I have a lot of card bought from various places and I sometimes would like an indication of the gsm (gram per square metre) of the card to help me with cut settings on my Silhouette Cameo. So I sat down today to work out the formula! If you don't follow the Maths, then just believe me, and I will give you the formula at the end.
1 square metre is 100 x 100 square cm = 10,000 square cm.
A4 card = 29.7 cm x 21 cm = 623.7 square cm
10,000/623.7 = 16.03
So the weight of your A4 card x 16.03 = the gsm
If you have accurate weigh scales, you need to weigh your piece of A4 card. Mine weighed in at 14g.
14g x 16.03 = 224gsm
The weight of your A4 sheet in g, multiplied by 16.03 = your gsm
I also have an A4 sheet of Kraft card weighing 18g
18 x 16.03 = 288 gsm
For 12" x 12" card (30.5 x 30.5cm = 930.25 sq cm)
10,000/930.25 = 10.75
I weighed some card stock that is labelled 200gsm. It weighed 18g on my scales.
18 x 10.75 = 194g which is close enough.
The weight of your 12" x 12" sheet in g, multiplied by 10.75 = your gsm
