Supply notes Minimal
Kraft Cardstock GSM Guide
Weighing twelve sheets sold as kraft and finding that the nominal weight was wrong on five of them — by as much as 31%. What that means for hinges, pockets and page bulk.
6 layers 6 min
Weigh, do not trust the label
Cut a 100 x 100 mm square, weigh it to 0.01 g, and multiply by 100. That is the real gsm. Of the twelve sheets tested, five were off by more than 10% and the worst was 31% heavier than its label. Kraft is sold by thickness class rather than by measured weight in most retail channels, and recycled content varies batch to batch, so the same product code can arrive 25% different two months later.
Weight and thickness are not the same thing
A 216 gsm virgin kraft measured 0.28 mm; a 200 gsm recycled kraft measured 0.31 mm. The recycled sheet is lighter and thicker, because recycled fibre bulks up. For hinges and pockets this matters more than weight: what you are fighting is bulk, and the lighter-looking sheet is the one that adds more spine width.
The bulk budget
A 200-page journal has about 100 sheets. Every 0.1 mm added per spread is 10 mm added to the spine. Adding a 0.31 mm kraft backing strip to twenty spreads costs 6.2 mm; adding a 0.28 mm one costs 5.6 mm. Decide the backing stock before you design the book, not after the spine has already stopped closing.
Tearing kraft for a torn edge
Kraft is a long-fibre sheet and tears in a wide wander by hand, up to 11 mm on 110 gsm in our testing. If you want the torn look, hand-tear and accept it. If you want a controlled edge, damp a line with water and wait 25 seconds; that brings the wander down to about 2 mm, which is the tightest any wet-tear method achieved in the test.
Assembly order
- 1Cut a 100 x 100 mm sample from the middle of each sheet, not the edge.
- 2Weigh to 0.01 g and multiply by 100 to get gsm.
- 3Measure thickness at three points with the micrometer and average.
- 4Tear each sample by hand and against a rule, and record the wander.
- 5Write the measured gsm on the sheet edge in pencil before it goes into storage.
About 6 minutes of bench time, plus whatever the adhesive needs.
Supplies
- Twelve kraft sheets nominal 80-300 gsm
- Digital scale 0.01 g resolution
- Steel rule and knife for 100 x 100 mm samples
- Micrometer 0.01 mm, flat anvil
Twelve sheets, label vs measured
| Nominal | Measured gsm | Thickness | Error |
|---|---|---|---|
| 80 gsm | 79 | 0.11 mm | -1% |
| 110 gsm | 142 | 0.19 mm | +29% |
| 160 gsm | 158 | 0.22 mm | -1% |
| 200 gsm recycled | 205 | 0.31 mm | +3% |
| 216 gsm virgin | 216 | 0.28 mm | 0% |
| 300 gsm | 393 | 0.42 mm | +31% |
Where this goes wrong
Kraft sheets are often calendered more on the felt side than the wire side, so the two faces have different surface energy and glue behaves differently on each. Test both faces before deciding which one is the front.
