Article — Density Converter - kg/m³, g/cm³, lb/ft³
The Density Converter
Density (ρ) is mass divided by volume. The SI unit is kg/m³; the most common alternative is g/cm³, which equals kg/L and g/mL. The density of water is 1,000 kg/m³ = 1 g/cm³. To convert kg/m³ to g/cm³ divide by 1,000. To convert kg/m³ to lb/ft³ multiply by 0.06243. Gold is 19,300 kg/m³, lead 11,340, mercury 13,546, aluminium 2,700.
Density is one of the few measurements in physics that crosses every discipline. Engineers use it for structural loads. Chemists use it for solution preparation. Geologists use it to classify rocks. Cooks use it (implicitly) when a recipe says to whisk cream until stiff or to pour oil over vinegar. The density of any solid, liquid, or gas at a given temperature and pressure is a fundamental material property — and converting between density units is one of the first things you need to do when working across systems.
What is density?
Density is the ratio of an object's mass to its volume: ρ = m / V. In words, it tells you how much matter is packed into a given amount of space. A cubic centimetre of gold weighs 19.3 grams; a cubic centimetre of water weighs 1 gram; a cubic centimetre of air weighs about 1.2 milligrams. Same volume, very different masses, and the difference is entirely captured by density.
Density depends on temperature and pressure, especially for gases. For solids and liquids the temperature dependence is usually small (a few percent across normal lab conditions) but matters in precision work. For gases it is large — air at 0°C is roughly 7% denser than air at 20°C. The pressure dependence is usually negligible for solids and liquids but significant for gases, where doubling pressure doubles density at constant temperature.
Osmium (22,590 kg/m³) and iridium (22,560 kg/m³) are the densest naturally occurring elements on Earth. The two values are so close that for decades scientists could not agree which was higher. Modern X-ray crystallography puts osmium just ahead, by about 0.1%.
Density units — SI vs imperial
The two unit systems use very different building blocks. SI builds density from kilograms and cubic metres; US customary uses pounds and cubic feet (or gallons, or cubic inches). The unit names are unrelated, but every value translates to every other through fixed multiplicative factors.
g/cm³ × 1,000kg/L × 1,000g/mL × 1,000lb/ft³ × 16.0185lb/US gal × 119.826oz/in³ × 1,729.994SI dominates scientific work because it lines up with everything else metric. g/cm³ is more popular than kg/m³ in chemistry because water comes out to exactly 1 g/cm³ — a convenient anchor. The imperial units appear mostly in US construction, freight, and industrial materials specifications.
kg/m³ to g/cm³ and other metric conversions
The metric-to-metric conversions are exact powers of ten. To go from kg/m³ to g/cm³, divide by 1,000. To go back, multiply by 1,000. The factor is the same because grams are 1/1,000 of kilograms and cubic centimetres are 1/1,000,000 of cubic metres — the ratio cancels to exactly 1,000.
g/cm³, kg/L, and g/mL all give the same numerical value for the same density. They are written differently because different fields prefer different volume units (litres in chemistry, millilitres in pharmacy, cubic centimetres in physics), but the underlying ratio is identical.
lb/ft³, lb/gal, and oz/in³ conversions
The US customary density units carry irrational-looking factors because pound and foot are not metric-derived. The exact factor between kg/m³ and lb/ft³ is built from the definition of the international pound (0.45359237 kg) and the international foot (0.3048 m), giving 0.45359237 / 0.3048³ = 16.0185… kg/m³ per lb/ft³.
lb/gal also depends on which gallon. The US gallon (3.785411784 L) gives the 119.826 factor. The Imperial gallon (4.54609 L) gives 99.776 instead. Both calculators on the web sometimes mix them up, so when you see a lb/gal figure in a datasheet, check the country of origin or the unit's full name.
