How to calculate VAT — adding it, and taking it off
Adding VAT is a multiplication; removing it is a division, and not the one most people reach for. The arithmetic for both, at 20% and 5%, with the mistake that costs money shown in full.
VAT CalculatorUpdated 16 Aug 20264 min read
Margin vs markup — the difference, and what it costs to confuse them
A 50% markup is a 33.3% margin. They are different numbers taken from different figures, and pricing off the wrong one quietly loses money on every sale. The conversion both ways, with a table.
Margin & Markup CalculatorUpdated 16 Aug 20264 min read
Hours and minutes to decimal — why 7h 30m is 7.5, not 7.30
Timesheets and payroll want decimal hours; clocks give you minutes. Writing one where the other belongs is the commonest way to be paid for the wrong number of hours. The conversion, and a table to keep.
Hours CalculatorUpdated 16 Aug 20264 min read
How to calculate percentages — the five sums people actually need
Percentage of a number, percentage increase and decrease, percentage difference, and working backwards to the original. Five different questions, each with its own formula and worked example.
Percentage CalculatorUpdated 16 Aug 20263 min read
The compound interest formula, explained with real numbers
A = P(1 + r/n)^(nt), taken apart one letter at a time — plus why 5% compounded monthly is really 5.116%, and why regular deposits matter more than the rate.
Compound Interest CalculatorUpdated 16 Aug 20263 min read
How to calculate BMI — and what the number can’t tell you
Weight divided by height squared, in metric and imperial, with the NHS categories — including the lower thresholds that apply to some ethnic backgrounds, and the limits of the measure.
BMI CalculatorUpdated 16 Aug 20263 min read
eBay, Amazon and Shopify fees — what each one actually takes
The fees that come off a sale on each platform, why the headline percentage is never the whole story, and how to work out what you really keep per unit.
Profit CalculatorUpdated 16 Aug 20263 min read
How to simplify a ratio, and how to share an amount in one
Reducing a ratio to its lowest terms, handling decimals, scaling to find a missing part, and dividing money in a given ratio — with the misreading that catches most people.
Ratio CalculatorUpdated 16 Aug 20262 min read
Working out the days between two dates — and your exact age
Counting days across months and leap years, why "how old am I" is harder than it looks, and the inclusive-or-exclusive question that changes every answer by one.
Age CalculatorUpdated 16 Aug 20262 min read
UK unit conversions — cm to feet, kg to stone, and the pints that differ
The conversions Britain actually uses, why a US pint is not a UK pint, and the temperature formula everyone half-remembers.
Unit ConverterUpdated 16 Aug 20263 min read