Cotton duck (from Dutch: doek, "linen canvas"), also simply duck, sometimes duck cloth or duck canvas, is a heavy, plain woven cotton fabric. Duck canvas differs from plain canvas in that the threads in the former are more tightly woven. There is also linen duck, which is less often used.
Duck is classified according to weight in a numerical system, with grade 1 the heaviest and grade 12 the lightest variety. Besides this, traditional names exist, which are rarely used today.
A numbering system is used to describe the various weights of duck cloth, based on the weight of a 36×22-inch piece. Weights below 19 ounces are called numbered duck. Those above 19 ounces are called naught duck. The grade of numbered duck refers to the number of ounces subtracted from 19 for a 36×22-inch piece of fabric. For example, a piece of #8 numbered duck with dimensions of 36"×22" weighs 11 ounces (19 − 8 = 11). 
Numbered duck is nominally made in weights from 1 to 12, but numbers 7, 9, and 11 are no longer used. Some typical uses of various grades (with weights in ounces) are:
- No. 1 (18 oz): hammocks, cots, sandbags
- No. 2 (17 oz): hatch paulins
- No. 3 (16 oz): heavy-duty bags
- No. 4 (15 oz): sea bags
- No. 5 (14 oz): heavy work clothes
- No. 6 (13 oz): large boat covers, heavy work clothes
- No. 8 (11 oz): work clothes, clothes bags
- No. 10 (9 oz): work clothes, shower curtains
- No. 12 (7 oz): light clothes
- "Tough Clothing for Tough Customers: A Guide to Workwear". Sierra Trading Post. Retrieved 30 March 2010.
- "duck canvas; duck". Sizes, Inc. 11 August 2004. Retrieved 23 June 2011.