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Significant figures calculator

Count the significant figures in a number, round it correctly, and apply measurement rounding rules to sums, differences, products and quotients.

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Significant figures calculator
Paste one or more numbers and this tool reports how many significant figures each one carries, then rounds it to the count you choose. It follows the rules taught in chemistry and physics labs: leading zeros never count, zeros between other digits always count, and trailing zeros count only when a decimal point is present — so "1500" reads as 1 significant figure but "1500.0" reads as 5.
Set Operation to Sum, Difference, Product or Quotient and the tool combines every number it finds — on the same line, or across the whole input when "Process each line separately" is off — and works out how precise the answer is allowed to be. Addition and subtraction keep as many decimal places as the least precise term; multiplication and division keep as few significant figures as the least precise factor, exactly what an instructor checks by hand. Round to the nearest digit ("round half up") or to the nearest even digit ("round half to even", the convention used in scientific computing to avoid a systematic upward bias). Switch Notation to scientific form when a plain decimal would hide which digits are significant.
Turn on "Show uncertainty" to see the absolute and relative uncertainty implied by a value's last significant digit, for writing up a lab result as a value plus or minus an error bound. A Unit field appends a label like "cm" to every result without affecting the calculation, and dividing by zero is reported as a calm message on that line rather than breaking the rest of the output.
Everything runs locally in your browser — numbers pasted from a lab notebook or a spreadsheet are never uploaded anywhere. Copy the result, download it as a .txt file, or send it back into the input to refine it further.
FAQ
How does the tool decide how many significant figures a number has?
It follows the standard convention: all non-zero digits count, zeros between two digits count, leading zeros never count, and trailing zeros count only when a decimal point is present. Scientific notation (like 1.50e3) makes every digit in the mantissa significant.
What is the difference between the two rounding modes?
"Round half up" always rounds an exact tie away from zero (2.5 becomes 3). "Round half to even" rounds an exact tie to whichever neighboring digit is even (2.5 becomes 2, 3.5 becomes 4) — the convention used in many scientific calculations to avoid rounding every tie the same direction.
Why does a sum use decimal places while a product uses significant figures?
That is the standard measurement rule. Addition and subtraction can only be as precise as their least precise term’s decimal places, while multiplication and division can only be as precise as the operand with the fewest significant figures.
What does "Process each line separately" change?
It only affects Sum, Difference, Product and Quotient. On, each line is its own calculation. Off, every number in the whole input is combined into a single result. It has no effect when Operation is set to "Analyze each number", which always evaluates every number on its own.
Is my data uploaded anywhere?
No. Every calculation runs locally in your browser using JavaScript — your numbers are never sent to a server.