Specific Gravity Calculator

What specific gravity tells you

Specific gravity is a plain number that says how heavy a material is compared with water. You take the density of the substance and divide it by the density of water, and the answer has no unit. A value of 1.000 means the stuff is exactly as dense as water, a value above 1.000 means it is heavier, and a value below 1.000 means it is lighter.

The handy part is that you never have to worry about grams or pounds. Because the unit cancels out, a number like 1.03 means the same thing whether you measured in a metric lab or on a kitchen scale. That is why doctors, brewers, jewelers, and pool owners all reach for specific gravity when they want a quick read on what is in a jar.

Think of it as a float test with a score. An object whose specific gravity is below 1.000 will bob on the surface of water, and an object above 1.000 will sink. A rubber duck sits at about 0.30, a log at about 0.70, and a rock at well past 2.00. The number tells you the story before you ever drop anything in a bucket.

This page does the division for you. Type a weight and a volume and it returns the density and the specific gravity using two common water references, plus the industrial cousins API gravity and Baumé degrees. You can also flip the problem around and ask how much a known volume of a known liquid weighs.

The formula and the water reference

The math is one line. Specific gravity equals the density of the sample divided by the density of water, written SG = density of sample / density of water. Density is mass divided by volume, so if you already know the weight and the volume, the calculator does both steps at once.

The catch is the water reference, because water itself changes density with temperature. Water is densest at 4 °C, where it is essentially 1.0000 grams per milliliter, and it thins to about 0.9982 grams per milliliter at 20 °C. The standard density tables published by the National Institute of Standards and Technology put water at 0.9982 grams per milliliter at 20 °C, so the same jar of liquid reads a slightly higher specific gravity against the warmer reference.

Most industries pick one reference and stick with it. Brewers and chemists often use 20 °C over 20 °C, petroleum uses 60 °F over 60 °F, and general science work uses the 4 °C maximum. The calculator lets you switch the reference so your number matches the one a lab report or a product sheet would print.

The table below shows how the water baseline itself moves. None of those values are specific gravity, they are the grams per milliliter that the sample density gets divided by, and they are the reason a careful measurement always names its temperature.

The numbers come from the NIST standard density tables for pure water, the same source the NIST ITS-90 water-density formulation is built on. The NIST glossary entry for specific gravity names water at 4 C as the standard, and the USGS Water Science School shows the hydrometer that reads it in the field.

Worked example: weight and volume to specific gravity

Suppose a beaker holds 400.0 milliliters of a clear liquid and the liquid weighs 412 grams. The density is the weight divided by the volume, so 412 ÷ 400 = 1.03 grams per milliliter. Against the 4 °C water reference that density is already the specific gravity, so SG at 4 °C = 1.03.

Against the 20 °C reference the water is only 0.9982 grams per milliliter, so the same liquid reads 1.03 ÷ 0.9982 = 1.0319. The gap is tiny here but grows for heavier samples, which is exactly why a lab sheet that reads 1.0319 should also say which water it used.

Two older scales are built from this same number. The petroleum API gravity is 141.5 ÷ 1.03 - 131.5 = 5.88, and the Baumé scale for liquids heavier than water is 145 - 145 ÷ 1.03 = 4.22. Both are just the specific gravity run through a fixed formula so the reading lands in a familiar range for a given trade.

The step-by-step table shows the chain the calculator runs. Every value printed on the page comes straight from these three inputs, so changing the weight or the volume changes all four outputs at once.

Running it backward: specific gravity to weight

Often you already know the specific gravity and you want the weight of a given volume. A quart of engine coolant with a specific gravity of 1.12 is a good case. One US quart is 946.3529 milliliters, so the weight is 1.12 × 946.3529 = 1059.9 grams, or about 2.34 pounds.

The reason this works is that the specific gravity is the density expressed as a multiple of water. At the 4 °C reference, where water is 1.0000 grams per milliliter, multiplying the specific gravity by the volume in milliliters gives the grams directly. At the 20 °C reference you would multiply by 0.9982 instead, which trims the answer by a fraction of a percent.

This backward path is what a beekeeper uses to check honey. Honey runs about 1.42 specific gravity, so a measuring cup of 236.588 milliliters holds about 336 grams, close to 11.85 ounces. A hydrometer that reads 1.42 tells the seller the jar is full of real, well-reduced honey rather than a watered-down mix.

