Digit Sum Calculator
Add up the digits of any number and repeat the process down to a single digit (the digital root) — free, instant and calculated entirely in your browser.
Signs, decimal points, spaces and thousands separators are ignored — every digit 0–9 is summed.
How to use the Digit Sum Calculator
- Enter a number. Type or paste any number into the box — it can be as long as you like, far beyond normal calculator limits.
- Read the results. The digit sum, the digital root (the repeated digit sum reduced to one digit) and the digit count update instantly as you type.
- Follow the steps. The step-by-step box shows each round of addition, such as 4 + 5 + 6 = 15 followed by 1 + 5 = 6, ready to copy into homework or notes.
About this tool
The digit sum of a number is simply all of its digits added together: for 456 it is 4 + 5 + 6 = 15. Repeating the process on the result — 1 + 5 = 6 — until a single digit remains gives the iterated digit sum, better known as the digital root. Digit sums appear in divisibility rules (a number is divisible by 3 or 9 exactly when its digit sum is), in checksum systems like the Luhn algorithm on credit cards and ISBN codes, and in numerology, where the digital root of a date or name is the headline number.
Because this calculator works directly on the digits as text rather than converting to a floating-point value, it handles numbers with thousands of digits without any rounding or overflow. Everything is computed locally in your browser — nothing you type is uploaded or stored, and the divisibility-by-3-and-9 check comes free with every result.
Frequently asked questions
What is the difference between the digit sum and the digital root?
The digit sum is one round of addition: 987 gives 9 + 8 + 7 = 24. The digital root repeats the process until one digit remains: 2 + 4 = 6. For any positive whole number the digital root equals 1 + (n − 1) mod 9, which is why it is also called the repeated or iterated digit sum.
How are negative numbers and decimals handled?
The minus sign and the decimal point are simply ignored — every digit present is summed. So -12.5 has a digit sum of 1 + 2 + 5 = 8. The divisibility-by-3-and-9 note is only shown for whole numbers, where the rule actually applies.
What is the digit sum used for?
The classic use is quick divisibility tests: a number is divisible by 3 or by 9 exactly when its digit sum is. Digit sums also power check digits on barcodes, credit cards and ISBNs, casting out nines to verify arithmetic, and digital-root calculations in numerology.
How large a number can I enter?
Practically unlimited — the tool adds digits as text, so a number with tens of thousands of digits is processed instantly with no overflow, something ordinary calculators cannot do.