Practical Calculators

Ohm's Law calculator: enter any two of voltage (V), current (A), or resistance (Ω) to instantly compute the third. Formula, worked examples, and theory.

V = I × R

Calculate voltage drop over a wire run by AWG gauge, wire length, current load, and conductor material. Outputs drop in volts and as a percentage.

V_drop = 2 × L × I × R_per_ft

Calculate the internal volume of a pipe or cylinder from diameter and length. Outputs in liters, cubic meters, and US gallons. For plumbing, HVAC, and process engineering.

V = π × (d/2)² × L

Calculate density from mass and volume, or solve for mass or volume given density. Formula: ρ = m / V. Includes common material densities.

ρ = m / V

Calculate volumetric flow rate from pipe diameter and fluid velocity, or from volume and time. Outputs in L/s, m³/h, and GPM. Includes Reynolds number context.

Q = A × v or Q = V / t

Convert horsepower to kilowatts or kW to hp. Supports mechanical hp, metric hp (PS), and electric hp. Formula, reference table, and worked examples.

kW = hp × W_per_hp ÷ 1000

Convert fuel economy between MPG (US), MPG (imperial/UK), and L/100km. Handles both directions. Notes on US vs imperial gallon difference.

L/100km = 235.214 / MPG(US)

Calculate BTU needed to heat or cool a room. Enter room dimensions, ceiling height, and sun exposure. HVAC sizing guide.

BTU = Area × 20 × ceiling factor × sun factor + (occupants × 600)

Calculate concrete volume for slabs, columns, and cylinders. Outputs cubic feet, cubic yards, and cubic meters. Includes bag count estimate.

Slab: V = L × W × D

Calculate square footage for rectangles, circles, and triangles. Also get square meters. Free online area calculator.

Rectangle: A = L × W | Circle: A = π × r² | Triangle: A = ½ × b × h

Calculate Body Mass Index (BMI) from height and weight. Supports metric (kg/cm) and imperial (lb/in) inputs. Shows BMI category and healthy weight range.

BMI = weight(kg) / height(m)²

Three percentage calculators in one: find what percent of a number is, calculate what percentage one number is of another, or compute percentage increase/decrease between two values.

Mode 1: result = (X / 100) × Y | Mode 2: result = (X / Y) × 100 | Mode 3: result = ((Y − X) / |X|) × 100

Calculate your exact age in years, months, and days from your date of birth. Free online age calculator with instant results.

Age = Today − Date of Birth

Calculate the number of days, weeks, months, and years between two dates. Handles leap years automatically.

Difference = End Date − Start Date (in days)

Convert cups to grams for common baking ingredients. Flour, sugar, butter, oats, and more. Ingredient-specific density lookup.

grams = cups × (grams per cup for ingredient)

Convert height between feet & inches and centimeters. Enter ft/in to get cm, or cm to get feet and inches.

cm = (feet × 12 + inches) × 2.54