Specific Gravity: Why Some Things Float and Others Sink

A Steel Ship Weighs 100,000 Tons. Why Doesn't It Sink?

The aircraft carrier USS Gerald R. Ford displaces about 100,000 tons of water. Steel has a specific gravity of 7.85 -- nearly eight times denser than water. By every intuition, it should plunge straight to the ocean floor. But it floats, and the reason involves the same property that makes ice cubes bob in your drink and oil slicks ride on the ocean surface: specific gravity.

Specific gravity (SG) is the ratio of a substance's density to the density of water. It is a dimensionless number -- no units attached -- which makes it immediately useful for comparisons. If SG is below 1.0, the material floats. Above 1.0, it sinks. A steel ship floats because its hull encloses enough air to bring the overall SG of the ship-plus-air system below 1.0. Punch a hole in the hull, flood those air spaces, and the ship's effective SG rises above 1.0. That is when it sinks.

The Water Baseline

Water at 4 degrees C (39.2 degrees F) has a density of exactly 1.0 g/cm cubed, which is why it serves as the universal reference. At 4 degrees C, water reaches its maximum density -- a fact with enormous consequences for life on Earth, since it means ice floats instead of accumulating on lake and ocean floors.

Everyday Floaters: SG Below 1.0

Wood (SG: 0.12 to 1.3)

Most woods float, but the range is enormous. Balsa wood (SG 0.12-0.20) is so light it was the primary structural material in the Kon-Tiki raft that Thor Heyerdahl sailed 4,300 miles across the Pacific in 1947. Ebony (SG 1.1-1.3) actually sinks -- it is one of the few woods denser than water, which is part of why it became prized for piano keys and chess pieces.

Some common wood specific gravities:

• Balsa: 0.12-0.20
• Pine: 0.35-0.60
• Oak: 0.60-0.90
• Ebony: 1.1-1.3

Oils and Fats (SG: 0.71 to 0.95)

Oils float on water because they are less dense, not because they "repel" water. Gasoline (SG 0.71-0.77) floats higher than olive oil (SG 0.91-0.92). This density difference is what makes oil spill cleanup possible -- booms and skimmers exploit the fact that crude oil (SG 0.87-0.95) stays on the surface.

Ice (SG: 0.917)

Water is one of very few substances that expands when it freezes. Ice's specific gravity of 0.917 means about 8.3% of an iceberg sits above water -- the origin of the phrase "tip of the iceberg." If water contracted when freezing like most substances, lakes would freeze from the bottom up, killing aquatic life each winter. Earth's climate would be fundamentally different.

Other Materials That Float

• Cork: 0.12-0.24
• Apples: about 0.80 (this is why bobbing for apples works)
• Most plastics: 0.90-0.97 (explaining the ocean plastic pollution problem)
• Human body: roughly 0.95-1.05 (saltwater makes floating easier because seawater's SG is about 1.025)

Sinkers: SG Above 1.0

Metals

• Aluminum: 2.70
• Iron/Steel: 7.85
• Copper: 8.96
• Lead: 11.35
• Gold: 19.32
• Osmium: 22.59 (the densest naturally occurring element)

To convert between grams and kilograms when calculating densities, use our grams to kilograms converter.

Rocks and Exceptions

Most rocks sink (granite: 2.65-2.75, quartz: 2.65). But pumice, the volcanic rock riddled with gas bubbles, has an SG of 0.25-0.90. After the 2012 eruption of Havre Seamount near New Zealand, a pumice raft the size of Belgium was spotted floating across the Pacific.

Where Specific Gravity Gets Used

Checking Car Batteries

The sulfuric acid solution inside a lead-acid battery changes density as the battery charges and discharges. A fully charged battery reads SG 1.265-1.299; a dead one reads 1.120-1.140. Mechanics stick a hydrometer into the battery, suck up some electrolyte, and read the float level. It takes ten seconds and costs nothing.

Brewing Beer and Wine

Brewers measure specific gravity before and after fermentation. The original gravity (OG, typically 1.040-1.090) drops to a final gravity (FG, typically 1.006-1.020) as yeast converts sugar to alcohol. The difference between OG and FG tells you the alcohol content. Every homebrew shop sells a hydrometer for exactly this purpose.

Identifying Gemstones

A gemologist can distinguish a real diamond (SG 3.50-3.53) from cubic zirconia (SG 5.50-5.95) without scratching or damaging the stone. The heavy liquid method or hydrostatic weighing reveals specific gravity precisely enough to catch fakes. Amber (SG 1.05-1.10) is easy to distinguish from plastic imitations because most plastics are noticeably lighter or heavier.

Medical Diagnostics

Urine specific gravity (USG) is a basic kidney function test. Normal range is 1.005-1.030. Below 1.005 suggests overhydration or possible kidney impairment. Above 1.030 signals dehydration. A urinalysis dipstick provides this reading in under a minute.

Industrial Quality Control

Petroleum refiners, dairy processors, sugar manufacturers, and chemical companies all use specific gravity as a fast check on product consistency. If a batch of fuel or milk reads outside the expected SG range, something is wrong -- contamination, incorrect concentration, or process failure.

Measuring Specific Gravity

The Hydrometer

The classic tool is a weighted glass tube that floats at different levels depending on liquid density. You drop it into a tall cylinder of your sample and read the scale at the liquid surface. Different scales exist for different applications:

• Brewing hydrometers: 0.990-1.170
• Battery hydrometers: 1.100-1.300
• Brix hydrometers: calibrated for sugar concentration

Digital Density Meters

Modern labs use oscillating U-tube densitometers. A small sample (often under 1 mL) fills a vibrating glass tube. The oscillation frequency changes with the liquid's density, giving readings accurate to five decimal places. These instruments cost $2,000-$15,000 but save time in high-volume testing.

Specific Gravity vs. Density

These terms are related but distinct:

Since water's density is 1.0 g/cm cubed at 4 degrees C, specific gravity numerically equals density in g/cm cubed under standard conditions. For volume conversions when working with density calculations, our milliliters to liters converter is useful.

Common Materials Reference

Why This Matters

Specific gravity is one of those measurements that seems academic until you need it. Brewers rely on it to know when fermentation is done. Mechanics use it to diagnose a failing battery without removing it from the car. Geologists use it in the field to identify minerals. Ship designers use it to calculate whether 100,000 tons of steel will float or founder.

The concept is simple -- compare a substance's density to water's -- but the applications reach into medicine, manufacturing, geology, food science, and marine engineering. Once you understand why some things float and others sink, you start noticing specific gravity everywhere.