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What does gsm mean?
Grammage (gsm, g/m²) is the weight of one square metre of paper in grams: 115 gsm coated is paper that weighs 115 grams per square metre. It stays the same whatever the sheet size, which makes it the clearest way to compare papers. In the US, paper is often described by basis weight in pounds instead, which depends on a reference sheet size per paper grade; gsm avoids that confusion.
Grammage is not thickness. At the same gsm, uncoated paper stands thicker and bulkier than coated; thickness is measured separately, in microns. When working out spine width or creep, go by the paper's own caliper, not its gsm.
The formula: how much does one sheet weigh?
Sheet area (m²) × grammage (g/m²) = the weight of one sheet in grams. Convert the size from centimetres to metres and multiply:
- 70 × 100 cm = 0.7 m². 115 gsm coated: 0.7 × 115 = 80.5 g.
- 64 × 90 cm = 0.576 m². 90 gsm uncoated: 0.576 × 90 ≈ 51.8 g.
- 50 × 70 cm = 0.35 m². 350 gsm board: 0.35 × 350 = 122.5 g.
- A4 (21 × 29.7 cm) ≈ 0.0624 m². 80 gsm copy paper: about 5 g.
How many kilos in a ream?
Once you know one sheet's weight, the ream or pack is a single multiplication: sheet weight × number of sheets. A ream is usually 500 sheets; heavy board often comes in smaller packs, so read the count off your supplier's label.
- 70 × 100, 115 gsm: 500 × 80.5 g = 40,250 g, i.e. 40.25 kg.
- 64 × 90, 90 gsm: 500 × 51.84 g ≈ 25.9 kg.
- Ream price = kilos per ream × price per kilo. If the supplier quotes per ream, work backwards to the price per kilo and compare it with other papers.
The other way: how many sheets in this many kilos?
To find how many sheets are in the stockroom, or in a one-tonne order, divide the total weight by the weight of one sheet:
- 1 tonne of 70 × 100, 115 gsm coated: 1,000,000 g ÷ 80.5 g ≈ 12,422 sheets.
- 250 kg of 70 × 100, 300 gsm board: 250,000 g ÷ 210 g ≈ 1,190 sheets.
- Round down: half a sheet doesn't print.
Paper for a job: kilos come after waste
For a job, the sheet count comes first: quantity ÷ pieces per sheet, rounded up, plus waste. The kilos are worked out from that number with waste included; adding waste after the weight means multiplying the error by the grammage.
Example: 600 sheets of 70 × 100, 115 gsm come to 600 × 80.5 g = 48,300 g, about 48.3 kg of paper. Pieces per sheet and the gripper margin are covered in a separate guide.
Which gsm for which job
The right grammage depends on what the job is, how it folds and how it feels in the hand. Typical ranges, roughly — they vary by shop and by paper:
- Forms, letterheads, book text: uncoated 70–100 gsm.
- Flyers and leaflets: coated 115–170 gsm.
- Catalogue and magazine text: coated 100–150 gsm; covers 200–300 gsm.
- Business cards: 300–400 gsm board or coated.
- Boxes and packaging: board, where thickness and folding matter as much as gsm.
The same sum gives the mailing weight
The weight of a job that goes by post or courier also comes from grammage. A 16-page A4 catalogue is 8 leaves; on 128 gsm coated each leaf is about 8 g, so the catalogue without a cover is about 64 g. A heavier cover, the envelope and packing come on top. Postage goes up in weight steps, so a few grams can mean the next step up.