What it does
Papers, lab reports and slides all want numbers written the same careful way: 1.23 × 10⁻⁴ rather than 0.000123456, and a result with its uncertainty rounded together, as (1.2 ± 0.4) × 10⁻⁴ m. This page does that for one number or a whole column, and gives you each result ready to paste:
- Unicode, for Word, PowerPoint, email and web pages: (1.2 ± 0.4) × 10⁻⁴ m.
- Plain text, for code and spreadsheets: (1.2 +/- 0.4)e-4 m.
- LaTeX maths:
$(1.2 \pm 0.4) \times 10^{-4}\,\mathrm{m}$. - siunitx (version 3):
\qty{1.2(4)e-4}{m}, which also keeps the spacing and the multiplication sign consistent through a thesis. - Concise form, as used for constants: 1.2(4) × 10⁻⁴ m, where the digits in brackets are the uncertainty in the last digits shown.
How to use it
- Numbers. Type or paste one number per line, or open a CSV or Excel file with one or two columns. An uncertainty can be written as
0.0123 ± 0.0004,12.3 +/- 0.4,1.234(12),(6.674 ± 0.015) × 10^-11, or as a second column. A unit after the numbers is kept. - Rounding. Numbers without an uncertainty are rounded to the significant figures you choose (0 keeps the figures as typed). With an uncertainty, the uncertainty is rounded first and the value to the same decimal place.
- Notation. Scientific puts one digit before the point. Engineering uses powers of ten that are multiples of three. SI prefix turns those powers into prefixes such as k, M, m, µ and n; type the unit (if it already has a prefix, such as µm or kHz, it is converted first). Plain decimal writes the number out.
- Copy. Copy any form for the first line, or a whole column for all lines, or download a CSV with every form.
How many figures on the uncertainty?
An uncertainty is itself only known roughly, so it is quoted to one or two significant figures, and the value to match. The PDG rule (the default) looks at the uncertainty’s first three digits: 100 to 354 keep two figures, 355 to 949 keep one, and 950 to 999 round up to 1000 and keep two. So 0.0119 becomes 0.012, 0.0827 becomes 0.08 and 0.0980 becomes 0.10. You can also choose two figures when the uncertainty starts with 1 or 2 and one otherwise (a common textbook rule), or always one or always two.
Worked example
Press Use the example. The first line, 0.000123456 ± 0.0000356 m, has an uncertainty starting 356, so it keeps one figure, 0.00004, and the value is rounded to the same place: (1.2 ± 0.4) × 10⁻⁴ m. With SI prefix the same line reads (120 ± 40) µm and \qty{120(40)}{\micro m}. The third line, 9.80665 ± 0.0951, starts 951, so the uncertainty rounds up to 0.10 and the result is 9.81 ± 0.10. The Avogadro constant, 6.02214076e23 to three figures, is 6.02 × 10²³.
FAQ
Why is 1.005 rounded to 1.01, when my spreadsheet says 1.00? The page rounds the number as you typed it, digit by digit, so a final 5 always rounds up (away from zero). Programs that store numbers in binary see 1.005 as 1.00499999… and round down.
Why were zeros added to my value? If the value has fewer decimals than the rounded uncertainty (1 ± 0.1), the page writes 1.00 ± 0.10 so both end at the same place, and says so.
Does siunitx need a setting? No. By default siunitx prints 1.2(4) × 10⁻⁴; add \sisetup{uncertainty-mode = separate} to print 1.2 ± 0.4 instead.
Is anything sent anywhere? No. Everything runs on this page and works offline.
Where the numbers come from
- Rounding and formatting are written by fibub, rounding digit by digit with halves away from zero. The uncertainty rule follows the Particle Data Group's published rounding rule (Review of Particle Physics, introduction); SI prefixes follow the SI Brochure (BIPM, 9th edition, with the 2022 prefixes).
- Checked in the tests against the Python packages uncertainties 3.2.3 (BSD licence) and quantiphy 2.23 (MIT licence) on 2,000 random values, and by typesetting the siunitx output with siunitx 3.3.
- The example lines are the defining values of the Avogadro constant and the speed of light, standard gravity, and made-up measurements.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.