Why Milliliters and Grams Aren't Always a 1:1 Match

If you have ever baked a cake or followed a scientific formula, you might have asked yourself: “Is 100 mL the same as 100 grams?”

The answer is: only if you are measuring water. For almost any other liquid or dry substance, 100 mL will weigh either more or less than 100 grams. To understand why, we need to look at the physics of measurement.

Volume vs. Mass: The Basics

To understand the difference, let’s define the terms:

• Volume (measured in milliliters, liters, cups, fluid ounces): This is a measure of how much three-dimensional space an object or substance occupies.
• Mass (measured in grams, kilograms, ounces): This is a measure of the amount of matter in an object, which we experience as weight under gravity.

Space and matter are not the same thing. You can have a very large space occupied by very little matter (like a cup of cotton candy) or a very small space packed with a lot of matter (like a gold coin).

The Water Baseline

Why does 1 mL of water equal 1 gram? This isn’t a coincidence; it was designed that way.

When the metric system was established in France in the late 18th century, the scientists wanted to unify all units of measurement. They defined the gram as the absolute mass of one cubic centimeter (which is equal to 1 milliliter) of pure water at the temperature of melting ice (4°C).

Because of this definition: $$\text{Density of Water} = 1.00\text{ g/mL}$$

Thus, for water (and liquids with a nearly identical density, like some vinegars or light broths), you can swap mL and grams on a 1:1 basis.

The Role of Density

For everything else, we must introduce density. Density is a measure of how tightly packed the molecules of a substance are. It is defined as mass per unit volume:

$$\text{Density} = \frac{\text{Mass}}{\text{Volume}}$$

If a substance is denser than water, its molecules are packed more tightly. A milliliter of it will weigh more than 1 gram.

• Honey has a density of about 1.42 g/mL. Therefore, 100 mL of honey weighs 142 grams.
• Milk has a density of 1.03 g/mL (fat and protein molecules make it slightly heavier than water). 100 mL of milk weighs 103 grams.

If a substance is less dense than water, its molecules are spread out or mixed with air. A milliliter of it will weigh less than 1 gram.

• Cooking Oil has a density of about 0.92 g/mL. 100 mL of cooking oil weighs only 92 grams. This is also why oil floats on top of water!
• All-Purpose Flour has a density of about 0.53 g/mL. 100 mL of flour weighs just 53 grams.

Conclusion

Next time you are converting milliliters to grams, remember: look up the density of your specific ingredient first. Swapping them 1:1 works for water, but for honey, oil, or flour, it will throw your measurements completely off!