Density Converter

The density converter translates a density value from any supported unit into every other supported unit in a single pass, using kilograms per cubic metre (kg/m³) as the canonical basis. Density is the mass per unit volume of a substance, and although the SI unit is unambiguous, real-world practice uses a confusing patchwork of units: laboratory chemistry often expresses solutions in g/cm³ or g/mL, brewing and winemaking use specific gravity (dimensionless), oil and gas uses API gravity derived from density at sixty degrees Fahrenheit, and engineering in the United States routinely reports density in lb/ft³ or lb per US gallon. The Density Converter handles all of these at once and lets you read the value in every other unit at the same time, so you do not have to keep mental track of conversion factors while comparing values across disciplines.

All of the factors used by this tool are derived from the SI prefix chain and the NIST conversion tables, and they are exact to the precision shown. There is no curve fitting, no rounding to a "nice" number, and no proprietary data. The same factors appear in the BIPM SI Brochure, NIST SP 811, the CRC Handbook of Chemistry and Physics, and ISO 80000-4 (the ISO standard that defines density and related quantities). If you find a discrepancy between this tool and a reference table, the discrepancy is in the rounding the reference table chose to print, not in the underlying factor.

How to Use the Density Converter

1. Enter a numeric value in the "Value" field. The field accepts integers, decimals, and scientific notation such as "1.225e3" or "1.225E3". An empty field, a single minus sign, or any input that does not parse as a number is highlighted as invalid and the results table clears.
2. Pick the unit of the value you typed in the "From unit" dropdown. The dropdown lists all eleven supported units, with their full names and any common aliases (for example, "g/cm³ (g/mL)" so you know the cubic-centimetre and millilitre conventions give the same numeric value).
3. Pick your preferred precision from the "Significant figures" dropdown. The options are 4, 6, and 8. The default is 6, which is precise enough for laboratory work but does not carry spurious trailing digits.
4. Read the live results table. Every supported unit appears with three columns: the chosen significant-figures value, the 4-decimal-place value, and the 8-decimal-place value. The row matching your from-unit is shaded and labelled "(source)" so you can find it instantly.
5. Optionally click a material preset button to drop in a standard reference density. The presets cover water at 4 °C, ice at 0 °C, air at sea level, seawater, ethanol, mercury, aluminium, steel, gold, and lead. Each preset sets both the value and the from-unit to kg/m³ in one click, so you can immediately read off lb/ft³, g/cm³, slug/ft³, and the rest.
6. Read the canonical kg/m³ value and the specific-gravity readout in the summary bar above the table. The specific gravity is the density divided by 1000 kg/m³ (the density of water at 4 °C), and it is dimensionless.

The Unit Definitions

Density is mass per unit volume. The SI unit of density is the kilogram per cubic metre, written kg/m³ or kg·m⁻³. By the BIPM SI Brochure §2.3.3, the kilogram is the unit of mass and the metre is the unit of length, so any density in SI is expressed as mass in kilograms over volume in cubic metres. The choice of cubic metres as the volume unit (rather than litres or cubic centimetres) is deliberate: it makes water at 4 °C exactly 1000 kg/m³, a value that is easy to remember and that scales cleanly with the SI prefixes.

Why are g/cm³ and kg/L numerically identical? Both are equivalent ways of writing 1000 kg/m³, and they coincide because of how the SI prefixes work: 1 g = 0.001 kg and 1 cm³ = 0.000001 m³, so 1 g/cm³ = 0.001 kg / 0.000001 m³ = 1000 kg/m³. Similarly, 1 kg/L = 1 kg / 0.001 m³ = 1000 kg/m³. The numeric value is the same, but the unit strings are different and the choice of unit communicates which instrument or convention produced the measurement. A bench chemist working with a 25 mL volumetric flask will reach for g/cm³ because that volume unit is convenient; a fuel-blending engineer working with a 1000 L tote will reach for kg/L.

Specific gravity is a dimensionless ratio: the density of a substance divided by the density of a reference substance, almost always water at 4 °C (where water reaches its maximum density of exactly 1000 kg/m³). Because the numerator and denominator share units, the ratio has no units and can be written as a plain number. Brewing and winemaking report progress in specific gravity because the ratio is what hydrometers actually read: a hydrometer is calibrated to sink to a marked depth in water, and the depth at which it sinks in a sugar solution gives the ratio of solution density to water density directly.

