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 &times; so to display the example above would require: 3 &times; 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
9,130,000,000,000 9.13×1012

See units.


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