Practical Calculators

Voltage, current, resistance, and power from any two known values

V = I × R

Voltage drop across a wire run from length, current, AWG, and material

V_drop = 2 × L × I × R_per_unit_length

Power in watts from current in amps and voltage in volts

P (W) = V × I × PF

Current in amps from power in watts and voltage in volts

I (A) = P / (V × PF)

Current in amps from power in kilowatts and voltage in volts

I (A) = (kW × 1000) / (V × PF)

Power in kilowatts from current in amps and voltage in volts

kW = (V × I × PF) / 1000

Energy in kilowatt-hours from power in watts and time in hours

kWh = (W × h) / 1000

Average power in watts from energy in kWh and time in hours

W = (kWh × 1000) / h

Electricity running cost from power, hours, and per-kWh rate

cost = (W × h × rate) / 1000

Cubic yards or cubic metres of concrete needed for a slab, footing, or pad

V = L × W × D (rectangular); V = π × r² × D (circular pier or post hole)

Internal volume of a pipe or tube from inside diameter and length

V = π × (ID/2)² × L

Volumetric flow rate from velocity and pipe diameter, or velocity from flow and diameter

Q = A × v = π × (d/2)² × v

Floor area in square feet from length and width, with circular and irregular-shape variants

A = L × W (rectangular); A = π × r² (circular); A = 0.5 × b × h (triangular)

BTU heating or cooling load for a room based on size, climate zone, and conditions

cooling BTUh = sq ft × 20 (base) + adjustments for sun, occupancy, ceiling

Cubic yards, tonnes, and number of bags of gravel for a given area and depth

volume = length × width × depth

Litres or gallons of paint needed for a wall or room

paint_volume = (wall_area × coats) / coverage_rate

Cubic yards, tonnes, and bags of topsoil for a given area and depth

volume = length × width × depth

Roofing squares, bundles, and waste allowance from roof area

roof_area = footprint × slope_multiplier

Board feet of lumber from thickness, width, and length

board_feet = (thickness × width × length_feet × pieces) / 12

Engine power conversion between mechanical horsepower, metric horsepower, and kilowatts

kW = mechanical hp × 0.7457

Human height between feet/inches mixed format and centimetres

cm = (feet × 12 + inches) × 2.54

Fuel economy conversion between miles per gallon (US/UK) and litres per 100 kilometres

L/100 km = 235.215 / mpg(US)

Cup-volume measurements converted to gram weights for common baking ingredients

grams = cups × ingredient_density (g per cup)

Density from mass and volume, or any of the three from the other two

ρ = m / V

Percent change between two values, plus markup, discount, and percentage-difference variants

percent change = (new − old) / old × 100

Age in years, months, days, and hours from a birth date to today or any reference date

age = years_diff(reference_date, birth_date), with month/day rollover

Number of days, weeks, months, weekdays, or years between two dates

days = julian_day(date2) − julian_day(date1)

Percent of a value, what percent X is of Y, and percent change

Mode 1: X% of Y = Y × (X/100) Mode 2: X / Y × 100 = ?% Mode 3: ((Y − X) / X) × 100 = % change

Add, subtract, multiply, and divide two fractions with simplified output

a/b + c/d = (ad + bc) / bd

Convert between standard form and scientific notation

a × 10ⁿ where 1 ≤ |a| < 10 (scientific) or n is a multiple of 3 (engineering)

Tip amount, total bill, and per-person split for restaurant bills

tip = bill × (tip% / 100)

Sale price, savings amount, and effective discount from price and percent off

savings = price × (discount% / 100)

Selling price, profit, markup percent, and margin percent from any two known values

markup% = (price − cost) / cost × 100

Body Mass Index from height and weight, with WHO category classification

BMI = weight (kg) / height (m)²

Total Daily Energy Expenditure from age, weight, height, sex, and activity level

BMR = 10w + 6.25h − 5a + s

Body-fat percentage using the US Navy circumference method

Men: 86.010·log10(waist−neck) − 70.041·log10(height) + 36.76 Women: 163.205·log10(waist+hip−neck) − 97.684·log10(height) − 78.387

Ideal body weight from height and sex using Devine, Robinson, Miller, and Hamwi formulas

Devine M: 50 + 2.3·(in over 60)

Training heart-rate zones from age and resting heart rate using the Karvonen formula

HRR = max_HR − resting_HR

Future value, total interest, and growth from compound interest with optional regular contributions

FV = P × (1 + r/n)^(nt)

Interest charged or earned without compounding

I = P × r × t

Equated Monthly Instalment, total interest, and amortisation for a loan

EMI = P × r × (1+r)^n / ((1+r)^n − 1)

Tax-inclusive and tax-exclusive prices for any sales-tax or VAT rate

Add tax: gross = net × (1 + rate/100)

Convert between Celsius, Fahrenheit, gas mark, and fan-oven equivalent

°F = °C × 9/5 + 32

Convert between fresh, active dry, and instant yeast quantities

fresh ÷ 3 = active dry