Calculators

About these calculators and converters

This hub is for the numbers that come up in normal life: what a quote looks like with VAT, how much a sale really takes off, what a mortgage might cost each month, how savings could grow, how far a pound goes abroad, or someone’s exact age on a given date. Each page has the working tool at the top and a guide underneath explaining what to enter and what the answer means.

They help sole traders writing invoices, people comparing mortgage deals or savings accounts, travellers budgeting for a trip, parents filling in school forms, and anyone who’d rather not build a spreadsheet for a two-minute question.

What’s in this hub

Money

The VAT Calculator adds or removes 20%, 5%, 0% or a custom rate, with a quantity box. The Mortgage Calculator covers repayment and interest-only loans, overpayments and a stamp duty estimate for England and Northern Ireland. The Compound Interest Calculator handles monthly additions and five compounding options, and the Percentage Calculator shows a worked line under each answer.

Everyday figures

The Unit Converter covers ten families, from length and temperature to data and pressure. The Age Calculator gives years, months and days plus the wait until the next birthday, and the BMI Calculator takes metric or stones and pounds.

Live rates and chance

The Currency Converter uses European Central Bank reference rates, and the Random Number Generator can draw a batch with no repeats.

How the tools work

Most of the maths happens in your browser. The figures you type, such as a salary, a deposit or a date of birth, aren’t sent to Catpool or saved, and the Random Number Generator uses your browser’s cryptographic randomness.

One page needs live data. The Currency Converter asks Frankfurter, a free service that publishes European Central Bank rates, for the pair you choose. Those rates update once each working day, so they’ll be a little behind a trading screen. It can’t move money.

• Results update as you type on most pages
• No sign-in and nothing saved between visits
• The currency page says where its rates come from

When to use them

Quoting £450 for a job and need the total with VAT? That’s £540 at 20%. A shop advertises 30% off with an extra 10% at the till: run it through the Percentage Calculator and you’ll find it’s 37% off, not 40%. Weighing up a 2-year fix: try both rates in the mortgage calculator, then see how much a £100 monthly overpayment would save. Booking a holiday: check the reference rate before you compare what a bureau offers. Drawing raffle winners at a school fair: set the range to your ticket numbers and tick no repeats.

Use the results for planning and checking, then confirm anything that matters. A lender’s illustration, HMRC’s guidance or your GP’s advice will always beat a general calculator.

Common questions

Are the results accurate enough to rely on?

The arithmetic is exact, but the answer is only as good as what you enter. Mortgage, savings and VAT figures are estimates for planning, not financial or tax advice.

Why doesn’t the currency rate match my bank?

Banks and bureaux add a margin and sometimes a fee. The converter shows the mid-market reference rate, so the amount you actually get will usually be a bit lower.

Does the mortgage calculator include stamp duty?

It gives an optional estimate using England and Northern Ireland residential bands from April 2025. Scotland and Wales have their own taxes, so check GOV.UK or your solicitor.

Can I use the BMI Calculator for a child?

No. It uses adult bands. Children’s BMI is read against age and sex charts, so ask your GP or health visitor.