How We Calculate
Last updated: May 2026
This page explains how Online Calculators keeps its calculators accurate, and where the figures behind them come from. We think you should be able to check our working - so every calculator page also shows the method it uses.
Our sources
We use primary, official sources wherever a figure is set by government or a recognised standard:
- Tax and National Insurance - HM Revenue & Customs (HMRC) published rates and thresholds, and, for Scotland, the Scottish Government's separate income tax bands.
- Vehicle tax - the Driver & Vehicle Licensing Agency (DVLA) Vehicle Excise Duty bands.
- Statutory pay - the published statutory rates for redundancy, sick pay, maternity pay and the National Minimum Wage.
- Health tools - NHS guidance and recognised clinical formulae, such as the Mifflin-St Jeor equation for metabolic rate.
- Conversions - internationally agreed conversion factors, with care taken over UK-specific units such as the imperial pint and gallon.
Our review process
Calculators whose results depend on a rate are reviewed at least once each tax year, and again whenever a Budget or official announcement changes a relevant figure. When a calculator is reviewed, the review date shown on its page is updated so you can see how current it is.
Calculators based on a fixed formula or a stable conversion factor - most maths tools and unit converters - change rarely, but are checked when the site is audited.
Showing the working
Every calculator page includes a plain-English explanation of how its result is reached. Where a standard formula is used, we name it. We would rather a page be slightly longer and fully transparent than short and opaque.
When we get something wrong
We aim to be accurate, but errors can happen. If you spot a figure that looks wrong, please tell us through the contact page. We investigate every correction and, where it is valid, fix it quickly and note the update.
What our calculators are not
Our tools provide estimates and general information. They are not financial, legal, medical or tax advice, and they cannot account for every individual circumstance. For decisions that matter, please consult a qualified professional. See our disclaimer for the full position.