Materials · Guide
Choose paper weight by how much water you use. 300gsm is the useful starting point for most work; lighter paper asks for more restraint, while 640gsm suits very wet work or large paintings.
5 min read
Paper weight is usually written in gsm: grams per square metre. It is not a quality grade and it does not tell you whether a paper is cotton, rough or highly absorbent. It tells you, broadly, how substantial the sheet is and how much dimensional stability it brings when water arrives.
Weight has a great deal to do with how stable your paper stays. Brand mostly decides how it feels once it does. For most painters, 300gsm is the sensible default. It is substantial enough for ordinary washes and repeated painting, without the cost and bulk of the heaviest sheets. If you work very dry, 190 or 200gsm can be perfectly useful. If you work on a large scale and very wet, 640gsm can remove some preparation and movement from the equation. Neither choice is an automatic rule.
The short answer
Choose 190/200gsm for sketches, studies and relatively dry work. Restrain or stretch it before sustained wet washes.
Choose 300gsm for most painting: it is the practical default for washes, glazing and ordinary reworking.
Consider 425gsm if you prefer a stiffer sheet or work wetter than usual without wanting the cost of 640gsm.
Choose 640gsm for very wet work, large paintings or repeatedly worked paintings where cost and bulk are acceptable.
Use a block for added restraint, not a promise of perfect flatness. It often reduces cockling during moderate wash work, but water load and the paper itself still matter.
What paper weight actually changes
When a sheet absorbs water, its fibres swell. They contract again as it dries. If the movement is uneven, the surface rises and falls: the familiar cockling that makes a flat wash drift into shallow pools.
A heavier sheet has more resistance to that movement, which is why weight is a useful first decision for broad washes, wet-in-wet passages or repeated glazing. The amount of water, the size of the sheet, how it is held down, its fibre composition, sizing and manufacture all affect the result alongside it.
Absorbency is a separate question again. Sizing and surface construction determine how quickly a wash sinks in or stays open on the surface, so a heavier sheet may feel more robust without being thirstier, more forgiving or better made.
190gsm and 200gsm: light paper for light work
Light paper is economical, portable and pleasant for drawing-led studies, colour testing and controlled work with limited water. It is not defective paper. It simply has less reserve when the painting becomes wet.
For a loose sheet, tape or stretch it before a sustained wash. If you prefer to work without preparation, accept that a wet passage may leave some movement behind. A guide to stretching watercolour paper covers the method; this page is concerned with the earlier decision of whether the paper weight makes that preparation worthwhile.
Light paper can also be useful when a study is meant to stay a study. There is no virtue in making every small experiment on an expensive, heavy sheet.
300gsm: the practical default
300gsm, also commonly sold as 140lb, is the weight most painters can use with confidence for ordinary watercolour work. It gives the sheet noticeably more stability than 190 or 200gsm, while remaining available in pads, blocks and full sheets at manageable cost.
It is the right place to start if you are learning wet-in-wet, building glazes or working with a moderate amount of water. A very wet wash, a large unsupported sheet or repeated soaking can still move even 300gsm paper; what changes is the margin, since most techniques stay well within it before the movement becomes disruptive.
For a first purchase, the beginner paper guide narrows the decision to a few appropriate papers and formats. Once you know the weight, choose the format according to your working method rather than assuming a block is always necessary.
425gsm: a preference rather than a requirement
425gsm occupies a quiet middle ground. It feels stiffer and more deliberate than 300gsm, while avoiding the price and weight of 640gsm. Some painters prefer it for larger sheets, broad washes or a working rhythm that involves more water.
Few painters need to take this step. If 300gsm already stays acceptably flat under your usual technique, 425gsm is a pleasure rather than a solution. If it does not, first consider whether the paper is restrained, how much water is being used and whether a block or stretching would answer the problem more economically.
640gsm: for very wet work or large paintings
640gsm, often described as 300lb paper, is thick enough to remain notably stable under demanding work. It makes sense for large paintings, generous washes and painters who want to return repeatedly to a wet surface without preparing a lighter loose sheet first.
The trade-off is simple: it costs more, takes up more space and is unnecessary for many small or moderately wet paintings. It is best chosen because the work asks for it, not because a heavier number seems inherently more professional.
Weight, blocks and stretching
Weight, format and stretching solve related but separate problems. A heavier loose sheet has more inherent stability. A watercolour block holds the sheet at all four edges while you paint. Stretching is a preparation method that gives a loose sheet restraint before it becomes wet.
These choices can work together. A 300gsm block commonly needs less preparation for moderate wash work than a 190gsm loose sheet. The properties of the paper still matter, and stretching remains useful when the scale or water load demands it. Choose the least complicated arrangement that keeps the surface stable enough for the way you paint.
Where brand fits into the decision
Brand becomes a more useful comparison once the weight is suitable. At the same gsm, papers can differ in fibre content, sizing, surface character and consistency. Those differences affect lifting, granulation, wash behaviour and the feeling of the brush on the surface.
They are not reasons to ignore weight. They are the next questions. If you are deciding whether a cotton sheet earns its additional cost, read cotton versus wood pulp watercolour paper. If you are deciding between formats, read blocks, pads and sheets. For supplier choice after the paper decision is made, use the guide to buying watercolour paper in the UK.
The practical verdict
Start with 300gsm unless you have a clear reason not to. Choose lighter paper for lighter, more economical work and be ready to restrain it when the washes become wet. Choose 640gsm when the scale or water load genuinely asks for a more stable surface.
There is no universal point at which paper cannot cockle. A suitable weight simply gives you more control over the conditions in which it does.