The difference between the US gallon and the Imperial gallon matters here because density conversions often route through volume units. The US gallon is defined as exactly 231 cubic inches, which equals about 3.785411784 litres. The Imperial gallon is defined as exactly 4.54609 litres, which is about 1.20095 US gallons. The two are close but not interchangeable: a liquid reported at "10 lb per gallon" without specifying which gallon can be off by twenty percent. This tool uses the US gallon (the only gallon encountered in most US engineering practice); if you need Imperial gallons, the conversion factor is different and you should consult a dedicated reference.

Worked Examples

1. Water at 4 °C. The density of water at its temperature of maximum density is, by definition, exactly 1000 kg/m³. In g/cm³ that is 1.000000, in lb/ft³ about 62.4280, in lb per US gallon about 8.3454, and in slug/ft³ about 1.94032. Specific gravity is exactly 1.

2. Mercury at 25 °C. Liquid mercury at 25 °C has a density of 13 534 kg/m³, which is 13.534 g/cm³ or about 844.71 lb/ft³. Specific gravity is 13.534, which is why a small amount of mercury feels surprisingly heavy when you pick up a sealed flask. The value is highly temperature-dependent: mercury at 0 °C is about 13 595 kg/m³ and at 100 °C about 13 352 kg/m³, so the same flask weighs measurably less when hot.

3. Air at sea level. The International Standard Atmosphere defines air at 15 °C and 101.325 kPa as having a density of exactly 1.225 kg/m³. In lb/ft³ that is about 0.07647, in g/L about 1.225, and in slug/ft³ about 0.002377. Specific gravity is 0.001225, which is dimensionless but obviously tiny: a balloon full of air at sea level weighs about a tenth of a gram less than the same balloon full of vacuum would weigh, which is why hot-air balloons are filled with a gas much less dense than ambient air.

4. Steel. Plain-carbon steel at 20 °C has a density of about 7850 kg/m³, or 7.85 g/cm³. In lb/ft³ that is about 490.07, and in lb per US gallon about 65.48. Knowing the density is what lets a structural engineer calculate the mass of a steel beam from its dimensions: a 6-metre beam of W-shape with a cross-section of 100 cm² has a volume of 0.06 m³ and therefore a mass of 0.06 × 7850 = 471 kg.

5. Gold. Pure gold at 20 °C has a density of 19 300 kg/m³, or 19.3 g/cm³. In oz/in³ that is about 11.046, which is the unit jewellers and refiners sometimes use for bullion accounting. Specific gravity is 19.3, which is why a small gold bar feels extraordinarily heavy for its size and why the classic "fools' gold" (iron pyrite, density about 5.0 g/cm³) feels wrong in the hand.

6. Seawater. Surface seawater at 25 °C and a typical salinity of 35 g/kg has a density of about 1025 kg/m³, or 1.025 g/cm³. In lb/ft³ that is about 63.99, and in lb per US gallon about 8.553. The value is critical for naval architecture: a ship's displacement in long tons is essentially the volume of water it displaces times the density of that water, and seawater's slight excess over fresh water is enough to change a ship's draft by several centimetres between river and ocean.

7. Ethanol at 20 °C. Anhydrous ethanol has a density of 789 kg/m³, or 0.789 g/cm³. In lb/ft³ that is about 49.26, and in lb per US gallon about 6.586. The low density (and therefore low specific gravity, 0.789) is what makes a hydrometer float higher in a spirit than in water, and it is also why fuel ethanol blends change a fuel's volumetric energy density in predictable ways.

Where Density Conversions Show Up

Materials engineering. Density is the first number on any material data sheet, and the choice of alloy, polymer, or composite depends on the mass a part must have for a given volume. Converting a spec sheet written in g/cm³ into lb/ft³ is a daily task for design engineers who quote in US units. Steel at 7850 kg/m³ and aluminium at 2700 kg/m³ give the same structural stiffness-to-mass ratio only after a unit-aware comparison.

Shipping and freight weight limits. Air freight and ocean freight both impose weight limits per unit volume, but the limits are quoted in different units. Volumetric weight in air freight is kg per 6000 cm³, while ocean freight uses metric tonnes per cubic metre. Density in lb/ft³ or kg/m³ tells the freight forwarder instantly whether the cargo is "heavy" (mass-limited) or "light" (volume-limited), and the right shipping rate depends on that.

