# Specific Gravity Calculator

Calculate specific gravity from substance density and water reference. Find if materials float or sink. Free density ratio calculator.

## What this calculates

Specific gravity (SG) is the ratio of a substance's density to the density of a reference substance, usually water at 4°C (1000 kg/m³ or 1 g/cm³). It is a dimensionless number. If SG is less than 1, the substance floats in water. If greater than 1, it sinks. Specific gravity is widely used in brewing, geology, materials science, and fluid engineering.

## Inputs

- **Substance Density** (kg/m³) — min 0
- **Reference Density** (kg/m³) — min 0 — Water at 4°C = 1000 kg/m³ (default reference).

## Outputs

- **Specific Gravity (SG)** — Dimensionless ratio of substance to reference density
- **Buoyancy in Water** — formatted as text — Whether the substance floats or sinks in the reference fluid
- **Substance Density** (g/cm³) — Substance density in g/cm³
- **Substance Density** (lb/ft³) — Substance density in lb/ft³

## Details

Here's the formula:

SG = Density of substance / Density of reference

Since the reference for liquids and solids is almost always water at 4°C (its maximum density), specific gravity numerically equals the density in g/cm³. For gases, the reference is usually air at standard conditions.

Common specific gravities:

  - Ice: 0.917 (floats in water)

  - Gasoline: 0.72-0.78 (floats)

  - Seawater: 1.025 (denser than fresh water)

  - Aluminum: 2.7

  - Iron: 7.87

  - Gold: 19.3

  - Mercury: 13.6 (iron floats on mercury)

In brewing, specific gravity readings track fermentation progress. Wort starts around SG 1.050 and finishes near 1.010 as yeast converts sugar to alcohol. In geology, specific gravity helps identify minerals. In medicine, urine specific gravity (1.002 to 1.035) indicates hydration status.

## Frequently Asked Questions

**Q: What is the difference between specific gravity and density?**

A: Density has units (kg/m³ or g/cm³) and tells you the mass per volume. Specific gravity is a dimensionless ratio comparing a substance's density to a reference (usually water). For solids and liquids referenced to water, specific gravity numerically equals the density in g/cm³, but it carries no units.

**Q: Why is water at 4°C used as the reference?**

A: Water reaches its maximum density (999.97 kg/m³, rounded to 1000) at 4°C. Using this standard temperature ensures consistent, reproducible measurements. At other temperatures water is slightly less dense, which would shift all specific gravity values slightly.

**Q: How is specific gravity used in brewing?**

A: Brewers measure original gravity (OG) before fermentation and final gravity (FG) after. The difference indicates how much sugar was converted to alcohol. For example, OG 1.050 and FG 1.010 gives about 5.25% ABV. A hydrometer or refractometer measures these values directly.

**Q: Can specific gravity be less than zero?**

A: No. Since density is always positive (mass and volume are both positive), specific gravity is always a positive number. A substance with SG less than 1 is lighter than the reference fluid and floats. A substance with SG greater than 1 is heavier and sinks.

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Source: https://vastcalc.com/calculators/physics/specific-gravity
Category: Physics
Last updated: 2026-04-08