The formula
weight of one sheet = area in m² × paper weight in g/m²
Where it comes from
The g/m² figure is literally what a square metre of that paper weighs: 80 g/m² means 80 grams per square metre. Since an A4 sheet is 0.210 × 0.297 metres, it covers 0.06237 m², and multiplying gives 4.99 grams. Hence the rule of thumb that a sheet weighs five grams.
How to work it out by hand
- Work out the area of the sheet in square metres
- Multiply it by the paper weight to get the weight of one sheet
- Multiply by the number of sheets
- Add the weight of the envelope
What is worth knowing
The ISO system has a very handy property for these sums: each size is half the one before, so two A5 sheets weigh what one A4 does and two A4 what one A3 does. On postage, the first band for a standard letter usually runs to 20 grams, which on 80 g/m² paper is three sheets and an ordinary envelope. The fourth sheet is normally what pushes it into the next band.