Calculators guides
Calculator guides: percentages, money and everyday maths
Everyday maths goes wrong in small, expensive ways: a percentage taken from the wrong base, a rate applied twice, a month that is not thirty days. These guides explain the method behind each calculation and link to free calculators that show their working.
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Calculators
A 50% markup is only a 33% margin, and mixing them up quietly costs you money
Margin divides the profit by the selling price and markup divides it by the cost. Here is a conversion table, both formulas, a worked pricing example and the mistakes to avoid.
All Calculators guides
Calculators
The monthly payment tells you what you can afford. Total interest tells you what the loan costs
How a payment is worked out, why the first payments are mostly interest, what extra payments really do and how to read a payment schedule, with examples you can check by hand.
Calculators
Why the same savings at the same rate end up so far apart: compound interest explained, with every number checked
Simple versus compound side by side, the formula, how little compounding frequency matters next to time, regular deposits, the effective rate and the rule of 72 with its error. An explanation, not advice.
Calculators
Counting the days between two dates is easy until it is not: the first day, weekends, holidays and month ends
One method for the number of days, the weekdays, the working days and the date a set number of days away, plus the rules that make two honest answers differ by one.
Calculators
A UK gallon is not a US gallon: the metric to imperial conversions people get wrong, with the exact factors
One table of the factors you actually need, then the traps in each family (gallons and pints, stone, temperature offsets, mpg versus L/100 km) and a mental shortcut for each, with its error.
Calculators
Take 20% off, then add 20% back, and you end up at 96, not 100. Percentages explained, with every trap worked out
The six questions behind almost every percentage problem, each with a formula and a worked example. Then the traps: stacking, reversing, and percent versus percentage points.
Free everyday calculators
Age Calculator
Exact age in years, months and days, with totals and the next birthday.
Open toolArea Converter
Convert between 10 area units, from square millimeters to square miles, with exact results and the working shown.
Open toolBreak-Even Calculator
Break-even units and sales from fixed costs, price and variable cost, with a profit target and a chart.
Open toolCompound Interest Calculator
Compound interest with any compounding frequency, regular deposits, a yearly table, a chart and the effective yearly rate.
Open tool