Free online calculators
Every calculator explains its own maths. Enter your numbers, then press Play to watch how the answer is built.
Everyday
Quick maths, with the working shown.
Online Calculator with Steps
Type an expression like 12 × (3 + 4) ÷ 2 + 5 and press =. It follows the order of operations (brackets, powers, multiply and divide, then add and subtract) and animates each step until only the answer is left.
Age Calculator
Your age is the number of complete years, months and days between your date of birth and today. Someone born on 15 May 1990 is 36 years, 4 months and 6 days old on 21 September 2026.
Days Between Dates Calculator
Subtract the earlier date from the later one to get the days between them. From 1 January to 25 December 2026 is 358 days, or 359 if you count both the first and last day.
Time Zone Calculator & World Clock
Pick a base city and a date and time, then pick a second city to compare it to. You'll instantly see the equivalent local time there, how many hours ahead or behind it is, and whether that puts it on a different calendar day.
Unit Converter
Pick a type, enter a value and choose the units. For example, 1 mile is 1.609 kilometres, 1 kilogram is 2.205 pounds and 1 US gallon is 3.785 litres.
Area Calculator & Size Comparison
Enter an area and a unit, and instantly see how many football pitches, tennis courts or parking spaces it's equivalent to, alongside the exact conversion between area units.
Tip Calculator & Bill Splitter
To calculate a tip, multiply the bill by the tip percent as a decimal. An 18% tip on a $50 bill is 0.18 × 50 = $9, for a total of $59. Split three ways, that is $19.67 each.
GPA Calculator
GPA is the total grade points across your courses (each grade's point value × its credit hours) divided by your total credit hours. Three courses graded A, B+ and B at 3, 3 and 4 credits work out to a 3.39 GPA.
Final Grade Calculator
Your overall grade is each category's score multiplied by its share of the total weight, added together. Leave one category blank — like a final exam — and enter a target grade instead, and the calculator solves for the score that category needs.
Time Duration Calculator
To find the time between two clock times, convert both to minutes past midnight and subtract. From 9:00 AM to 5:30 PM is 8 hours 30 minutes.
Fraction Calculator
To add or subtract fractions, first convert them to a common denominator. To multiply, multiply straight across. To divide, multiply by the second fraction flipped upside down. This calculator shows every step and simplifies the result.
Roman Numeral Converter
Roman numerals are built from I (1), V (5), X (10), L (50), C (100), D (500) and M (1000), taking the largest values first — 1994 becomes MCMXCIV (1000 + 900 + 90 + 4).
Percentage Calculator
To find a percentage of a number, divide the percent by 100 and multiply by the number: 15% of 200 is 0.15 × 200 = 30.
Health & pregnancy
BMI, calories, due date and fertility.
Pregnancy Due Date Calculator
Your estimated due date is 280 days (40 weeks) after the first day of your last period. For example, a last period starting March 1 gives a due date of about December 6.
BMI Calculator
BMI is your weight in kilograms divided by your height in metres squared. A 70 kg adult who is 1.70 m tall has a BMI of 24.2, which is in the healthy range of 18.5 to 24.9.
Calorie Calculator (TDEE)
Your daily calorie need is your basal metabolic rate (BMR) multiplied by an activity factor. For a 30-year-old man who is 180 cm, weighs 80 kg and exercises 3 to 5 days a week, that is about 2,760 calories a day.
Ovulation Calculator
Ovulation usually happens about 14 days before your next period starts. In a 28-day cycle that is cycle day 14, and your most fertile days are the five days before it plus ovulation day itself.
Body Fat Calculator
The US Navy method estimates body fat from your neck, waist and (for women) hip measurements, using a formula built from log-scaled circumference differences. A 32-inch waist and 15-inch neck at 5'8" puts a man at roughly 14–15% body fat, in the athletic-to-fitness range.
Macro Calculator
Multiply your daily calorie target by each macro's percentage, then divide by its calories per gram: protein and carbs are 4 kcal/g, fat is 9 kcal/g. On a 2,000-calorie, 30/40/30 split, that's 150 g protein, 200 g carbs and 67 g fat.
One-Rep Max Calculator
Your estimated one-rep max is the weight you lifted × (1 + reps ÷ 30). Lifting 100 kg for 5 reps gives an estimated 1RM of about 116.7 kg.
Water Intake Calculator
A common baseline is 30–35 ml of water per kilogram of body weight per day, plus more for exercise and hot weather. A 70 kg adult with 30 minutes of exercise in a temperate climate needs roughly 2.8 litres a day.
