Water vs. Oil: Why Oil Floats and How Their Densities Compare

It’s one of the most famous science experiments we learn as children: mix oil and water in a glass, and no matter how hard you shake it, the oil will stubbornly rise to the top. But why does this happen? The answer comes down to a fundamental property of matter: density.

In this guide, we'll explain the exact density differences between water and various types of oil, why they separate, and how to convert their volumes to weights.

What is Density?

Density measures how much mass is packed into a specific volume. The standard metric unit for liquid density is kilograms per liter (kg/L) or grams per milliliter (g/mL).

• If a substance has a high density, it feels heavy for its size.
• If a substance has a low density, it feels light for its size.

Pure water is the baseline for the metric system. At 4°C, water has a density of exactly 1.000 kg/L. This means exactly 1 kilogram of water fits perfectly into a 1-liter container.

The Density of Oil

Unlike water, "oil" is not a single chemical. It's a broad category of non-polar liquids, ranging from the olive oil in your kitchen to the motor oil in your car. However, almost all everyday oils share one trait: they are less dense than water.

Here are the typical densities of common oils compared to water:

Because every one of these values is less than 1.000 kg/L, they are lighter than the equivalent volume of water.

Why Oil Floats on Water

When two immiscible (unmixable) liquids are poured together, the denser liquid will sink to the bottom, pushing the less dense liquid to the top. This is a principle known as buoyancy.

graph TD
    subgraph Glass
    A[Oil Layer: ~0.91 kg/L] --- B[Water Layer: 1.00 kg/L]
    style A fill:#F4D03F,stroke:#D4AC0D
    style B fill:#3498DB,stroke:#2980B9
    end

Because olive oil (0.91 kg/L) has about 9% less mass packed into the same space as water (1.00 kg/L), gravity pulls harder on the water. The water forces its way underneath the oil, lifting the oil to the surface.

Why don't they mix?

Even if you shake them vigorously, they separate again because of their chemical structures. Water molecules are polar (they have a positive and negative end, like tiny magnets), so they attract each other strongly. Oil molecules are non-polar and have no charge. The water molecules basically squeeze the oil molecules out of the way so they can stick to each other.

Converting Liters to Kilograms for Oil

Because oil is lighter than water, you can't assume that 1 liter of oil weighs 1 kilogram. If a recipe calls for 1 kg of oil, and you measure out 1 liter, you will not have enough!

To find the weight of oil from its volume, use the formula: Kilograms = Liters × Density

Example 1: Cooking You have a 5-liter jug of vegetable oil (density 0.915 kg/L). 5 L × 0.915 kg/L = 4.575 kg Your 5 liters of oil weigh just under 4.6 kilograms. (If it were water, it would weigh 5 kilograms).

Example 2: Automotive You are buying 20 liters of SAE 30 motor oil (density 0.880 kg/L) for your garage. 20 L × 0.880 kg/L = 17.6 kg

If you don't want to do the math by hand, you can use the interactive calculator below to instantly convert any volume of oil, water, or 20+ other substances into accurate weights.

For more deep-dives into liquid properties, read about why temperature affects liquid density or check out our guide on how to accurately measure cooking liquids.