The unit conversions are exact. The two scales built on top of them — specific gravity and API — are where the trouble is.
Calculator
The SI unit, and the one engineering and materials data use everywhere.
Try:
850 kg/m³ is
850.00 kg/m³
Closest familiar material: Diesel at 832.00 kg/m³ — this is 1.02× that.
Every unit
Kilograms per cubic metre850.00kg/m³
Grams per cubic centimetre0.850000g/cm³
Grams per millilitre0.850000g/mL
Kilograms per litre0.850000kg/L
Pounds per cubic foot53.0638lb/ft³
Pounds per cubic inch0.030708lb/in³
Pounds per US gallon7.0936lb/US gal
Pounds per Imperial gallon8.5191lb/Imp gal
Ounces per cubic inch0.491331oz/in³
Tonnes per cubic metre0.850000t/m³
These conversions are exact — every factor comes from the exact pound, foot, inch and gallon. Note that lb/in³ is 1728 times lb/ft³, and that g/cm³, g/mL, kg/L and t/m³ are all the same number, which is why metric density work rarely needs a converter at all.
Specific gravity — against which water?
Standard laboratory conditions; common for polymers and general materials data.
Specific gravity0.8515vs water at 20 °c
Across the five references in common use this same substance ranges from 0.8500 to 0.8525 — a 0.29% spread produced by nothing but the choice of water. Specific gravity is dimensionless, which makes it look unambiguous; it is not, and a figure quoted without its reference is incomplete.
On the API scale
API gravity34.8°at 60/60 °F
Gradelightfloats on water
The API scale runs backwards. A higher number is a lighter liquid, because it is defined as 141.5 ÷ SG − 131.5. And water lands on exactly 10 by construction, which makes 10 the float-or-sink line rather than an arbitrary point. Light crude. WTI is about 39.6 and Brent about 38.3; these command the highest prices.
API is defined at 60/60 °F specifically, so this figure uses that reference regardless of the one selected above — computing it against any other water would not be API gravity.
For scale
Air at 20 °C1.2040706.0× lighter than yours
Balsa wood160.005.3× lighter than yours
Petrol745.001.1× lighter than yours
Diesel832.001.0× lighter than yours
Water at 20 °C998.21
Seawater1,025.00
Concrete2,400.00
Aluminium2,700.00
Steel7,850.00
Lead11,340
Gold19,300
Density spans four orders of magnitude between air and gold, which is why a converted figure is worth sanity-checking against something familiar before trusting it.
This converts the units of a density, not a quantity. If the question is what a given volume of something weighs, that needs the substance rather than a factor — and the answer is a range, because real materials vary. The Volume to Weight Converter handles that case and says so.
What this converter covers
Ten density units, five water references for specific gravity, and the API scale with its float line made explicit.
kg/m³, g/cm³, kg/L, t/m³, lb/ft³, lb/in³, lb/gal and oz/in³
Specific gravity against each water reference in common use
How much the reference alone moves the answer
API gravity, its inversion, and why water is exactly 10
Familiar densities for sanity-checking a converted figure
API runs backwards SG needs a reference Ten units Metric and imperial
850 kg/m³ is SG 0.8500 against 4 °C water and 0.8525 against 25 °C · water is API 10 exactly
Educational estimate
Planning support from the values you enter — not professional advice.
API gravity runs backwards
Every density unit gets larger as the substance gets heavier. API gravity gets smaller. It is the one scale in the family that is inverted, and the inversion is deliberate.
The definition is API = 141.5 ÷ SG − 131.5, with SG measured at 60/60 °F. Because SG is in the denominator, a lighter liquid — lower SG — produces a higher API number.
The same liquids on both scales
Specific gravity
Density
API
What it is
0.78
780 kg/m³
49.9
Condensate
0.83
830 kg/m³
39.0
Light crude (WTI, Brent)
0.89
890 kg/m³
27.5
Medium crude
0.93
930 kg/m³
20.6
Heavy crude
1.00
1000 kg/m³
10.0
Water
1.05
1050 kg/m³
3.3
Bitumen — sinks
Read the API column downward and it falls while the density rises. So “40 API” is lighter than “20 API”, and anyone reading it as a density has it exactly backwards.
