There are two common measurements called ounces. One is the regular (Avoirdupois) ounce, which is a measure of weight equal to 28.35 grams. The other is the fluid ounce, which is a measure of volume sued for liquids and is equal to 29.57 millilitres (also called cubic centimetres). Since a fluid ounce is a measure of volume and a gram is a measure of weight, you can't actually convert grams into fluid ounces. However, if you know the density of a liquid, you can figure out how many grams of the liquid are in a fluid ounce.
Know how many grams of pure water are in one fluid ounce. When the modern metric system was developed starting in 1790s France, the gram was defined as the mass of cubic centimetre of water. A cubic centimetre is a millilitre (abbreviated ml) when referring to liquids. Since there is exactly 1.00 gram of water in a millilitre, there are 29.57 grams of water in one fluid ounce.
Understand how density of a substance is measured. By definition, water has a density of 1.00g/ml. The density of other substances is measured with this as the standard. For example, mercury has a density of 13.534 grams/ml, so its 13.534 times as dense as water.
Determine the density of a liquid. This is very easy to do. Measure out a convenient amount of the liquid, such as 100 ml, into a container and then weigh it using a balance scale (a chemistry lab scale is perfect). It's better to use a large amount like 100 ml and then divide the results by 100 to get grams/ml because a single millilitre is so small it's hard to measure out accurately. If you already know what a liquid is, you can also look up its density in a standard table (see Resources).
Calculate the number of grams of a liquid contained in 1 fluid ounce. This is a simple multiplication exercise once you know the density of a liquid. Just multiply the density in grams/ml by 29.57. For example, ethyl alcohol has a density of 0.79 grams/ml. Multiplying 0.79 by 29.57 grams/ml gives you 24.36 grams/ml. That's the number of grams of ethyl alcohol in 1 fluid ounce.
Exercise caution when handling liquids other than water. Some, like mercury, are toxic. Others, including ethyl alcohol, are flammable. Learn and follow the appropriate safety precautions to handle any liquid.