How Do I Find Density of an Object? Easy 3-Step Method

If you need to know how to find density of an object, the answer is straightforward: divide the object’s mass by its volume. Density measures how much matter is packed into a given space. You can find it with a scale, a ruler or graduated cylinder, and one simple calculation. The formula is density = mass / volume.

What Density Tells You

Density describes how compact a substance is. A dense object, like a lead fishing weight, feels heavy for its size. A less dense object, like a foam packing peanut, feels light. This property helps identify materials, predict whether something will float, and solve practical problems.

Pure water has a density of about 1 gram per cubic centimeter (g/cm³) at room temperature. An object with a density lower than 1 g/cm³ floats in water. One with a higher density sinks. Encyclopaedia Britannica notes that density is a characteristic property, meaning it stays the same for a given pure substance regardless of sample size.

Step 1: Measure the Mass

Mass is the amount of matter in the object. You measure it with a balance or a digital scale. For most small objects, grams are the practical unit.

• Place the object on the scale and record the reading.
• Make sure the scale is on a flat, level surface and set to zero before you start.
• If you are measuring a liquid, first weigh the empty container, then weigh the container with the liquid. Subtract the empty weight to get the mass of the liquid alone.

A typical kitchen scale measures to the nearest gram, which is accurate enough for many purposes. For scientific work, a laboratory balance measures to fractions of a gram.

Step 2: Measure the Volume

Volume is the amount of space the object takes up. The method depends on whether the object has a regular shape or an irregular shape.

Regular-Shaped Objects

If the object is a cube, a rectangular prism, a sphere, or a cylinder, you can calculate its volume from its dimensions.

• Rectangular prism (box): Measure length, width, and height in centimeters. Multiply them: volume = length × width × height.
• Sphere: Measure the diameter, divide by 2 to get the radius, then use the formula volume = (4/3) × π × radius³.
• Cylinder: Measure the radius of the circular base and the height. Use volume = π × radius² × height.

All measurements must be in the same unit. If you measure in centimeters, the volume will be in cubic centimeters (cm³). One cubic centimeter equals one milliliter.

Irregular-Shaped Objects

For an object that does not have a simple shape, use water displacement. This method works for solids that do not dissolve in water and are denser than water (they sink).

1. Fill a graduated cylinder or narrow measuring cup with enough water to fully submerge the object.
2. Record the starting water level.
3. Gently tilt the cylinder and slide the object in. Avoid splashing.
4. Record the new water level.
5. Subtract the starting level from the final level. The difference is the object’s volume.

For example, if the water starts at 50 mL and rises to 62 mL after you add the object, the volume is 12 mL, which is 12 cm³.

For a small object that floats, you can push it under with a thin needle or use a sinker (a dense object whose volume you already know) to hold it down. Subtract the sinker’s volume from the total displaced volume.

Step 3: Calculate the Density

Now you have both numbers. Divide the mass by the volume.

Worked example:

• You have a small rock.
• Mass on the scale: 45 grams.
• Volume by water displacement: 15 mL (which is 15 cm³).
• Density = 45 g / 15 cm³ = 3 g/cm³.

The rock’s density is 3 grams per cubic centimeter. This is typical for many common rocks, which have densities between 2.5 and 3.5 g/cm³.

If you are working with liquids, the same formula applies. A 250 mL cup of cooking oil might have a mass of 220 grams. Its density would be 220 g / 250 mL = 0.88 g/mL, which is why oil floats on water.

Units Matter

Density is often expressed in grams per cubic centimeter (g/cm³) for solids and grams per milliliter (g/mL) for liquids. These two units are numerically equal: 1 g/cm³ = 1 g/mL. For larger objects, you might use kilograms per cubic meter (kg/m³). The National Institute of Standards and Technology provides standard reference data for the densities of many pure substances.

Common Mistakes to Avoid

• Mixing units. If you measure mass in kilograms and volume in cubic centimeters, the numbers will be off. Convert everything to the same unit system before dividing.
• Air bubbles. When using water displacement, air bubbles stuck to the object add extra volume. Tap the cylinder gently or use a thin wire to dislodge them.
• Water absorption. Porous objects like wood or unglazed pottery will soak up water, giving a false volume reading. Seal the object with a thin coat of clear varnish or wrap it in plastic wrap before measuring.
• Scale drift. A digital scale can drift if the battery is low. Check the zero reading between measurements.

Using Density to Identify Unknown Materials

Density is a reliable identifier because it is an intensive property. Intensive properties do not change with the amount of material. Color, melting point, and density are intensive. Mass and volume are extensive properties because they change with sample size.

If you find an unknown metal object, measuring its density can narrow down what it might be. For example, if you calculate a density around 7.9 g/cm³, the metal could be steel or iron. A density near 2.7 g/cm³ suggests aluminum. A density close to 8.9 g/cm³ points to copper or nickel. These values are well-established reference points used in materials science and recycling.

You can check your measured density against published tables. The National Institute of Standards and Technology maintains standard reference data for the densities of thousands of materials. Keep in mind that alloys and impure samples will have densities that fall between the values of their components.

Density and Buoyancy: The Float Test

Archimedes’ principle states that an object immersed in a fluid experiences an upward force equal to the weight of the fluid it displaces. This principle explains why density determines whether an object floats or sinks.

A ship made of steel floats because its hull encloses a large volume of air. The overall density of the ship (steel plus air) is less than the density of water. If the hull is breached and fills with water, the overall density increases, and the ship sinks.

You can test this at home. A solid ball of clay sinks in water. If you shape the same clay into a boat-like form, it floats. The mass stays the same, but the volume increases, lowering the overall density below 1 g/cm³. This is the same principle that allows steel ships to float despite steel itself having a density of about 7.8 g/cm³.

Key Takeaways

• Density equals mass divided by volume.
• Measure mass with a scale. Measure volume by calculation for regular shapes or by water displacement for irregular shapes.
• Use consistent units, typically grams and cubic centimeters (or milliliters).
• Density is a characteristic property that helps identify materials and predict behavior like floating or sinking.
• Avoid common errors like mixing units, ignoring air bubbles, or measuring porous objects without sealing them.