Reverse checks like this are also how a service writer confirms antifreeze. Coolant that should read 1.12 but weighs far less than 1059.9 grams per quart has been diluted with water, and the freeze protection it promises is no longer the number on the bottle.

API gravity and Baumé degrees

Specific gravity is the parent, but two child scales show up on industrial labels, and the calculator prints both. API gravity is the scale the petroleum world uses, defined as 141.5 ÷ SG - 131.5. It runs backward from intuition: a heavier oil has a lower API number, so light gasoline sits near 60 API while heavy crude sits near 10 API.

Baumé degrees come in two forms. For liquids heavier than water the formula is 145 - 145 ÷ SG, and for liquids lighter than water it is 140 ÷ SG - 130. Syrup makers, acid sellers, and tanneries still quote Baumé on equipment, so a reading of 4.22 Baumé lines up with the 1.03 specific gravity from the worked example above.

The point of both scales is a friendlier number. A specific gravity of 0.74 and an API gravity of about 60 describe the same gasoline, but the API figure spreads the useful range across bigger, easier-to-read marks on a dial. The calculator keeps the parent number front and center and lists the two children so nothing gets lost in translation.

Notice that once a liquid gets much heavier than water, API gravity turns negative, which is why the scale is mostly quoted for fuels and light oils rather than syrups and salts.

What floats and what sinks

The single most useful thing specific gravity tells you is whether something floats. Water is the 1.000 line. Anything under that number floats, anything over sinks, and the farther from 1.000 the faster and more decisively it moves.

A gallon of gasoline at 0.74 rides high on a puddle and a spill spreads across the top instead of mixing. Ethanol at 0.789 is lighter than water, though it mixes with water completely, so it never forms a layer on top. Vegetable oil at 0.92 forms a layer on top of water and never blends without shaking.

Seawater at 1.025 is why the ocean holds you up a little better than a lake, and glycerin at 1.260 sinks straight to the bottom of a sink full of water. Honey at 1.42 drops like a stone, and a chunk of aluminum at 2.70 sinks, though it is light enough to foil by hand. The chart below lines up the common suspects.

Mercury is the dramatic outlier at about 13.55, heavy enough that a steel ball floats on its surface, but it is a lab hazard rather than a kitchen curiosity, so keep it out of any home project.

Where specific gravity shows up at home

You do not need a lab to use this number. A battery hydrometer reads the specific gravity of the acid, and a healthy car battery sits near 1.265 while a discharged one drops toward 1.120. The calculator can turn that reading into the weight of a known volume so you know what you are handling.

Aquarium owners use it to mix saltwater. Reef salt is added until the tank reads about 1.025, the specific gravity of seawater, and a cheap floating hydrometer is enough to keep fish alive. Brewing and winemaking lean on it even harder, because the sugar in the wort pushes the reading up near 1.050 before yeast, and the falling number tells the brewer when fermentation is done.

In the kitchen, a quick specific gravity check separates real maple syrup from table syrup, and it confirms that the honey in the jar matches its label. Pool and spa care uses it too, through a device called a hydrometer, to track how much dissolved solids the water carries after a hot summer of evaporation.

Each of these jobs shares one idea: a denser liquid reads a higher number, and the change from one reading to the next is the change you care about, whether that is a battery losing charge, sugar turning to alcohol, or salt going into a tank.

Limits and good measuring habits

Specific gravity is exact only when the temperature is known, because both the sample and the water reference shift with heat. A reading taken at a warm bench will not match the same sample at a cool one, so always note the temperature or use the reference the label expects.

The number also says nothing about what a substance is, only how dense. Sugar water and salt water can land on the same reading, and two different alcohols can overlap, so specific gravity is a check, not an identity test. Pair it with a second measurement when the answer has to be certain.

On the instrument side, a home hydrometer is good to about plus or minus 0.001, which is fine for a battery or a brew but not for a lab report. Calibrate it in pure water first, where it should read exactly 1.000 at the temperature printed on the stem, and throw it out if it does not.

Finally, the result is only as good as the volume you measured. A graduated cylinder beats a drinking glass, and a scale that reads to a tenth of a gram beats a bathroom scale. Feed the calculator clean numbers and it will hand you a clean specific gravity in return.