Specific Gravity Calculator

What Specific Gravity Actually Measures

Specific gravity is the ratio of a substance's density to the density of a reference material, water for liquids and solids, air for gases, measured under a defined set of conditions. Because it is one density divided by another, the units cancel out entirely, Omni Calculator's specific gravity reference notes it is a unitless quantity, unlike density itself, which always carries units such as g/mL or kg/m³. A specific gravity above 1 means the substance is denser than water and will sink in it, below 1 means it is less dense and will float, exactly at 1 means it is essentially neutral.

This calculator covers the two most common ways people arrive at a specific gravity figure, working it out from a measured mass and volume, or comparing two already-known densities directly, plus a dedicated brewing mode for the original-gravity-to-final-gravity calculation homebrewers run constantly.

How to Calculate Specific Gravity

The formula is SG = ρ_substance ÷ ρ_reference. If you are starting from a raw measurement, first find the substance's density with ρ = mass ÷ volume, then divide that figure by the reference density. CalcTool's specific gravity calculator and reference tables walk through the same two-step process: a 150 g sample occupying 120 mL has a density of 1.25 g/mL, and against a reference of 1.0000 g/mL at 4°C that comes out to a specific gravity of exactly 1.25, meaning the sample is 25% denser than water.

If you already know both densities, for example from a materials data sheet, skip straight to the division, the Two Densities tab above handles that case directly without needing a mass or volume input at all.

Why the Reference Temperature Matters

Water's density is not a fixed number, it peaks at almost exactly 1.0000 g/mL at 4°C and drops slightly as temperature rises, to roughly 0.9982 g/mL at 20°C and 0.9970 g/mL at 25°C. Wikipedia's relative density article explains why this means a specific gravity figure is technically incomplete without stating both temperatures, the substance's and the reference water's, since two labs measuring the identical sample at different reference temperatures can report slightly different specific gravity values for the same material. Our calculator's reference temperature dropdown lets you match whichever standard your source material used, 4°C is the traditional scientific default, 20°C is common in modern lab work, and 60°F is the petroleum industry's standard reference point.

Specific Gravity in Brewing and Winemaking

Homebrewers check specific gravity constantly, a hydrometer reading before fermentation starts (original gravity) and again once it finishes (final gravity) tells you how much sugar the yeast converted to alcohol. Brewer's Friend's OG/FG calculator guidance confirms the standard estimate is ABV = (OG − FG) × 131.25, so a beer starting at 1.050 and finishing at 1.010 comes out to roughly 5.25% ABV. That simple formula loses accuracy in stronger beers and meads, where the fermentable sugar itself measurably changes the liquid's density beyond what the linear formula assumes, which is why our Brewing tab also shows the Gosset alternate formula alongside the simple one for higher-gravity batches. If you are diluting a high-gravity must or wort down to a target starting gravity before pitching yeast, our Dilution Calculator handles working out how much water to add.

Specific Gravity in Medicine: Urine Testing

Urine specific gravity is a routine clinical test that compares how concentrated a urine sample is against pure water. MedlinePlus's urine concentration test overview lists a typical normal range of roughly 1.005 to 1.030, with a provider's dipstick or lab measurement helping identify dehydration, kidney concentrating ability, or conditions such as diabetes insipidus. A reading toward the low end means the kidneys are producing dilute urine, toward the high end means urine is highly concentrated, both patterns a clinician interprets alongside the rest of a patient's fluid status and lab work, this calculator is not a substitute for that clinical interpretation.

Specific Gravity in Petroleum: API Gravity

The oil and gas industry converts specific gravity into a separate scale called API gravity, using API = 141.5 ÷ SG − 131.5. CalcTool's API gravity calculator explains the constants were deliberately chosen so that water sits at exactly 10°API, meaning any crude oil above 10°API floats on water and is classified light, while anything below sinks and is classified heavy. Once you have a specific gravity figure from either mode of this calculator, that formula converts it directly to the degrees API figure used throughout petroleum trading and refining specifications.

Common Mistakes When Measuring Specific Gravity

The most frequent error is comparing a substance measured at one temperature against a reference density measured at a different one, since even a few degrees can shift the third decimal place enough to matter for precise work. Dilutions Calculator's specific gravity guide also flags trapped air bubbles in a hydrometer or graduated cylinder as a common source of error, since a bubble inflates the apparent volume and quietly deflates the calculated density. Reading a hydrometer at eye level from the bottom of the meniscus rather than the top, and letting a sample fully reach the reference temperature before measuring, are the two habits that fix most reported inconsistencies. If you need the resulting figure expressed as a concentration in molarity, percent, or ppm instead of a raw ratio, our Concentration Calculator handles that conversion.