The 141.5 and 131.5 are not arbitrary. They are chosen so that water lands on exactly 10 — substitute SG = 1 and the arithmetic gives 141.5 − 131.5 = 10. That makes API 10 the float-or-sink line: above it a liquid floats on water, below it sinks. It is the single most useful thing to remember about the scale, and it is invisible unless someone points it out.
Commercially the direction matters because lighter crude yields more of the valuable light products per barrel, so higher API sells at a premium. A cargo described as 39 API and one described as 22 API are different products at different prices, and the numbers say so in the opposite direction to intuition.
Specific gravity hides its reference
Specific gravity is dimensionless, which makes it feel like a clean, unambiguous number. It is a ratio — the substance’s density divided by water’s — and water’s density depends on temperature.
One substance at 850 kg/m³, against each water reference
Reference
Water density
Specific gravity
Used by
4 °C
999.97
0.8500
Chemistry; water’s density maximum
15 °C
999.10
0.8508
ISO petroleum and gas
60 °F
999.02
0.8508
The API 60/60 convention
20 °C
998.21
0.8515
Standard laboratory conditions
25 °C
997.05
0.8525
The other common lab reference
The spread is 0.29%, produced by nothing except which water somebody had in mind. Small, and larger than the tolerance on plenty of commercial specifications.
Note also that water is densest at about 4 °C rather than at freezing, which is why that reference gives the smallest specific gravity for any substance. It is also why ice floats and why deep lakes do not freeze solid — the same anomaly, showing up in a unit conversion.
The practical rule is the same one this site keeps arriving at: a figure whose reference is unstated is not yet a specification. “SG 0.85” should be written “SG 0.85 at 20/20 °C” or “0.85 at 60/60 °F”. Where precision matters, quoting the density in kg/m³ with its temperature sidesteps the ambiguity entirely.
Three different pounds per volume
Metric density is easy: g/cm³, g/mL, kg/L and t/m³ are all the same number, so a substance at 0.85 g/cm³ is 850 kg/m³ and 0.85 t/m³ and there is nothing to get wrong. Imperial has three units that all get called “pounds per” in conversation.
Water, in each of them
Unit
Water
Steel
In kg/m³
lb/ft³
62.3
490
16.02
lb/in³
0.0361
0.284
27,680
lb/US gal
8.33
65.5
119.8
lb/Imp gal
10.0
78.6
136.5
The relationships are exact integers where you would expect them: lb/in³ is exactly 1728 times lb/ft³, because a cubic foot is 12³ cubic inches. The gallon versions are 231 cubic inches per US gallon, and the Imperial gallon is a further 20.095% larger.
Two anchors worth memorising. Water is 62.3 lb/ft³, which is the number US structural and bulk-material work runs on. And water is almost exactly 10 lb per Imperial gallon — not a coincidence, since the Imperial gallon was defined in 1824 as the volume of ten pounds of water.
The machining number is the odd one: steel at 0.284 lb/in³ looks impossibly small until you notice the unit. A cubic inch of steel really does weigh about four and a half ounces.
What this does not answer
There are two questions that sound the same and are not, and it is worth being explicit about which one this page handles.
“Convert 850 kg/m³ to lb/ft³” is a unit conversion. The quantity is a density either way, the factors are exact, and the answer is 53.1 with no uncertainty attached. That is this page.
“How much does a gallon of this weigh?” is not a unit conversion. It crosses from volume to mass, which needs the substance, and the honest answer is a range: a gallon of petrol is about 6.2 lb, a gallon of water 8.3 lb, a gallon of honey nearly 12 lb. The Volume to Weight Converter handles that, and returns a range rather than inventing a single number.
The distinction shows up constantly in practice. A specification that says “density 0.85” has told you a material property. One that says “8 lb per gallon” has told you the same thing in disguise, but only if you know which gallon and at what temperature. Converting a property is safe; converting a quantity requires knowing what is in the container.
Densities worth remembering
Density spans four orders of magnitude between air and gold, and a converted figure is much easier to trust once it can be placed against something familiar.
