Quick answer: weight = volume × density. Metric makes it trivial — 1 litre of water = 1 kg, and any material's density in g/cm³ is directly its weight in kg per litre. In imperial, multiply cubic feet by pounds per cubic foot.
Density reference table
Densities in g/cm³ (equivalently kg per litre) and lb per cubic foot:
| Material | g/cm³ | lb/ft³ | lb per US gal |
|---|---|---|---|
| Liquids | |||
| Petrol / gasoline | 0.75 | 46.8 | 6.26 |
| Ethanol | 0.789 | 49.2 | 6.58 |
| Diesel | 0.85 | 53.1 | 7.09 |
| Motor oil | 0.90 | 56.2 | 7.51 |
| Water (fresh) | 1.00 | 62.4 | 8.34 |
| Seawater | 1.025 | 64.0 | 8.55 |
| Milk | 1.03 | 64.3 | 8.59 |
| Honey | 1.42 | 88.6 | 11.84 |
| Metals | |||
| Aluminium | 2.70 | 168.5 | 22.5 |
| Titanium | 4.51 | 281.5 | 37.6 |
| Steel | 7.85 | 490.1 | 65.5 |
| Brass | 8.50 | 530.6 | 70.9 |
| Copper | 8.96 | 559.3 | 74.7 |
| Lead | 11.34 | 707.9 | 94.6 |
| Gold | 19.32 | 1,206 | 161.2 |
| Building materials | |||
| Pine (softwood) | 0.50 | 31.2 | 4.17 |
| Oak (hardwood) | 0.75 | 46.8 | 6.26 |
| Ice | 0.917 | 57.2 | 7.65 |
| Brick | 1.92 | 119.9 | 16.0 |
| Concrete | 2.40 | 149.8 | 20.0 |
| Glass | 2.50 | 156.1 | 20.9 |
| Loose & bulk | |||
| Bark mulch | 0.40 | 25.0 | 3.34 |
| Topsoil (screened) | 1.28 | 80.0 | 10.7 |
| Sand (dry) | 1.60 | 100.0 | 13.4 |
| Gravel (loose) | 1.60 | 100.0 | 13.4 |
| Foods & dry goods | |||
| Flour | 0.59 | 36.8 | 4.92 |
| Rice (uncooked) | 0.85 | 53.1 | 7.09 |
| Granulated sugar | 0.85 | 53.1 | 7.09 |
| Table salt | 1.20 | 74.9 | 10.0 |
Two entries deserve a second look. Ice at 0.917 is less dense than water, which is why it floats and why frozen pipes burst — water is one of very few substances that expands on freezing. And gold at 19.32 is nearly 2.5 times denser than steel, which is why a gold bar that fits in your hand weighs more than you expect.
Solid density versus bulk density
The most common mistake with this conversion is using the wrong density for loose material. Solid limestone rock is about 165 lb/ft³, but crushed limestone delivered to your driveway is around 100 — the difference is air between the pieces. The same applies to sand, soil, mulch, grain and any powder.
Bulk densities also shift with moisture and compaction: damp sand can be 20% heavier than dry, and a compacted load weighs more per unit of finished volume than the loose material you ordered. For construction ordering, the construction calculators use loose delivered figures throughout.
Worked examples
Steel bar stock
A 2-inch square steel bar, 6 feet long: 2 × 2 × 72 = 288 in³ ÷ 1,728 = 0.167 ft³. At 490 lb/ft³ that is 81.7 lb — which is why steel is priced and shipped by weight, not length.
A tank of diesel
A 500-gallon tank filled with diesel at 7.09 lb per gallon holds about 3,547 lb of fuel, over one and a half tons. The tank's supporting structure has to carry that, not just the empty vessel.
Water in an aquarium
A 75-gallon tank holds 75 × 8.34 = 626 lb of water alone, before glass, substrate and rock. Add roughly 25% for those and you are near 800 lb on a stand's footprint.
Frequently asked questions
How do I convert volume to weight?
Multiply the volume by the material's density: weight = volume × density. In metric this is effortless — a litre of water at 1.0 g/cm³ weighs 1 kg. In imperial, multiply cubic feet by pounds per cubic foot: 10 ft³ of concrete at 150 lb/ft³ weighs 1,500 lb.
What is density?
Density is mass per unit volume — how much matter is packed into a given space. It is what makes a cubic foot of lead weigh 708 pounds while a cubic foot of pine weighs 31. Density is also why some things float: anything less dense than water, below 1.0 g/cm³, floats in it.
Why do fluid ounces and ounces differ?
A fluid ounce measures volume and an ounce measures weight. They happen to coincide for water, where one fluid ounce weighs about one ounce, but they diverge for everything else. Eight fluid ounces of honey weighs about 12 ounces because honey is 42 percent denser than water.
Does bulk material density differ from solid density?
Very much so. Solid limestone is about 165 lb per cubic foot, but crushed limestone delivered loose is only around 100 because of the air gaps between the pieces. Always use the bulk or loose density for aggregates, sand and soil, not the density of the parent rock.
How does temperature affect density?
Most materials expand as they warm and so become slightly less dense. Water is densest at 4 °C, and near boiling it is about 4 percent lighter per unit volume. For everyday calculations the effect is negligible, but it matters for fuel measurement and laboratory work, where volumes are usually corrected to a standard temperature.