Brewing and winemaking. The original gravity of a wort (sugar content before fermentation) and the final gravity (after fermentation) are both reported in specific gravity, and the difference between them multiplied by an empirical factor gives the alcohol by volume. A hydrometer reads specific gravity directly, but the values are often cross-checked against a refractometer reading that needs a density correction at 20 °C.

Concrete mix design. The density of aggregates (sand, gravel, crushed stone) is what sets the mass of a cubic metre of concrete and therefore the dead load on a slab or beam. Lightweight aggregates (expanded clay, pumice) have densities below 1000 kg/m³; normal-weight aggregates are around 2600 to 2700 kg/m³; heavy aggregates (barite, magnetite) are above 3000 kg/m³ and are used for radiation shielding.

Oil and gas API gravity. The American Petroleum Institute gravity is a scale derived from density at 60 °F (15.56 °C): API = (141.5 / SG) − 131.5, where SG is the specific gravity relative to water at 60 °F. Crude oils with API above 31.1 are "light", below that are "heavy", and below 10 are "extra heavy". To use this scale you must first get the density at the reference temperature, which is exactly what a density converter with a temperature note enables.

Laboratory solutions. Concentrations in mg/L and kg/m³ are numerically identical for water-based solutions at room temperature (because 1 L of water weighs almost exactly 1 kg). For non-aqueous solvents the conversion needs the actual solvent density, which is where a material preset like "ethanol 789 kg/m³" or "mercury 13534 kg/m³" lets a bench chemist prepare a 1000 mg/L standard in the correct solvent volume.

Common Mistakes

Confusing density with specific gravity. Density has units (kg/m³ or lb/ft³); specific gravity is dimensionless. A substance with SG = 1.025 has a density of 1025 kg/m³ in SI units, but the same SG in US customary units corresponds to about 63.99 lb/ft³, not 1.025 lb/ft³. Multiplying a density by a number labelled "specific gravity" without checking the units is one of the most common sources of off-by-large-factor errors.

US vs Imperial gallon. A US gallon is 3.785411784 L; an Imperial gallon is 4.54609 L. They are about 20 % different, which means a liquid reported at "10 lb per gallon" without specifying the gallon system can be off by 20 % depending on which gallon is meant. The lb/US-gal unit in this tool is the US gallon only; never use it interchangeably with Imperial gallons.

Forgetting temperature dependence. Almost every density value is temperature-dependent. Water at 4 °C is 1000 kg/m³; at 20 °C it is 998.2 kg/m³; at 100 °C it is 958.4 kg/m³. Air at 15 °C is 1.225 kg/m³; at 25 °C it is about 1.184 kg/m³. Steel expands with heat too: its density drops by roughly 0.5 % from 20 °C to 200 °C. When a reference value is quoted, the reference temperature matters as much as the number itself.

Mixing mass and weight. Density is mass per volume, not weight per volume. On Earth, mass and weight are proportional, but the proportionality constant (g, about 9.81 m/s²) varies with location and is zero in free fall. An object with a density of 1000 kg/m³ on Earth still has a density of 1000 kg/m³ on the Moon; its weight, however, is only about one-sixth as much. Engineering tables that quote "weight density" in lb/ft³ are using a different quantity from mass density, and the difference matters anywhere the local gravity is not exactly 9.81 m/s².

Frequently Asked Questions

What is the SI unit of density? The SI unit of density is the kilogram per cubic metre, written kg/m³ or kg·m⁻³. By the BIPM SI Brochure §2.3.3, the kilogram is the SI unit of mass and the metre is the SI unit of length, so density in SI is mass in kilograms over volume in cubic metres. Water at 4 °C is exactly 1000 kg/m³ by definition, which makes the SI density of water a memorable round number. Every other density unit in this tool converts to kg/m³ first, so the SI value is always available in the canonical readout above the results table.

What is specific gravity, and how does it relate to density? Specific gravity is the ratio of a substance's density to the density of a reference substance, almost always water at 4 °C (where water reaches its maximum density of 1000 kg/m³). Because both densities share units, the ratio is dimensionless. A substance with specific gravity 1.025 has a density of 1025 kg/m³ in SI units, but the same number "1.025" in US customary units corresponds to about 63.99 lb/ft³, not 1.025 lb/ft³. The Density Converter shows the specific gravity alongside the canonical kg/m³ value so you can read it directly.

