Convert ounces (yarn/fiber weight) to grams.
1 oz = 28.35 g.
Yarn and fiber are sold by weight, and the weight is shown in ounces in some countries and in grams in others. One ounce is about 28.35 grams, so you multiply the ounces by 28.35 to get grams. A skein of 3.5 ounces is about 99 grams, which is the same as the 100 gram skeins sold in many places.
A small kitchen scale that reads in both ounces and grams is useful for crafts. Weigh your leftover yarn to see how much you have left, or weigh a new ball and your project as you go to see how much a pattern uses.
Do not confuse ounces of weight with fluid ounces, which measure volume. The ounces here are ounces of weight, as used for yarn, fiber, and fabric. Check which one a recipe or pattern means before you convert.
Round the answer sensibly. Yarn weights are approximate, since balls vary a little, so rounding to the nearest gram is plenty. When you are matching a pattern that gives weights in grams, buy whole balls and keep the extra for later.
Handy conversions: 1 ounce is 28.35 grams, 2 ounces is 56.7 grams, 3.5 ounces is 99.2 grams, 8 ounces is 226.8 grams, and 16 ounces, one pound, is 453.6 grams. A quick estimate is to multiply the ounces by 28. Weighing a ball of yarn before and after you work shows exactly how much a project used, which is useful when you plan the next one.