Plus minus sign (±) in HTML
The plus minus sign states a tolerance or an uncertainty around a value. Its named reference is ± and its code point is U+00B1.
At a glance
- Character
- ±
- Named entity
- ±
- Decimal reference
- ±
- Hex reference
- ±
- Unicode code point
- U+00B1
- Unicode name
- PLUS-MINUS SIGN
- In the entity reference
- Math, and pinned in quick access
In detail
The sign carries two related meanings that people read differently. In an engineering tolerance it defines a permitted range, so a length given as one hundred with a tolerance of two means anything between ninety eight and one hundred and two is acceptable. In a measurement it states an uncertainty, which is a statement about confidence rather than a hard boundary, and the interval usually corresponds to a stated number of standard deviations.
In algebra it means both roots at once, which is where most people first meet it in the quadratic formula. That usage produces two answers rather than a range, and reading it as a range in an equation is a genuine misunderstanding rather than a shade of meaning.
There is a reversed character, the minus plus sign at U+2213, used when two expressions must take opposite signs together. It is not in the entity reference tool's list, so write it as a numeric reference. Outside trigonometric identities it is rare enough that spelling the relationship out is usually clearer than relying on the reader to notice which way round the glyph is.
Spacing follows the convention for a binary operator when it joins two quantities: a thin space on each side reads better than either a full space or none at all. Keep the value and its tolerance on one line with a non-breaking space, because a tolerance that wraps to the next line reads as a separate figure entirely.
Examples
<p>Shaft diameter 12.00 ± 0.05 mm</p>
<p>Mean 4.7 ± 0.3 (one standard deviation)</p><p>cos(a ± b) = cos a cos b ∓ sin a sin b</p>∓ is U+2213, the minus plus sign, which takes the opposite sign to the one before it.
Things that catch people out
- Writing plus or minus as words in a data table makes the column unparseable and wider than it needs to be. The character is the standard notation.
- A tolerance without a unit is ambiguous. Repeat the unit or make it unmistakable from the surrounding column header.
- An uncertainty means nothing without a coverage statement. Say whether it is one standard deviation, two, or a confidence interval.
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The division sign at U+00F7, its entity forms, why standards bodies discourage it, and the slash and fraction characters that replace it in published maths.
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Tools for this
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