How Specific Gravity Is Calculated
Specific gravity (also called relative density) compares the density of a substance to the density of a reference substance — almost always water at 4°C for solids and liquids, or dry air for gases. The formula is SG = ρsubstance / ρreference, where ρ (rho) denotes density. Because density divided by density lets the mass and volume units cancel, specific gravity is always a pure, dimensionless number: an object with SG = 2.70 is 2.70 times as dense as the reference fluid, regardless of which unit system was used to measure it.
Finding density from mass and volume
This calculator first computes the substance's density from your mass and volume inputs using ρ = m / V. It converts your mass and volume into consistent SI units (kilograms and cubic meters) behind the scenes, so you can mix unit systems — for example, entering mass in pounds and volume in liters — without converting by hand. It then divides that density by the reference fluid density you enter (water at 4°C ≈ 1000 kg/m³ by default) to produce the final specific gravity.
Common mistakes
- Using the wrong reference temperature: water's density changes slightly with temperature (about 998.2 kg/m³ at 20°C versus ≈1000 kg/m³ at 4°C), so lab reports sometimes state "SG 20°/4°C" to mean the sample was measured at 20°C against water's density at 4°C. For everyday use, 1000 kg/m³ (1.000 g/cm³) is close enough.
- Trapped air or gaps in volume measurement: for irregular solids measured by water displacement, air bubbles clinging to the surface inflate the apparent volume and understate density and SG.
- Confusing mass with weight: specific gravity is a ratio of densities (mass per volume), not weights — though in the classic Archimedes method (weight in air versus weight submerged in water), the two ratios come out numerically equivalent because gravity cancels out of the division.
Real-world applications
- Gemology and mineralogy use specific gravity to help identify unknown minerals and gemstones, since SG is a fixed physical property of a pure substance.
- Brewing and winemaking track the specific gravity of the liquid before and after fermentation to measure sugar consumed and estimate alcohol content.
- Automotive and marine technicians measure battery electrolyte and antifreeze specific gravity with a hydrometer to check charge state or freeze protection.
- Geotechnical and materials engineers use the SG of soil particles and aggregates to calculate void ratios, porosity, and material quantities.
Frequently Asked Questions
What is specific gravity?
Specific gravity (SG), or relative density, is the ratio of a substance's density to the density of a reference substance — usually water at 4°C for solids and liquids, or air for gases. It's calculated as SG = density of substance ÷ density of reference and has no units, since the density units cancel out in the division.
What's the difference between density and specific gravity?
Density is mass per unit volume and carries units, such as kg/m³ or g/cm³. Specific gravity is density divided by a reference density, which cancels the units and leaves a pure number. A substance with a density of 2700 kg/m³ has a specific gravity of about 2.70 relative to water (1000 kg/m³).
Why is water used as the reference for solids and liquids?
Water is the conventional reference because its density is well known, stable, and conveniently close to 1000 kg/m³ (1.000 g/cm³) at its point of maximum density, around 4°C. Using water as the baseline makes specific gravity values easy to compare across labs and industries without restating units.
How does temperature affect specific gravity?
Both the substance and the reference fluid expand slightly as temperature rises, which lowers their density. Precise work records both temperatures — for example "SG 20°/4°C" means the sample was measured at 20°C against water's density at 4°C. For general use, treating water as 1000 kg/m³ (1.000 g/cm³) is accurate enough.
Common Tip Percentages
| Service | Typical Tip Range | Notes |
|---|
| Restaurant (sit-down) | 15–20% | 20%+ for exceptional service |
| Food delivery | 10–20% | Consider extra for long distances |
| Taxi / rideshare | 10–20% | More for assistance with luggage |
| Hotel housekeeping | $2–5/night | Leave daily, not just at checkout |
| Barber / hair salon | 15–20% | On pre-discount total |
| Pizza delivery | 10–15% or $3–5 minimum | Higher in bad weather |
General Household Budget Ratios (50/30/20 Rule)
| Category | % of Net Income | Examples |
|---|
| Needs | 50% | Rent, groceries, utilities, insurance |
| Wants | 30% | Dining out, entertainment, hobbies |
| Savings / Debt | 20% | Emergency fund, retirement, extra payments |
Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |