300 gsm means that one square meter of the paper has a nominal mass of 300 grams. It does not mean that every sheet weighs 300 grams, and it does not specify the thickness of a sheet. Use the number to compare paper weight per area, then check the surface, composition and format separately.
Read gsm as grams per square meter
The letters stand for grams per square meter; you may also see the notation g/m². Canson distinguishes this measurement from paper thickness, which it describes in microns. Two labels can therefore tell you the mass per area without telling you that the sheets have the same thickness.
Imagine cutting a one-meter by one-meter square from a sheet with a nominal weight of 300 gsm. The label corresponds to 300 grams for that area. Cut a smaller piece and the calculated mass becomes smaller, while the gsm stays the same.
How much would an A4 sheet weigh?
An A4 sheet is 210 × 297 mm. Convert those dimensions to meters and multiply by the gsm:
0.210 m × 0.297 m × 300 g/m² = 18.711 g
That is approximately 18.7 grams of paper for one A4 sheet at the stated nominal weight.
This is arithmetic from the label, not a measurement of a particular product. Actual sheet mass can differ from the nominal calculation. A pad also includes items such as its cover and backing, so this formula does not calculate its total shipping weight.
| Paper label | Calculation | Approximate mass per sheet |
|---|---|---|
| 200 gsm | 0.210 × 0.297 × 200 | 12.5 g |
| 300 gsm | 0.210 × 0.297 × 300 | 18.7 g |
| 640 gsm | 0.210 × 0.297 × 640 | 39.9 g |
What the number does not tell you
- The surface. Check whether the specific paper is hot press, cold press or another surface. Use our hot press vs cold press guide to decide which sample to try.
- The composition. Read the manufacturer’s fiber specification rather than inferring a cotton percentage from gsm.
- The format. The number alone does not tell you whether you are buying a loose sheet, pad or block.
- Your painting result. The label cannot stand in for a trial with your paint, water amount and working method.
Does 300 gsm mean the paper cannot buckle?
No such guarantee follows from the measurement. Canson discusses 300 g/m² as a compromise weight in its watercolor guidance; it also discusses weight and how paper is supported. That is not evidence that every 300 gsm sheet will stay flat in every painting process.
When comparing products, record the sheet format and the manufacturer’s preparation guidance alongside gsm. If your main concern is what happens during a wet wash, run a small trial using your usual setup before committing a larger piece of paper to a finished work.
A short checklist before you buy
- Write down the sheet dimensions and the number of sheets so that differently sized packs are not treated as equal quantities.
- Record the gsm, surface and stated composition as separate fields.
- Check the format and any preparation instructions for the particular product.
- If you are uncertain, compare a small sample using a familiar exercise. Our printable comparison sheet has space for both the label information and your observations.
Source and calculation notes
Canson: Selecting your watercolor paper supports the definition of grammage, its distinction from thickness and the qualified discussion of weight. Sheet masses in the table are our own calculations using the stated A4 dimensions; they are not product specifications or measured results.
Source checked September 8, 2026. This is a research-based explanation, not a product test. See how these guides are made.