Free online calculators handle the kind of arithmetic that is slow, error-prone, or simply tedious to do by hand. Finding every prime factor of a large number, working out the 40th Fibonacci number, or checking a body mass index takes seconds with the right tool and several careful minutes without one. This guide is a map of the calculators on this site: the number and math tools, the sequence and constant tools, and the everyday calculators, with a note on when each one earns its place.
In this guide
When a calculator beats doing it by hand
Some arithmetic is quick in your head. Some is not. The work that genuinely needs a tool tends to share three traits: it repeats the same step many times, a single slip ruins the whole answer, or the numbers are simply too large to track.
Finding the prime factors of 4,294,967,295 is not hard in principle, it is just dozens of trial divisions where one mistake sends you down the wrong path. A calculator removes that risk. The tools below all run in your browser, process the numbers on your own device, and need no account, so reaching for one costs nothing.
Number and digit calculators
The most basic calculators work on the digits and structure of a number. The digit sum calculator adds up the individual digits of a number, a step used in checksums and in simple divisibility tests. The digit product calculator multiplies them instead.
For working across a whole set of numbers, the number sum calculator and the sum of integers calculator total a list or a range, and the number divisors calculator lists every number that divides cleanly into a given value.
Prime and factor calculators
Prime numbers are the building blocks of every other whole number, and they are tedious to work out by hand. The prime numbers calculator lists the primes up to a limit you set. The prime factors calculator breaks a single number down into the primes that multiply to make it, for example showing that 360 is 2 by 2 by 2 by 3 by 3 by 5.
Prime factorisation sits behind a lot of practical maths, from simplifying fractions to the key exchange that protects web traffic, so a reliable tool for it is worth keeping handy.
Sequence calculators
A sequence is a list of numbers built by a fixed rule, and the famous ones grow fast enough that hand calculation becomes impractical within a dozen steps. The Fibonacci calculator generates the sequence where each number is the sum of the two before it, the pattern that appears throughout nature and design.
The Lucas numbers calculator handles a closely related sequence, and the factorial calculator works out factorials, the running product 1 by 2 by 3 and onward, which grow so quickly that 20 factorial already passes two quintillion.
Math constant calculators
Some numbers never end and never repeat, and you sometimes need them to a specific precision. The pi digits calculator produces the digits of pi to as many places as you ask for. The e digits calculator does the same for Euler’s number, the base of natural logarithms, and the phi digits calculator produces the golden ratio. These are useful for testing, for teaching, and for any work that needs a constant carried to high precision.
Everyday calculators
Not every calculator is about pure maths. A few solve common practical problems. The BMI calculator works out body mass index from height and weight, a quick screening figure worth understanding properly, which our guide to BMI covers in full.
The age calculator works out an exact age in years, months, and days from a birth date, and the aspect ratio calculator resizes width and height while keeping their proportion fixed, which is the everyday fix for images and video that would otherwise stretch. The full set lives in the math tools category and the number tools category.
Frequently asked questions
Are these calculators free?
Yes. Every calculator on the site is free, runs in your browser, and needs no account. The numbers are processed on your own device.
What is prime factorisation used for?
It breaks a number into the primes that multiply to make it. It is used to simplify fractions, find common denominators, and it underpins parts of modern encryption.
How large a number can these tools handle?
The calculators handle everyday and large values comfortably. Extremely large inputs, such as a factorial of a very high number, can take longer because the result itself becomes enormous.
What is the difference between digit sum and number sum?
A digit sum adds the individual digits inside one number, so the digit sum of 142 is 7. A number sum adds several separate numbers together.
Why would I need the digits of pi?
Most often for testing, teaching, or any calculation that must carry a constant to a set precision. The pi digits calculator produces as many places as you request.