Why a Cup of Oil Weighs Less Than a Cup of Water (and How to Measure Liquids)
Why a cup of oil, milk, water and honey all weigh different amounts, how fluid ounces differ from weight ounces, and the right way to measure liquids in cups or on a scale.
"1 cup of oil" and "1 cup of water" take up exactly the same space, but they do not weigh the same. If you have ever converted a liquid to grams and been surprised that the number was not what you expected, this guide explains why, and shows how to get the right amount whether you use a measuring cup or a scale.
Volume is space; grams are weight
A US cup is a measure of volume: about 237 ml of space. Grams measure how much matter fills that space. Water is the reference point, because the metric system was built around it: 1 ml of water weighs about 1 g. So a cup of water is about 237 g.
Other liquids are denser or lighter than water. Oil is lighter, which is why a cup of vegetable oil is 218 g, about 8% less than water. Honey is much denser, so a cup of honey is 340 g, about 43% more. Same cup, different contents.
A cup of common liquids, in grams
| Liquid (1 cup) | Grams | Compared with water |
|---|---|---|
| Vegetable oil | 218 g | Lighter |
| Water | 237 g | Reference |
| Heavy cream | 238 g | About the same |
| Sour cream | 240 g | Slightly heavier |
| Milk | 245 g | Slightly heavier |
| Greek yogurt | 245 g | Slightly heavier |
| Maple syrup | 315 g | Much heavier |
| Honey | 340 g | Much heavier |
Milk is a little heavier than water because of the proteins, sugars and minerals dissolved in it. Cream is a mix of fat, which is lighter than water, and water, so it lands close to water. Oil is nearly all fat, which is why it comes in lightest.
Fluid ounces are not the same as ounces
This is the most common source of confusion. A fluid ounce measures volume: there are 8 fluid ounces in a US cup. An ounce on a scale measures weight: there are about 28.35 g in an ounce. The two happen to line up for water, where 8 fluid ounces weighs about 8.4 oz, which is close enough that people mix them up. They do not line up for anything else.
| 1 cup (8 fl oz) of | Weighs |
|---|---|
| Vegetable oil | about 7.7 oz |
| Water | about 8.4 oz |
| Milk | about 8.6 oz |
| Maple syrup | about 11.1 oz |
| Honey | about 12.0 oz |
So when a recipe asks for "8 oz of honey", check which it means. If it is a weight, that is about 227 g, and a cup of honey is a good deal more. A kitchen scale set to grams avoids the problem entirely. See choosing a kitchen scale if you are shopping for one.
Common smaller amounts
These are the amounts people search for most often, calculated from the cup weights above and rounded to the nearest gram.
| Amount | Oil | Water | Milk |
|---|---|---|---|
| 1/4 cup | 55 g | 59 g | 61 g |
| 1/3 cup | 73 g | 79 g | 82 g |
| 1/2 cup | 109 g | 119 g | 123 g |
| 2/3 cup | 145 g | 158 g | 163 g |
| 3/4 cup | 164 g | 178 g | 184 g |
| 1 cup | 218 g | 237 g | 245 g |
Because oil is lighter, a recipe that says "1/3 cup oil" needs about 73 g, not the 79 g you would use for water. The difference is small in one spoonful, but it adds up in a recipe that uses several liquids.
How to measure liquids in a cup
- Use a liquid measuring cup. This is the clear jug with a spout and markings below the rim. Dry cups are made to be filled to the brim and levelled, which is hard to do with liquid without spilling or under-filling.
- Put it on a flat surface. Holding the jug in the air tilts it and gives a false reading.
- Read it at eye level. Bend down so your eyes are level with the line. Looking from above makes the liquid seem higher than it is, so you pour too little.
- Read the bottom of the curve. Water and milk curve slightly where they meet the glass. Use the bottom of that curve as the level.
For small amounts, spoons are more accurate than a large jug. Our tablespoons and teaspoons guide has the conversions.
Thick liquids: yogurt, sour cream, honey
Thick ingredients behave more like solids in the cup. Yogurt and sour cream trap air pockets unless you press them in, and honey clings to the sides and leaves a lot behind when you tip it out. Weighing sidesteps all of that. For honey and syrups, the tricks in our guide to measuring sticky ingredients help if you must use a cup: lightly oil the cup first, or measure the oil in the recipe in the same cup beforehand, so the honey slides out.
The easiest way: weigh the liquid
Set your bowl on a scale, press tare, and pour until the display shows the grams you want. You do not have to read a line or wash a jug, and you can add several liquids to the same bowl, taring between each. To convert a recipe, find each ingredient on its chart, or follow our guide to converting any recipe to grams. For the reasons weighing wins more broadly, see why bake by weight.
A note on temperature
Liquids expand a little when heated, so a cup of hot water weighs slightly less than a cup of cold. For cooking and baking, the difference is too small to matter, and the figures here are typical room-temperature values.