Ideal Weight Calculator
The Devine formula estimates ideal weight from height alone: 50 kg (men) or 45.5 kg (women) at 5 feet tall, plus 2.3 kg for every inch above that. At 175 cm, that's about 70.5 kg for a man.
Heart Rate Zone Calculator
Using the Karvonen method, your target zones are based on your heart rate reserve — the gap between your estimated max heart rate (220 minus your age) and your resting heart rate.
Weather
How hot, cold or humid it really feels.
Temperature Converter (°C, °F, K)
To convert Celsius to Fahrenheit, multiply by 9/5 (1.8) and add 32: 20 °C is 68 °F. To convert Fahrenheit to Celsius, subtract 32 and multiply by 5/9.
Heat Index Calculator
The heat index combines air temperature and humidity to show how hot it feels. At 90 °F (32 °C) and 70% humidity it feels like about 106 °F (41 °C), which is in the Danger range.
Wind Chill Calculator
Wind chill shows how cold the wind makes exposed skin feel. At 20 °F (−7 °C) with a 15 mph (24 km/h) wind it feels like about 6 °F (−14 °C).
Dew Point Calculator
The dew point is the temperature at which air becomes saturated and dew forms. At 25 °C (77 °F) and 60% humidity the dew point is about 16.7 °C (62 °F), which feels slightly humid.
Frost Date Calculator
Pick your nearest city to see the typical date of the last spring frost, the first fall frost, and how many frost-free days usually fall in between — plus, if you enter a date, the estimated chance frost still lands on it.
Sunrise & Sunset Calculator
Sunrise and sunset times depend on your latitude, longitude and the date. In London on the June solstice the sun rises at about 4:43 AM and sets at about 9:21 PM, giving roughly 16 hours 38 minutes of daylight.
Wind Speed Converter & Beaufort Scale
To convert m/s to km/h, multiply by 3.6. To convert km/h to mph, multiply by 0.6214. A 10 m/s wind is 36 km/h, 22.4 mph or 19.4 knots, which is Force 5 (a fresh breeze) on the Beaufort scale.
Absolute Humidity Calculator
Absolute humidity is the mass of water vapour in each cubic metre of air. At 20 °C and 50% relative humidity, air holds about 8.6 g of water per m³, half of the 17.3 g/m³ it could hold when saturated.
Rainfall Converter & Rain Volume Calculator
One millimetre of rain over one square metre is exactly one litre. One inch of rain is 25.4 mm, and over one acre it is about 27,154 US gallons (102,790 litres).
Moon Phase Calculator
Enter any date to see the Moon's phase, how much of it is illuminated, and how many days until the next full and new moons — worked out from how many days have passed since a known new moon, divided by the Moon's roughly 29.53-day cycle.
Tide Calculator
Pick a coastal station and a date to see NOAA's predicted high and low tide times and heights for that day, the tidal range between them, and whether the Moon phase points to a bigger spring tide or a smaller neap tide.
Finance
Loans and growing your money.
Compound Interest Calculator
Compound interest means you earn interest on your interest. $10,000 at 7% for 20 years with no deposits grows to about $40,400 with monthly compounding.
Loan & Mortgage Calculator
A $300,000 loan at 6.5% over 30 years costs about $1,896 a month, and roughly $382,600 of that total is interest.
Savings Goal Calculator
To hit a savings goal, this calculator works backwards from compound interest: enter your goal, starting balance, interest rate and either a deadline (to find the monthly deposit needed) or a monthly deposit (to find how long it will take).
Currency Converter
Enter an amount and a currency, and see it converted into a dozen major world currencies at once, using the latest official exchange rates — updated daily from the European Central Bank.
Home & electrical
Volts, amps, watts and running costs.
Electrical Calculator (Volts, Amps, Watts, Ohms)
Enter any two of voltage (V), current (A), power (W) or resistance (Ω) and the calculator finds the other two using Ohm's law (V = I × R) and Watt's law (P = V × I). A 120 V circuit drawing 10 A is using 1,200 W.
Electricity Cost Calculator
Multiply the appliance's kilowatts by the hours used and by your electricity rate to get the cost. A 1,500 W heater run 2 hours a day at $0.16 per kWh costs about $0.48 a day, or roughly $14.40 a month.
Power Load Calculator
Add up each appliance's watts to find your running load, add headroom for the largest motor starting up to size a generator, project your daily energy use into a monthly usage and cost, and multiply it by how many backup days you want to size a battery bank or solar array.
Auto & fuel
Trip costs and fuel economy.