Density and Specific Gravity: Converting Between Weight and Volume
October 21, 2026
Density is mass per unit volume, usually expressed in kilograms per cubic meter (kg/m³), grams per milliliter (g/mL), or pounds per cubic foot (lb/ft³). It lets you convert between the volume of a material, which is often easy to measure, and its weight, which determines shipping cost, structural load, or how much a purchase will weigh. The formula is: mass = density × volume, or volume = mass ÷ density.
Specific gravity is the ratio of a material's density to the density of water (1,000 kg/m³ or 62.4 lb/ft³ at about 4°C). It has no units. A material with a specific gravity of 2.4, like concrete, is 2.4 times as dense as water.
Skip the math and get your answer instantly:
Open the Weight Converter →Densities of Common Materials
Values vary with composition, moisture, and compaction, so use these as typical figures. Always check the supplier's data for precise work.
| Material | Density (kg/m³) | Density (lb/ft³) | Specific gravity |
|---|---|---|---|
| Water | 1,000 | 62.4 | 1.00 |
| Gasoline | 740 | 46.2 | 0.74 |
| Diesel | 850 | 53.1 | 0.85 |
| Dry sand | 1,600 | 100 | 1.6 |
| Topsoil (dry) | 1,300 | 81 | 1.3 |
| Concrete | 2,400 | 150 | 2.4 |
| Steel | 7,850 | 490 | 7.85 |
| Aluminum | 2,700 | 169 | 2.7 |
| Oak (dry) | 750 | 47 | 0.75 |
Weight From Volume
How heavy is a slab of concrete 10 feet by 10 feet and 4 inches thick? Volume = 10 × 10 × (4 ÷ 12) = 33.3 cubic feet. Weight = 33.3 × 150 = 5,000 pounds, or 2.5 tons. A truck carrying 10 cubic yards (270 cubic feet) of moist topsoil weighing 100 lb/ft³ carries 27,000 pounds, 13.5 tons, which may exceed the capacity of a small truck. For liquids, a 15-gallon gasoline tank holds 15 × 3.78541 × 0.74 = 42 kg, or 92.6 pounds, of fuel. A gallon of water weighs 8.345 pounds.
Remember that bulk materials like gravel and sand have bulk density, which includes air spaces between particles.
Volume From Weight
If you have 500 kg of sand with a density of 1,600 kg/m³, the volume is 500 ÷ 1,600 = 0.3125 cubic meters. If a recipe needs 200 grams of honey (density 1.42 g/mL), the volume is 200 ÷ 1.42 = 141 mL. For 100 pounds of diesel (53.1 lb/ft³), the volume is 1.88 cubic feet, or 14.1 gallons.
Shipping costs often depend on the greater of actual weight and dimensional weight, which is derived from volume divided by a standard density factor. A large, light box can be charged as if it were heavy.
Temperature and Moisture Effects
Densities change with temperature. Water is densest near 4°C; fuel expands as it warms, so a gallon of fuel contains slightly less mass on a hot day. Moisture raises the weight of soil and sand dramatically: wet sand may weigh 120 lb/ft³ compared with 100 lb/ft³ dry. Concrete with steel reinforcement is heavier. When estimating loads for a structure or trailer, use the heavier, wet value for safety.
Specific Gravity in Practice
Brewers measure specific gravity with a hydrometer to estimate alcohol content: a wort at 1.050 fermented to 1.010 yields about 5.25% alcohol by volume, using the formula ABV ≈ (OG − FG) × 131.25. Mechanics check battery acid and antifreeze with hydrometers. Geologists use specific gravity to identify minerals. In each case, the idea is the same: compare density to water.
Frequently Asked Questions
What is the formula for density?
Density = mass ÷ volume.
What is specific gravity?
The ratio of a substance's density to that of water, with no units.
How much does a gallon of gasoline weigh?
About 6.2 pounds (2.8 kg).
How much does a cubic foot of concrete weigh?
About 150 pounds.
Why do wet materials weigh more?
Water fills the spaces between particles and adds its own mass.