Significant Figures Calculator (Sig Fig Counter & Rounder)

Type a number to see how many significant figures it has, which digits count, and the number rounded to 1-6 sig figs. The second calculator adds, subtracts, multiplies or divides two measurements and rounds the answer by the textbook rules, with exact decimal math (no floating-point errors).

FreeNo sign-upRuns in your browser: your data is not uploaded

Built by · Updated

Calculate with significant figures

The rules for counting significant figures

  1. Non-zero digits always count. 3.14 has 3.
  2. Zeros between non-zero digits count ("captive" zeros). 505 has 3, 1.0203 has 5.
  3. Leading zeros never count. They only place the decimal point: 0.00450 has 3 (the 4, the 5 and the final 0).
  4. Trailing zeros after a decimal point count. 12.30 has 4; 100.0 has 4. They show the measurement was precise to that place.
  5. Trailing zeros in a whole number without a decimal point are ambiguous. 1200 could be 2, 3 or 4 significant figures. Most textbooks treat it as 2. Write 1200. (with a point) for 4, or use scientific notation: 1.2 × 10³ (2), 1.20 × 10³ (3), 1.200 × 10³ (4).
  6. In scientific notation, every digit of the number before × 10ⁿ counts. 6.022 × 10²³ has 4.

The calculator colours the digits: green ones are significant, grey leading zeros are placeholders, and yellow trailing zeros are the ambiguous kind from rule 5, shown as a range (2 to 4).

Rounding to a number of significant figures

Count from the first non-zero digit, keep n digits and look at the next one: 5 or more rounds up, less than 5 leaves it. 0.0045678 to 2 sig figs is 0.0046; 123.456 to 2 is 120, which is ambiguous on paper, so the table also shows 1.2 × 10². Rounding can carry into a new digit: 9.996 to 3 sig figs becomes 10.0, still three significant figures. A half (2.5 to 1 sig fig) is rounded away from zero to 3, the convention most school courses use; some lab software rounds halves to the even digit instead.

Sig figs in calculations

  • Multiplication and division: the answer keeps as many significant figures as the input with the fewest. 4.56 × 1.4 = 6.384 → 6.4 (2 sig figs, because 1.4 has 2).
  • Addition and subtraction: what matters is the decimal place, not the count. Round to the least precise place among the inputs. 12.11 + 18.0 = 30.11 → 30.1 (tenths, because 18.0 stops at tenths). 1200 + 35.6 = 1235.6 → 1200 (hundreds, because 1200 is only known to the hundreds).
  • Multi-step problems: keep all digits in the intermediate results and round only the final answer. Rounding at every step adds error.
  • Exact numbers (counted items, defined constants such as 100 cm in 1 m) have unlimited significant figures and never limit the answer. Leave them out of the rule.

The calculator works on the decimal digits exactly (it uses whole-number arithmetic internally), so 0.1 + 0.2 really is 0.3 here, unlike a floating-point calculator that shows 0.30000000000000004.

FAQ

How many significant figures does 0.00450 have?
Three: the 4, the 5 and the final 0. Leading zeros only show where the decimal point is; the trailing zero after the decimal point was measured, so it counts.
How many sig figs are in 1200?
By the usual rule 2, because trailing zeros in a whole number without a decimal point are not counted. It is really ambiguous (2 to 4). Write 1200. for 4 or 1.20 × 10³ for 3 to make it clear.
What is the sig fig rule for multiplication?
Keep as many significant figures as the input with the fewest. 2.0 × 3.456 = 6.912, rounded to 6.9 because 2.0 has two significant figures.
What is the sig fig rule for addition?
Round to the least precise decimal place among the numbers, not to the fewest significant figures. 1.0 + 2.3456 = 3.3456, rounded to 3.3 (tenths).
Do zeros between digits count as significant?
Yes. Zeros between non-zero digits always count: 505 has three significant figures and 1.007 has four.

Embed this calculator on your website

Put the live calculator on your own page (blog post, HR portal, client site). Paste this where you want it to appear; it is free, loads from HeadsNTail and always uses the current rates. Keep the credit line under it.

Embed code (HTML)
<iframe src="https://headsntail.com/embed/sig-fig-calculator" title="Significant Figures Calculator" loading="lazy" style="width:100%;max-width:720px;height:760px;border:0;border-radius:16px;background:#0f141b" allow="clipboard-write"></iframe>
<p style="font-size:13px"><a href="https://headsntail.com/tools/sig-fig-calculator">Significant Figures Calculator</a> by HeadsNTail</p>

Works in WordPress (Custom HTML block), Blogger, Webflow, Notion (embed block with the iframe URL) and plain HTML. Height can be changed in the code.

Found it useful? Put a link in your docs, README or forum answer so others can use it too:

HTML
<a href="https://headsntail.com/tools/sig-fig-calculator">Significant Figures Calculator</a> (free, by HeadsNTail)
Markdown (GitHub, Stack Overflow, dev.to)
[Significant Figures Calculator](https://headsntail.com/tools/sig-fig-calculator)

Need a website, HR or business system? HeadsNTail builds websites, HR and payroll apps and business tools. See services or ask a question.

More free tools

Click Speed Test (CPS, 10 seconds)How many clicks per second can you do? 1, 5, 10, 30, 60 and 100 second modes with personal best and history. Grade Calculator (Weighted + Final Exam)Weighted grade from homework, quizzes and exams (%, points or letters), letter grade and GPA, and the final exam score you need. Cron Job Generator for cPanel & LaravelBuild or explain a cron expression, see the next run times, and copy the exact cPanel line for the Laravel scheduler or a queue worker. Bcrypt Hash Generator & Laravel APP_KEYCreate a PHP/Laravel compatible bcrypt hash, check a password against a hash, or generate a new APP_KEY.