Guides Behind the Calculators

The real UK data, sourced and cited, behind each of our 20 calculators — not filler, the actual explanation.

Health
Why BMI Gets Athletes Wrong (and What Doesn’t)
BMI can’t tell muscle from fat. Here’s what the number actually measures, and what to check instead.
Finance
Margin of Safety: The Number That Matters More Than Break-Even
Two businesses, same profit, very different risk — why margin of safety beats break-even alone.
Health
The Old Calorie Formula Overestimates by 74 kcal a Day
Harris-Benedict dates to 1919. Mifflin-St Jeor is what dietitians use now — the real gap, worked out.
Home
The 0.5m³ Line: When Bagged Concrete Stops Making Sense
Below half a cubic metre bagged mix wins. Above it, ready-mix starts winning on cost and your back.
Finance
A £3,000 Balance on Minimum Payments: Still Unpaid After 50 Years
Real simulation: minimum payments on £3,000 at 24.9% APR cost over £13,500 in interest — and counting.
Home
What Your Appliances Actually Cost — Not What They Feel Like
A kettle costs 23p a year. An EV charger costs £2,668. Real annual costs for 5 common appliances.
Home
Why a 20m Fence Never Comes Out to Exactly 20m
Standard panel widths force a round-up — and posts don’t double the way people assume.
Finance
"Divide Your Salary by 220" Underprices Freelancers by 57%
The common day-rate shortcut ignores tax, non-billable days and costs — real numbers on the gap.
Home
Why Your Gas Meter Reading Never Quite Matches Your Bill
The real m³-to-kWh conversion behind every UK gas bill, and what typical households actually pay at the current price cap.
Finance
2% vs 3.36%: Why "Just One Point" of Inflation Matters
Over 20 years on £10,000, the gap between the BoE target and recent UK inflation is worth £4,500.
Finance
SDLT vs LBTT: Where Scotland Is Cheaper, and Where It Isn’t
Real 2026/27 figures across four price points — the crossover where Scottish stamp duty flips from cheaper to more expensive.
Home
Why Your Paint Coverage Never Quite Matches the Tin
What actually shifts paint coverage away from the tin’s claimed 12-14 m² per litre.
Finance
Starting Your Pension at 25 vs 35: £24k More In, £153k More Out
The same monthly contribution, started 10 years earlier, produces vastly more — real numbers on why.
Finance
50% Return Sounds Great — Until You Ask How Long It Took
The same 50% return means wildly different things depending on the years it took to get there.
Home
A 45° Roof Has 41% More Surface Than Its Footprint
Footprint-based roofing estimates get dramatically worse as pitch increases. Real numbers at 4 pitches.
Home
Measuring an L-Shaped Room Without Getting It Wrong
The "full rectangle minus the missing corner" method that turns an awkward shape into two measurements.
Finance
Salary & Tax Calculator
Free salary and tax calculator. Calculate your take-home pay after income tax, National Insurance and pension contributions.
Home
Same Solar System, Different Country: Scotland vs South England
An identical 4kWp system pays back nearly 2 years faster in South England — purely down to sunlight.
Home
Why Buying Extra Tiles Now Beats Matching Them Later
Tile dye lots vary between batches — why the spare box you skip today can become impossible to match.
Finance
The Flat Rate Scheme Has a Break-Even Point — Here’s Where It Is
Real numbers across four expense levels showing exactly where the Flat Rate Scheme stops paying off.