Everyday maths, with the working shown
Most of the calculations people do in daily life are not difficult — they are just easy to get slightly wrong. Is a rise from 80 to 100 a 20% increase or a 25% increase? Does "30% off, plus an extra 10%" mean 40% off? How many days are there really between two dates when February is involved? The calculators on this page answer those questions quickly and, just as importantly, show how the answer was reached.
Percentages and prices
The Percentage Calculator handles five separate questions: a percentage of a number, what percent one number is of another, percentage increase or decrease, increasing or decreasing a number by a percentage, and percentage difference. Each shows the formula with your numbers plugged in. The Discount Calculator is built for shopping: it turns a percentage off into a real saving, handles "extra" discounts applied on top of sale prices, and adds sales tax to give you the total you will actually pay.
If percentages have always felt slippery, our guide on how to calculate percentage walks through every formula with worked examples and the mistakes that trip people up most often, such as confusing percent with percentage points.
Dates and ages
Date calculations are harder than they look because months have different lengths and leap years add a day. The Age Calculator gives an exact age in years, months and days on any date — useful for eligibility rules that depend on age on a specific day — along with total days and a countdown to the next birthday. The Date Difference Calculator measures the span between any two dates in days, weeks, calendar months and years, and can include the end date or count weekdays only.
Calendar months vs average months: dividing a number of days by 30 does not give an exact number of months. Our date tools count real calendar months and explain when an approximate figure is being used instead.
Converting units
The Unit Converter covers length, weight, temperature, area, volume, speed, time and data storage, switching between metric and imperial units. It uses exact international conversion factors, handles temperature scales correctly, and distinguishes between decimal storage units (GB) and binary ones (GiB) — the reason a new drive often seems smaller than advertised.
Accuracy and responsibility
These calculators use standard formulas and are tested against worked examples, but no tool can know your exact situation. For money, contracts, tax, medicine or anything official, double-check important results and follow the rules that apply to you — for example, how your local tax is calculated, or whether a legal deadline counts the first day. Our disclaimer explains this in more detail.
Which calculator should I use?
| Question | Calculator |
|---|---|
| What is 15% of 240? | Percentage Calculator |
| How much did a price go up in percent? | Percentage Calculator |
| What will I pay after 25% off plus tax? | Discount Calculator |
| How old will someone be on a given date? | Age Calculator |
| How many days until a deadline? | Date Difference Calculator |
| How many miles is 10 km? | Unit Converter |
Frequently asked questions
Do the calculators store what I enter?
No. All calculations run in your browser and nothing you type is sent to our server.
How precise are the results?
Calculations use full precision internally. Results are rounded for display — money to two decimal places, other values to a sensible number of digits.
Can I use these for financial or legal decisions?
They are helpful for checking figures, but important financial, legal or official numbers should always be verified independently.