Why are g/cm³ and kg/L numerically the same? Both g/cm³ and kg/L equal 1000 kg/m³, which is why the numeric value is identical in either unit. The reason is the SI prefix chain: 1 g = 0.001 kg and 1 cm³ = 0.000001 m³, so 1 g/cm³ = 1000 kg/m³. Similarly, 1 kg/L = 1000 kg/m³ because 1 L = 0.001 m³. The unit strings differ because they describe different physical measurements (a chemist with a 25 mL flask reaches for g/cm³; a fuel engineer with a 1000 L tote reaches for kg/L), but the numeric value is the same.

Does density change with temperature? Yes, density is strongly temperature-dependent for almost every substance. Water at 4 °C is 1000 kg/m³, at 20 °C is 998.2 kg/m³, and at 100 °C is 958.4 kg/m³. Air at 15 °C is 1.225 kg/m³, at 25 °C is about 1.184 kg/m³. Steel, aluminium, mercury, ethanol, and every other material in the presets row shifts measurably with temperature. The reference temperature of each preset is shown in the button label (water at 4 °C, ice at 0 °C, mercury at 25 °C, and so on), and any laboratory measurement at a different temperature should be reported at the temperature it was actually taken.

What is the difference between a US gallon and an Imperial gallon? A US gallon is exactly 231 cubic inches, which equals about 3.785411784 L. An Imperial gallon (used in the United Kingdom and some Commonwealth countries) is exactly 4.54609 L, which is about 1.20095 US gallons. The two differ by roughly twenty percent, which is a large enough error to be visible on any engineering calculation. The lb/US-gal unit in this tool uses the US gallon only, and you should never use it interchangeably with Imperial gallons without conversion.

What is the slug/ft³ unit used for? The slug is a unit of mass in the US customary system that gives the convenient property that weight in pounds-force equals mass in slugs multiplied by the local gravity in ft/s² (about 32.174). In density calculations, slug/ft³ is therefore the "natural" US-customary density unit when you want to multiply by volume and get a force. The conversion factor is exact: 1 slug/ft³ = 515.3788183932961 kg/m³, derived from 1 slug = 32.1740485564 lb (exactly) and 1 ft³ = 0.028316846592 m³ (exactly).

How accurate are the conversions? The conversion factors used by this tool are exact to the precision shown. They come from the SI prefix chain (g, kg, mg, t, all exact) and from the NIST SP 811 conversion tables (lb, oz, slug, all exact to the listed number of significant figures). A discrepancy between this tool and a printed table is therefore always a rounding decision in the printed table, not an error in the underlying factor. The results table offers three precisions (4/6/8 significant figures) so you can pick the precision that matches your downstream use without carrying spurious trailing digits.

Is this the same as API gravity? No. API gravity is a derived scale specific to the petroleum industry: API = (141.5 / SG) − 131.5, where SG is the specific gravity at 60 °F (15.56 °C). A Density Converter gives you the specific gravity at any reference temperature; you can then plug it into the API formula if you need the API value. Crude oils with API above 31.1 are classified as "light" and below that as "heavy". A density converter does not compute API directly, but it is the necessary first step.

References

BIPM SI Brochure, 9th edition (2019), §2.3.3, definition of the kilogram, the metre, and units derived from them. The SI Brochure is the authoritative statement of how SI units are defined and how they relate to other units; density in SI is kilograms per cubic metre by direct derivation. The 9th edition is the current version at the time of writing.

NIST SP 811, Guide for the Use of the International System of Units (SI). NIST Special Publication 811 is the US national guidance on SI usage and contains the factor tables that this tool draws on for lb/ft³, lb/in³, lb/US-gal, oz/in³, and slug/ft³. SP 811 also contains the rules for rounding and significant figures that the tool's precision control implements.

CRC Handbook of Chemistry and Physics, the standard desk reference for physical constants and material properties. The densities of water, ice, mercury, ethanol, seawater, and the metals in the presets row are all taken from the CRC tables at the temperatures noted in the button labels. Different editions may quote slightly different values; this tool uses the 97th-100th edition numbers.

ISO 80000-4, Quantities and units, Part 4: Mechanics. ISO 80000-4 defines density as mass divided by volume, gives the SI unit as kg/m³, and gives specific gravity as a dimensionless quantity. ISO 80000-4 is the international standard that aligns the SI definition of density with the practical units encountered in engineering and science.