- 1 lb/ft³ = 16.0185 kg/m³
- 1 lb/US gal = 119.826 kg/m³
- 1 lb/UK gal = 99.776 kg/m³
- 1 oz/in³ = 1,729.994 kg/m³
- water in US units = 62.43 lb/ft³ = 8.345 lb/gal
- steel in US units ≈ 490 lb/ft³
Density of water as the reference point
Water is the universal density anchor. At 4°C — its temperature of maximum density — pure water has a density of exactly 1,000 kg/m³ (1 g/cm³, 1 kg/L). The choice was deliberate: when the kilogram and metre were defined by the French in the 1790s, the kilogram was tied to a litre of water at this temperature, making water's density unit-by-design.
Specific gravity (also called relative density) measures density relative to water. A material with specific gravity 2.7 (aluminium) is 2.7 times denser than water. Specific gravity is dimensionless — convenient for quick comparisons. Most density tables in chemistry and metallurgy give both an absolute kg/m³ value and the specific gravity.
If you see a density value but no unit, guess g/cm³ first. Most chemistry and materials sources default to that. A value of 7.85 is steel; 1.0 is water; 13.5 is mercury; 19.3 is gold. None of those are kg/m³ values — those would be ×1,000 larger.
Common material densities
A small set of densities covers most engineering and science use cases. Memorising the top eight or ten anchors makes fast sanity checks possible:
- Hydrogen (gas) 0.0899 kg/m³ — the lightest substance
- Air (sea level) 1.225 kg/m³
- Cork 240 kg/m³ — floats easily
- Wood (pine to oak) 500–750 kg/m³
- Ice 917 kg/m³ — less dense than water
- Water 1,000 kg/m³ — the anchor
- Concrete 2,400 kg/m³
- Aluminium 2,700 kg/m³
- Steel 7,850 kg/m³
- Copper 8,960 kg/m³
- Lead 11,340 kg/m³
- Mercury 13,546 kg/m³ — the densest liquid at room temperature
- Gold 19,300 kg/m³
- Platinum 21,450 kg/m³
- Osmium 22,590 kg/m³ — densest stable element
How density changes with temperature
For most materials, density falls as temperature rises because thermal expansion enlarges the volume while mass stays constant. A linear approximation works well within ordinary temperature ranges:
ρ(T) = ρ₀ × [1 − β(T − T₀)]
Here β is the volumetric thermal expansion coefficient. For water near 20°C, β ≈ 0.000206 per K. For aluminium, β ≈ 6.9 × 10⁻⁵ per K. For air at constant pressure, β ≈ 3.4 × 10⁻³ per K — far larger, because gases expand much more than solids.
Water is the famous exception. Below 4°C it expands as it cools, so ice at 0°C is less dense than water at 0°C. That single anomaly explains why lakes freeze top-down rather than bottom-up, why ice floats, and why aquatic life can survive winter even in cold climates.
Most density tables quote values at 20°C unless they note otherwise. Water is sometimes given at 4°C (1,000.000 kg/m³) and sometimes at 20°C (998.207 kg/m³) — a 0.2% difference that matters in pharmacy and analytical chemistry. Always check the reference temperature before substituting a tabulated value into a precise calculation.
Common density conversion mistakes
Most density errors are unit-confusion errors — mixing g/cm³ with kg/m³, or US gallons with Imperial gallons. The factors are simple; the bookkeeping is what trips people up.
- off by 1,000 — writing 7.85 kg/m³ instead of 7.85 g/cm³ for steel (true value 7,850 kg/m³)
- US vs UK gallons — lb/gal differs by 20% between the two; check which gallon the datasheet means
- g/mL vs g/cm³ — identical values, but some chemistry homework still marks them as different
- specific gravity treated as kg/m³ — specific gravity 7.85 means 7,850 kg/m³, not 7.85 kg/m³
- ignoring temperature — water at 20°C is 998 kg/m³, not 1,000; the difference matters when calibrating instruments
- mixing mass and weight — density uses mass (kg, g), not weight (newtons). On the Moon a 1 kg block still has 1 kg of mass but a different weight