Guides
Longer explainers for the questions behind the tools — methods, worked examples, and what the numbers do not include. Written by Josh, who builds Utiio.
How to calculate a percentage (and percentage change)
Worked examples for finding a percentage of a number, percentage increase and decrease, and reverse percentages — the same methods used in Utiio’s percentage calculator.
About 4 min read
Read guideHow UK VAT works: adding and removing 20%
A plain-English guide to adding VAT to a net price, removing VAT from a gross price, and when reduced or zero rates apply — with the 20% worked examples used in Utiio.
About 4 min read
Read guideHow mortgage repayments are calculated
The amortising loan formula behind monthly mortgage payments, what deposit and term change, and what a simple calculator leaves out — fees, tax, and affordability.
About 4 min read
Read guideWhat BMI means — and what it does not
How Body Mass Index is calculated, the adult WHO categories, why athletes and some ethnic groups need extra context, and how to use Utiio’s BMI calculator as a screening number only.
About 4 min read
Read guideHow compound interest grows savings
What compounding means, why frequency and regular deposits matter, and how to read a projection from Utiio’s compound interest and savings calculators.
About 4 min read
Read guideHow to estimate paint for a room
A practical method for wall area, doors and windows, coats, and tin coverage — the same approach as Utiio’s paint calculator, with room for wastage.
About 4 min read
Read guideHow to estimate daily calorie needs
What BMR and TDEE mean, how Mifflin–St Jeor is used, and why activity multipliers are only a starting point — plus how Utiio’s calorie and macro calculators fit together.
About 4 min read
Read guideHow Stamp Duty Land Tax works in England
How SDLT is charged in bands on residential purchases in England and Northern Ireland, first-time buyer relief, and surcharges — and why Utiio’s figures are a guide, not a completion statement.
About 4 min read
Read guideWorking days vs calendar days: how to count them
When to count every calendar day, when to skip weekends, and when to exclude public holidays — including how UK regional bank holidays differ in Utiio’s calculators.
About 4 min read
Read guideHow to work out a tip and split a bill
How to calculate a tip percentage, split equally between people, and handle UK vs US customs — using the same method as Utiio’s tip calculator.
About 4 min read
Read guide