. We re-derive every calculator's worked example by hand and cross-check it against HMRC's own examples before publishing.
Documented simplifications
Some real-world rules are more granular than a single web form can sensibly capture. Where we simplify, we say so on the page and in its FAQ. Common examples:
- National Insurance is calculated on an annualised basis rather than per pay period, which can differ by a small amount from a specific payslip.
- Calculators assume a standard tax code and England/Northern Ireland Income Tax bands unless a Scotland/Wales option is offered.
- Loan and mortgage tools use a standard annuity formula; lenders may round differently.
Primary sources
We rely on primary UK government sources. The main ones, by calculator area:
| Area | Official source |
|---|---|
| Income Tax bands, rates & personal allowance | HMRC — Income Tax rates and Personal Allowances |
| National Insurance (Class 1, 2, 4) rates & thresholds | HMRC — National Insurance rates and categories |
| VAT rates (standard, reduced, zero) | HMRC — VAT rates on different goods and services |
| Stamp Duty Land Tax (SDLT) bands | HMRC — Stamp Duty Land Tax |
| Dividend tax rates & allowance | HMRC — Tax on dividends |
| Holiday entitlement (statutory) | gov.uk — Holiday entitlement |
| Making Tax Digital for Income Tax (MTD ITSA) thresholds | HMRC — Making Tax Digital for Income Tax |
Corrections & update process
We review rates at the start of each tax year and after fiscal events. If you find a figure that's out of date or wrong, email [email protected] with the calculator and the value you'd expect; we correct verified errors promptly and log material changes on the relevant calculator. More about who we are on the About page.
These calculators are informational and do not constitute tax or financial advice. Always verify with HMRC or a qualified accountant — see our Terms of Use.