Density Converter
Convert density between metric and imperial units — kg/m³, g/cm³, lb/ft³ and pounds per gallon — with the US and Imperial gallon kept firmly apart and specific gravity derived.
Density Converter
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Density varies with temperature and, for gases, strongly with pressure; a single figure implies a stated reference condition.
- Specific gravity here references water at 4 °C — other disciplines reference 20 °C or 60 °F, which shifts the result slightly.
- API gravity is defined for petroleum liquids at 60 °F and is nonlinear; it is derived, not convertible.
- The float-or-sink judgement compares material density against water and ignores enclosed air, hull form and surface tension.
In short: Density conversion routes through the kilogram per cubic metre. The convenient anchor is that 1 g/cm³ = 1,000 kg/m³, which is very nearly the density of water. The trap is pounds per gallon: the US gallon is 231 in³ and the Imperial gallon is 4.54609 L, about 20% larger, so lb/gal means two different things. Water peaks at 999.972 kg/m³ at 4 °C, not exactly 1,000.
Formula
Each factor is the number of kilograms per cubic metre in one of that unit. Specific gravity is the dimensionless ratio to water at its maximum density, 999.972 kg/m³ at 4 °C. API gravity is a nonlinear rescaling of specific gravity used in the petroleum industry, which is why it is derived here rather than offered as a convertible unit.
Worked Example
- Identify the source factor. One g/cm³ is 1,000 kg/m³.
- Convert to the SI unit. 1 × 1,000 = 1,000 kg/m³.
- Identify the target factor. One lb/ft³ is 16.018463 kg/m³.
- Divide. 1,000 ÷ 16.018463 = 62.427961 lb/ft³.
- Name the gallon. The same density is 8.345 lb per US gallon but 10.022 lb per Imperial gallon.
Why water is not exactly 1,000. The litre was once defined as the volume of one kilogram of water at maximum density, which would have made water exactly 1 kg/L by construction. That definition was abandoned in 1964 in favour of defining the litre as exactly one cubic decimetre. Water at 4 °C is therefore 999.972 kg/m³ — close to, but not exactly, 1,000.
Strengths & Limits Of This Model
Where this engine is strong
- Always shows both the US and Imperial pound-per-gallon figures, never one alone.
- References specific gravity to the true 999.972 kg/m³ rather than a rounded 1,000.
- Derives API gravity properly instead of faking it as a linear unit.
- Every imperial factor is derived exactly from the defined pound, foot and inch.
Where it stops
- Cannot know the temperature or pressure at which your figure was measured.
- Does not carry material presets — it converts a density you supply.
Practical Use Cases
Fuel and chemical handling
Density governs the conversion between volume delivered and mass invoiced. Because the US and Imperial gallons differ by about twenty per cent, a lb/gal figure without a named gallon is a genuine commercial hazard.
Materials selection and structural take-offs
Steel at about 7,850 kg/m³, aluminium at 2,700 and concrete at roughly ed 2,400 drive dead-load calculations — pair with the Volume Converter to turn dimensions into mass.
Petroleum classification
API gravity sorts crude oil into light, medium and heavy grades, and the scale is deliberately inverted so that lighter, more valuable crude scores higher. Because it is nonlinear, it must be computed rather than scaled.
Laboratory and formulation work
Specific gravity is the standard dimensionless expression of density in chemistry, brewing and pharmacy, always referenced against water at a stated temperature.
Methodology & Editorial Standards
All conversions route through the kilogram per cubic metre. Imperial factors are derived exactly from the defining values: the pound per cubic foot is 0.45359237 divided by 0.3048 cubed, giving 16.018463373960138 kg/m³, and the pound per cubic inch follows the same way from the exact inch. The US gallon is exactly 231 cubic inches, or 0.003785411784 m³; the Imperial gallon is exactly 4.54609 litres as defined in 1985. Both pound-per-gallon readings are always shown together, because a lb/gal figure with an unnamed gallon is ambiguous by about twenty per cent. Specific gravity is referenced to water at its maximum density of 999.972 kg/m³ at 4 °C rather than to a rounded 1,000, since the litre has been defined as exactly one cubic decimetre since 1964 and the old water-based definition no longer holds. API gravity is computed from specific gravity by the standard petroleum formula and is deliberately not offered as a convertible unit, because the relationship is nonlinear and a linear factor would be wrong at every point but one. All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Density Converter — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How do I convert kg/m³ to lb/ft³?
Divide by 16.018463373960138. So 1,000 kg/m³, roughly the density of water, is 62.427961 lb/ft³. The factor is exact, since it is simply the exact pound divided by the exact foot cubed. Going the other way, multiply lb/ft³ by 16.018463.
Is water exactly 1,000 kg/m³?
Very nearly, but not exactly. Water reaches maximum density at about 4 °C, where it is 999.972 kg/m³. At 20 °C it is 998.207. The round figure of 1,000 comes from the original definition of the litre as the volume of one kilogram of water, which was abandoned in 1964 when the litre was redefined as exactly one cubic decimetre.
What is specific gravity?
The dimensionless ratio of a substance's density to that of a reference, almost always water. Because it is a ratio it carries no units and is the same in every measurement system, which is why it is preferred in chemistry, brewing and petroleum work. A specific gravity above one means the substance sinks in water.
Why are there two different pounds per gallon?
Because there are two different gallons. The US liquid gallon is exactly 231 cubic inches, about 3.785 litres. The Imperial gallon is exactly 4.54609 litres, roughly twenty per cent larger. Water is about 8.345 lb per US gallon but 10.022 lb per Imperial gallon, so any lb/gal figure is meaningless unless the gallon is named.
What is API gravity?
A petroleum industry scale defined as 141.5 divided by specific gravity, minus 131.5, measured at 60 °F. It is deliberately inverted so that lighter, generally more valuable crude oils score higher: water is 10 API, light crude is above about 31.1 API and heavy crude below 22.3. Because the relationship is nonlinear it cannot be applied as a simple conversion factor.
What is a slug per cubic foot?
The density unit of the US customary gravitational system, equal to 515.378818 kg/m³. It appears in aerodynamics and fluid mechanics done in imperial units, where air density at sea level is quoted as about 0.002377 slug/ft³ rather than 1.225 kg/m³.
Why does a steel ship float?
Because buoyancy depends on the average density of the whole vessel, not the density of the material it is made from. A steel hull encloses a large volume of air, and the average density of hull plus air is well below that of water. Solid steel at about 7,850 kg/m³ sinks immediately.
Does density change with temperature?
Yes, for essentially all substances, because volume changes while mass does not. Water is unusual in having a density maximum at 4 °C rather than at its freezing point, which is why ice floats and why lakes freeze from the top down. Petroleum densities are always quoted at a reference temperature for this reason.
What is the difference between density and specific weight?
Density is mass per unit volume, in kg/m³. Specific weight is force per unit volume, in N/m³, and equals density multiplied by local gravity. The distinction mirrors the one between mass and weight, and matters in fluid statics where pressure gradients depend on specific weight rather than density.
How dense are common materials?
Approximate values in kg/m³: air at sea level 1.225, cork 240, ice 917, water 1,000, concrete 2,400, aluminium 2,700, steel 7,850, lead 11,340, gold 19,300 and osmium about 22,590, the densest naturally occurring element. Dividing any of these by 1,000 gives specific gravity directly.