Density Converter

What the units measure

Density is mass divided by volume, and every unit on this page is just that ratio with a different pair of mass and volume units attached. The kilogram per cubic meter, kg/m3, is the coherent unit of the International System, and the NIST guide to SI units lists it as the derived unit for mass density. The gram per cubic centimeter, g/cm3, is the same ratio scaled down, and because 1 cm3 is 1 milliliter, g/cm3 and g/mL are two spellings of one unit.

The rest of the page's units come from everyday measures. Pounds per cubic foot, lb/ft3, is common in US shipping and building trades. Pounds per gallon, lb/gal, shows up on fluid containers and in brewing. None of these changes the material; only the packaging of the ratio moves. Water is 1000 kg/m3, and it is equally 1 g/cm3, 62.428 lb/ft3 and 8.345 lb per US gallon.

The converter above takes one number and one unit and fills in all the others at once, so there is no chain of hand conversions to get wrong.

The conversion factors, and where they come from

Every factor on this page traces back to two exact definitions: 1 inch is 25.4 millimeters and 1 pound is 453.59237 grams. The NIST SI units page and the OpenStax Chemistry measurement chapter both walk through how prefixed and US customary units hang off those definitions. Cubing the inch gives the cubic foot: 1 ft3 = (12 x 25.4 mm)3 = 28316.846592 cm3, so 1 lb/ft3 = 453.59237 g / 28316.846592 cm3 = 0.0160185 g/cm3.

The pound per gallon factor follows the same road: 1 US gallon is 3.785411784 L, so 1 lb/gal = 453.59237 g / 3785.411784 mL = 0.119826 g/cm3, which is 119.826 kg/m3. The cubic inch is the small one: 1 lb/in3 = 27.6799 g/cm3, because a cubic inch packs 16.387 cm3.

Water, air and common materials

Water is the reference point for everything else. At about 4 C, water reaches its maximum density of 0.99997 g/cm3, which rounds to 1.000 for daily work, or 1000 kg/m3, 62.428 lb/ft3 and 8.345 lb/gal. Seawater runs about 1.025 g/cm3 because of dissolved salt, and ice at 0.917 g/cm3 is why about 92 percent of an iceberg sits below the waterline.

Metals cluster far above water. Aluminum is 2.70 g/cm3, iron about 7.87, copper 8.96, lead 11.34 and gold 19.32. In lb/in3, gold is 0.698, which is why a one cubic inch gold bar weighs close to 10 troy pounds of ordinary weight. On the light side, gasoline is near 0.74 g/cm3 and cooking oils near 0.92, so both float on water. Air at sea level sits at 1.225 kg/m3, which converts to 0.0765 lb/ft3: a cubic foot of air weighs about 1.23 ounces.

Wood shows why the converter earns its keep in the shop. Oak runs 0.70 to 0.75 g/cm3, pine near 0.50. A board foot of pine at 0.50 g/cm3 holds 144 in3 = 2359.7 cm3, so it weighs about 1180 g, or 2.6 lb.

Specific gravity, floating and freight

Specific gravity is a density with the units stripped off: the material's density divided by the density of water, both in the same unit. Because the units cancel, 1.025 seawater means 1.025 g/cm3, 1025 kg/m3, 64.09 lb/ft3 and 8.55 lb/gal all at once. That is why hydrometers and battery testers read a bare number and still pin down the density in any unit you like.

The float test falls out of the same comparison. Anything under 1 g/cm3 floats on fresh water; anything over it sinks. A 0.92 g/cm3 cooking oil floats with about 8 percent of its volume above the surface. In salt water at 1.025, the same oil sits higher. Swimmers feel this directly: salt water at 1025 kg/m3 buoys a body more than a pool at 998.

Freight and packaging use density in lb/ft3 to set handling classes, and the conversion matters there because a quoted 25 kg/m3 crate is only 1.56 lb/ft3. Aquarium and pond work leans on the same numbers: every cubic foot of fresh water in a tank adds 62.43 lb to the stand.

Mistakes that move the answer by 1000

The classic error is a factor of 1000 between g/cm3 and kg/m3. A reading of 1.2 g/cm3 entered as 1.2 kg/m3 lands in the air range instead of the plastic range. The reverse slips in too: 2700 kg/m3 for aluminum typed as 2700 g/cm3 would be denser than any stable element.

The imperial gallon deserves a second look because both countries say gallon. One imperial gallon is 4.54609 L, so water weighs 10.022 lb per imperial gallon against 8.345 lb per US gallon. Fuel economy figures, brewing recipes and tank charts that cross the Atlantic need that distinction stated, not assumed.

Temperature is the smaller but real effect. Liquids expand as they warm, so the same mass spreads through more volume and the density drops. If a datasheet quotes density at 20 C and your shop sits at 30 C, expect the measured value to come in a few tenths of a percent light.

Three worked cases

Work the converter through three real cases. First, an oil with a datasheet value of 0.92 g/cm3: multiply by 1000 for 920 kg/m3, then by 0.062428 for 57.43 lb/ft3, and by 0.0083454 for 7.68 lb/gal. A 5 gallon pail of that oil therefore holds about 38.4 lb of oil, plus the weight of the pail.

Second, a shipping quote lists a crate density of 12 lb/ft3. Multiply by 16.0185 to get 192.2 kg/m3, which is 0.1922 g/cm3, well under water. Third, a gold claim of 1204 lb/ft3: 1204 x 16.0185 = 19286 kg/m3, about 19.29 g/cm3, which matches the accepted 19.32 within rounding, so the claim passes the sniff test.

A quick habit keeps conversions honest: convert to water first. Divide any density by the water value in the same unit to get specific gravity, sanity check that the number matches the material, then convert outward. Air lands near 0.0012, oils near 0.9, water at 1, rocks near 2.7, iron near 7.9 and gold near 19.3. If a converted answer places a familiar material in the wrong neighborhood, one of the factors moved a decimal point.

Densities you will meet in the kitchen and the shop

A feel for a few reference values makes conversion mistakes obvious before they cost you anything. The table lists density at room temperature unless noted, all in kilograms per cubic meter, grams per cubic centimeter, and pounds per cubic foot. The conversion between the first two is trivial: 1 g/cm3 equals 1000 kg/m3, so the numbers just move the decimal three places.

The ice row explains itself the moment you picture a drink: ice at 917 kg/m3 sits about 8 percent lighter than water, so roughly one tenth of a floating cube shows above the surface. Oil floats higher still. On the other end, gold at 19.3 g/cm3 is over 19 times denser than water, which is why a small ingot takes two hands and why panning for gold works at all.