A single sheet of ordinary A4 office paper weighs just under 5 grams. A full ream of it comes to about 2.5 kilograms. Both numbers fall straight out of the paper’s area and its gsm rating.
The formula
sheet weight in grams = area in square metres × gsm
That is what gsm means: grams per square metre. An A4 sheet is 0.0624 m², so at 80 gsm it weighs 0.0624 × 80 = 4.99 g. The reason this is so tidy is the A series itself: A0 is exactly one square metre, so an A0 sheet of 80 gsm paper weighs exactly 80 grams, and every fold halves it.
Weight of one sheet
| Size | Area | 70 gsm | 80 gsm | 90 gsm | 100 gsm | 120 gsm | 160 gsm | 200 gsm | 250 gsm | 300 gsm |
|---|---|---|---|---|---|---|---|---|---|---|
| A3 | 0.1247 m² | 8.73 g | 9.98 g | 11.2 g | 12.5 g | 15.0 g | 20.0 g | 24.9 g | 31.2 g | 37.4 g |
| A4 | 0.0624 m² | 4.37 g | 4.99 g | 5.62 g | 6.24 g | 7.49 g | 9.98 g | 12.5 g | 15.6 g | 18.7 g |
| A5 | 0.0311 m² | 2.18 g | 2.49 g | 2.80 g | 3.11 g | 3.73 g | 4.98 g | 6.22 g | 7.77 g | 9.33 g |
| A6 | 0.0155 m² | 1.09 g | 1.24 g | 1.40 g | 1.55 g | 1.86 g | 2.48 g | 3.10 g | 3.88 g | 4.65 g |
| Letter | 0.0603 m² | 4.22 g | 4.82 g | 5.43 g | 6.03 g | 7.24 g | 9.65 g | 12.1 g | 15.1 g | 18.1 g |
| Legal | 0.0768 m² | 5.38 g | 6.14 g | 6.91 g | 7.68 g | 9.22 g | 12.3 g | 15.4 g | 19.2 g | 23.0 g |
| Tabloid | 0.1206 m² | 8.44 g | 9.65 g | 10.9 g | 12.1 g | 14.5 g | 19.3 g | 24.1 g | 30.2 g | 36.2 g |
Weight of a ream
Multiply by 500. A ream of A4 at 80 gsm is 2,495 g, near enough 2.5 kg. A ream of Letter at the same weight is slightly lighter, because Letter has marginally less area than A4. If you are shipping boxes of paper, five reams to a box is the usual packing, so a box runs around 12.5 kg.
The counting units themselves are covered on sheets, quires and reams.
Why postal weight is the real question
Most people arrive at this looking for a postage threshold rather than a physics answer. The arithmetic is the same: weigh the envelope, add roughly 5 g per A4 sheet at 80 gsm, and check the result against your carrier’s bands. Three sheets and a DL envelope comfortably clear the lightest UK band; six or seven start to push it.
Card behaves very differently. A 300 gsm sheet weighs almost four times what an 80 gsm sheet does, so a handful of invitations can outweigh a thick letter.
Converting between gsm and pounds
North American paper is sold by basis weight in pounds, which is measured on a different parent sheet size for each paper type. That makes the conversion type-dependent rather than a single constant, which is why 20 lb bond and 50 lb text are the same paper.
The paper weight converter handles the type differences. The concepts are in the glossary under basis weight and caliper.
Common questions
How much does one sheet of A4 paper weigh?
4.99 g at 80 gsm, 4.37 g at 70 gsm, and 6.24 g at 100 gsm. Standard office paper is usually 80 gsm.
How much does a piece of Letter paper weigh?
About 4.83 g at 80 gsm. Letter is 0.0603 m², slightly smaller than A4.
How much does a ream of paper weigh?
Around 2.5 kg for 500 sheets of A4 at 80 gsm, or about 2.4 kg for Letter.
Does thicker paper always weigh more?
Usually, but not reliably. Bulky uncoated stock can be thicker than a coated sheet of the same gsm, because gsm measures mass and caliper measures thickness. Two sheets can share a weight and feel completely different.
More guides
- Standard paper sizeLetter or A4: which one is printer paper where you live, what it weighs, and how to set it.
- Paper size equivalentsThe nearest ISO size for every US and ARCH sheet, and the exact scale to print one on the other.
- Paper sizes in pixelsEvery size at 72, 96, 150, 200, 300 and 600 dpi, and the arithmetic behind it.
- Sheets, quires and reamsHow paper is counted, and why old books disagree about the quire.
- Portrait and landscapeThe same sheet, turned. Both orientations for every common size.
- Foolscap explainedThree different sheets share the name. Which one someone means, and why.
- Postcard sizesWhat posts as a postcard, from A6 to the US 4 × 6, with mailing limits.
- Poster sizesEvery standard poster size, what each is used for, and what to send the printer.