When applied to countable objects, spell out numbers below 10:
- There are four large detectors at the LHC.
- The LHC construction project had three phases.
For 10 and above, use digits. Numbers up to 9999 do not take a comma. Numbers with five digits take a comma every three digits. So it’s:
One, nine, 10, 100, 1001 and 9999, but 10,000 or 564,381 or 1,057,983.
Try to avoid putting numbers at the start of a sentence, but if it’s unavoidable, spell them out.
It is permissible to spell out round numbers such as one hundred, thousand, million when they are being used to convey a rough order of magnitude rather than a precise count:
Thousands of people turned up to the event.
Quantities (numbers that apply to a unit)
Always use numerals for numbers relating to quantities (such as SI units or sums of money) or numbers on a scale, even when less than 10:
- It takes 8 minutes for sunlight to reach us
- A single dose of vaccine costs $3
- There are nine telescopes all in a straight line, set 2 kilometres apart
- A 2-kilogram meteorite was found in the garden
- The subjects gave the food a bitterness rating of 4 on a scale of 1 to 10
Avoid starting sentences with numbers attached to a unit
Large numbers and tiny decimals
For numbers above one million or below the decimal fraction 0.001 scientific notation may be more appropriate. This takes the form:
3 × 1015
Note: The multiplication symbol is not just a normal "x" on the keyboard. In HTML, it is written
× so to display the example above would require:
3 × 10<sup>15</sup>
Be careful when converting large numbers from decimal to scientific notation, ensure you count the zeroes accurately:
|Decimal notation||Scientific notation|
|6,720,000,000||6.72 × 109|
|0.000 000 007 51||7.51×10−9|