Everyday densities, in the units each trade uses
Material
kg/m³
lb/ft³
Times water
Air at 20 °C
1.2
0.075
0.0012
Balsa
160
10
0.16
Petrol
745
46.5
0.75
Water
998
62.3
1.00
Concrete
2400
150
2.4
Aluminium
2700
169
2.7
Steel
7850
490
7.9
Lead
11340
708
11.3
Gold
19300
1205
19.3
A few of these are worth keeping in mind because they catch errors. Water is 1000 kg/m³ and 62.3 lb/ft³. Concrete is about 2.4 times water, which makes a cubic metre of it 2.4 tonnes — the number that stops people underestimating a slab. Steel is 7.9 times water and 2.9 times aluminium, which is the ratio behind every weight-saving substitution in engineering.
And gold at 19.3 times water is the one that surprises people physically. A gold bar the size of a paperback weighs more than a car battery, which is why film props are always visibly too light and why the real thing is handled the way it is.
Related calculators
Related quantities, including the one this page deliberately does not answer:
Volume to WeightGallons to pounds, cubic yards to tons, litres to kilograms — by substance, with an honest range rather than one invented number.
WeightKilograms, pounds, ounces, stone and tonnes, listing the short, long and metric ton separately — three masses, one word.
VolumeLitres, gallons, pints and cubic units — with US and imperial named apart, because a UK gallon is 20% larger than a US one.
Flow RateL/min, m³/h, GPM, CFM and barrels a day — with the two gallons, standard against actual CFM, and mass flow.
Textile WeightYarn counts between tex, denier, Nm and Ne, and fabric weight between GSM and ounces — keeping them apart, because no factor connects them.
Pressurepsi, bar, kPa, atm, torr and mmHg — plus why gauge and absolute differ by an offset that no factor can bridge.
The unit factors are all derived from exact definitions rather than tabulated, so the conversions carry no error of their own. The water densities are measured values and the API constants are a trade convention fixed by the petroleum industry — which is precisely why those two parts of the page need sources and the arithmetic does not.
CIPM — the density of water as a function of temperatureInternational Committee for Weights and Measures · verified 2026-09-07 · The water densities at 4, 15, 20 and 25 °C used as specific-gravity references here, and the fact that water’s density maximum sits at about 4 °C rather than at 0
Conversion note
Density is a property of a material at a stated temperature and pressure, and the figures offered here for familiar materials are nominal values at ordinary conditions. Real materials vary: alloys differ by composition, concrete by mix and cure, fuels by blend and season, and every liquid expands as it warms — diesel by roughly 0.08% per degree, which is enough to matter over a tanker load. Gases vary far more, since their density is proportional to pressure and inversely proportional to absolute temperature, so a gas density without both stated is not a measurement. The API classification bands are industry conventions rather than a standard with legal force, and grades are defined slightly differently by different bodies. For custody transfer, contractual quality specifications or anything someone is billed on, use a measured density at the contractual reference conditions rather than a converted or tabulated one.
Published the Density Converter: ten density units, specific gravity against five water references, and the API scale.
Explains that API gravity is inverted. Defined as 141.5 divided by specific gravity minus 131.5, a higher API number is a lighter liquid -- so 40 API is lighter than 20 API, opposite to every density unit beside it.
Points out that the constants are chosen so water lands on exactly 10, which makes API 10 the float-or-sink line rather than an arbitrary point on the scale.
Makes the specific-gravity reference a visible required control, because SG is dimensionless and therefore looks unambiguous while being a ratio to water whose density depends on temperature. The same 850 kg/m3 substance ranges from 0.8500 to 0.8525 across the five references in common use.
Notes that water is densest at about 4 C, which is why that reference gives the smallest specific gravity for any substance -- the same anomaly that makes ice float.
Computes API from the 60/60 F reference specifically regardless of the reference selected above, and says so, because computing it against any other water would not be API gravity.
Sets out that the three imperial pounds-per-volume units differ by exact integer factors: lb/in3 is exactly 1728 times lb/ft3, and the Imperial gallon version puts water at 10.0 lb per gallon, which is where that gallon came from in 1824.
States explicitly what the page does not answer, and links to the Volume to Weight Converter for it: converting a density unit is exact, while asking what a gallon of something weighs needs the substance and honestly returns a range.
Verified by 66 automated cases, including that API falls at every step as density rises, that all five water references give genuinely different answers, and that lb/in3 is exactly 1728 times lb/ft3.
Add this calculator to your site
Responsive embed — and private: nothing your visitors type leaves their browser.