# Maths - Quantitative units

When writing and working with very large or very small numbers, we use .

Standard form shows the size of numbers as powers of ten.

Standard from numbers are written as:

A × 10n

where: A is a number greater than 1 but less than 10 and n is the index or power.

We use standard form with:

UseExample
Large numbersA population of 120,000,000 microorganisms could be written as 1.2 × 108
Small numbersA red blood cell's diameter of 7 μm or 0.000007 m could be written as 7 × 10−6 m
CalculationsWhen multiplying: add powers. When dividing: subtract powers.
Four small numbers you need to know:
1. milli (10−3) 0.001
2. micro (10−6) 0.000001
3. nano (10−9) 0.000000001
4. pico (10−12) 0.000000000001