Calculators that show their working

Mortgages, loans, cards, savings and the arithmetic of what things really cost.

• $Money

  Compound interest calculator: how a balance and deposits grow

  A starting balance and a stream of deposits grow under two different formulas. This page works both out by hand and shows what the rate, the term and the frequency each add.

• $Money

  Cost Per Use Calculator: What an Item Really Costs You

  Cost per use is the net cost of ownership -- price plus upkeep minus resale -- divided by an honest count of uses. The formula is simple; the usage estimate is where the comparison is won or lost.

• $Money

  Credit card payoff calculator: months to clear and interest paid

  A fixed payment behaves like an installment loan. A shrinking minimum payment does not. This page shows the arithmetic behind that difference and what it costs.

Salary conversions, take-home pay, overtime, freelance rates and paid leave.

• WWork & Pay

  Freelance hourly rate calculator: from take-home to price

  Start from the money you need to keep, gross up for self-employment tax, add expenses, and divide by the hours you can genuinely sell to find the rate that works.

• WWork & Pay

  Hourly to salary calculator: convert pay in both directions

  Turn an hourly rate into annual, monthly and per-paycheck pay, convert a salary back to an hourly rate, and see exactly what unpaid weeks off do to the comparison.

• WWork & Pay

  Overtime pay calculator: what a long week is actually worth

  Split a week into regular, overtime and double-time hours, price each correctly, and see the premium the overtime rule is really worth once bonuses join the regular rate.

Ages, date differences and hours worked, calculated exactly.

• TTime & Dates

  Age calculator: exact age in years, months and days

  Calendar age is three subtractions with borrowing, not one division. Here is the algorithm, a worked example across a month boundary, and why decimal age differs.

• TTime & Dates

  Date difference calculator: days between two dates

  Two dates have more than one correct gap. This explains inclusive against exclusive counting, the weekday algorithm, and why months between dates is ambiguous.

• TTime & Dates

  Time card calculator: hours worked, breaks and overtime

  Payroll runs on decimal hours: 7:45 is 7.75, not 7.45. Here is the conversion, overnight shifts, paid against unpaid breaks, and why the week drives overtime.

Running costs, solar payback, paint quantities and room measurements.

• HHome & Energy

  Electricity cost calculator: what an appliance costs to run

  Watts are a rate, kilowatt-hours are what you buy. This shows the conversion step by step, how to find your effective rate, and why nameplate wattage misleads.

• HHome & Energy

  Paint calculator: how many gallons your room actually needs

  Wall area is exact arithmetic; coverage per gallon is a manufacturer's best case. This page separates the two so your paint estimate survives contact with a real wall.

• HHome & Energy

  Room area calculator: floor area, perimeter, wall area and volume

  Floor area, perimeter, wall area and volume from three measurements, in both unit systems, with the square-unit conversion error that ruins most flooring orders explained.

Fuel costs, economy conversions, EV comparisons and what a commute really costs.

• DDriving & Travel

  Commute Cost Calculator: What Getting to Work Really Costs

  Fuel is the visible cost and the smallest one. Here is how to build a real per-mile figure, price the hours, and turn a commute into a salary difference between two jobs.

• DDriving & Travel

  EV vs Gas Cost Calculator: What Each Car Costs Per Mile

  Put gasoline and electricity on the same footing: two cost-per-mile formulas, a worked annual comparison, and the charging and weather factors that move the answer.

• DDriving & Travel

  Fuel cost calculator: what a trip actually costs in fuel

  Distance, consumption and price give the trip cost. The annualized rows and the mpg illusion are where the arithmetic changes what you thought you knew.

BMI, daily energy needs and running pace, with the formulas shown.

• FHealth & Fitness

  BMI Calculator: How the Index Is Calculated and What It Shows

  BMI divides weight by height squared. This page derives the formula in both unit systems, works an example by hand, and sets out the limits of the index.

• FHealth & Fitness

  Calorie Calculator: How BMR and Daily Expenditure Are Estimated

  The Mifflin-St Jeor equation, written out and worked step by step, plus what BMR, RMR and TDEE each mean and how much error the activity multiplier introduces.

• FHealth & Fitness

  Running Pace Calculator: Pace, Splits and Race Predictions

  Pace is time divided by distance; a race prediction is a fitted curve. This page works both by hand and shows how far Riegel's formula can honestly be stretched.