A calculator gives you a number with a confidence it hasn't always earned. Two tools can give different answers to the same question, and both can look authoritative. Before you rely on a result, whether it's your take-home pay, a mortgage payment or how long your savings will last, it's worth knowing what's behind it.
Check the rules are current
Anything involving tax depends on the tax year. The UK's Personal Allowance has been £12,570 for several years, but National Insurance rates, student loan thresholds, Scottish bands and many benefits change each April. A calculator still using last year's figures can be out by hundreds of pounds without any obvious sign. Our tax calculators state the tax year they use, and every UK figure comes from GOV.UK, HMRC or the relevant scheme's published rates, which we review each tax year.
Look for the assumptions
Every calculation rests on assumptions, and the honest ones tell you what they are. A take-home pay figure assumes a tax code, a pension type and a student loan plan. A savings projection assumes a rate of return and, often, no inflation. A walking time assumes a pace and a terrain. When a result surprises you, the assumptions are usually the reason. Changing one input at a time is the quickest way to see which assumption matters most for your situation.
Investment projections deserve particular care. A calculator that applies one average return every year produces a smooth line that real markets never follow. Several of our investment tools test your plan against real historical market sequences instead, so you can see the range of outcomes rather than a single figure.
Know what the number can't tell you
A calculator answers the question it's asked. It can tell you how much of a pay rise you'll keep, but not whether the new job is right for you. It can show the interest saved by overpaying a mortgage, but not whether you'd be better off keeping that money as an emergency fund. Use the result as one input to a decision, alongside the things that don't fit in a form: job security, health, family plans and how much risk you're comfortable with.
That's also why the tools on this site come with explanations, worked examples and links to the official sources. We'd rather you understood how a number was reached than simply trusted it.
When to get advice
For everyday questions, a good calculator and a clear explanation are usually enough. For decisions that are large, hard to reverse, or depend on your personal circumstances, such as pension transfers, complex tax situations or major investment choices, speak to a regulated financial adviser. Free, impartial guidance is also available from MoneyHelper and, for pensions, from Pension Wise.
Where to start
Most visitors arrive with a specific question. The take-home pay calculator is the most used tool on the site. For savings and investing, try the compound interest calculator. For everything else, browse the categories above or read one of our guides.