Specific Gravity Calculator
Weigh it dry, then in water. The units cancel, so grams or ounces give the same answer — it is a ratio, not a weight.
What that weight belongs to
Minerals in the identification key whose specific gravity covers your reading.
The measurement that doubles your odds
Across the 39 minerals in the identification key on this site, a scratch test alone pins down 2 of them. Add specific gravity and it becomes 21 of 39 — more than half, from one extra measurement, and 11 times what hardness manages by itself. It is the single most useful thing you can do to an unknown rock.
It sounds like lab work and is not. Weigh the sample. Hang it from a thread so it is fully under water and touching nothing, and weigh it again. The formula is Archimedes' and has not changed:
specific gravity = weight in air ÷ (weight in air − weight in water)
The units cancel, which is why this page never asks which you used. Grams, ounces, grains — the answer is the same number, because it is a ratio against water rather than a weight. The only thing that matters is using the same unit twice.
Getting a reading you can trust
- The sample must hang freely. Resting on the bottom transfers weight to the container and gives a submerged weight that is far too low, which inflates the answer.
- Knock the bubbles off. Air clinging to a rough surface lifts the sample and reads as a lighter submerged weight. Tap it or wet it first.
- Porous rock will drink. Anything that soaks up water reads heavy and unstable; soak it fully first or accept that the number is approximate.
- The thread counts. Use the thinnest you have, and zero the scale with the thread already hanging if you can.
- Warm water is slightly lighter than cold, but the difference is a fraction of a per cent — far smaller than the error from a bubble, and not worth correcting for outside a lab.
The heavy ones
Specific gravity is how you tell these apart from things that look identical.
| Mineral | Specific gravity | Hardness | |
|---|---|---|---|
| Cinnabar | 8–8.2 | 2–2.5 | Very heavy for how soft it is. Mercury ore — wash your hands. |
| Galena | 7.2–7.6 | 2.5–2.75 | Startlingly heavy, breaks into perfect cubes. |
| Wolframite | 7.1–7.5 | 4–4.5 | Very heavy; tungsten ore. |
| Cassiterite | 6.8–7.1 | 6–7 | Very heavy for a non-metallic-looking mineral. Tin ore. |
| Hematite | 5–5.3 | 5–6.5 | The red-brown streak identifies it whatever the specimen looks like. |
| Magnetite | 5.15–5.2 | 5.5–6.5 | Strongly magnetic, which settles it in one second. |
For comparison, ordinary rock-forming minerals sit between about 2.5 and 3.0 — quartz is 2.65 and feldspar about 2.57. Anything reading over 4 is unusual enough that the number alone narrows the field sharply, which is exactly why the measurement is worth the two minutes.
How to use
- Weigh the dry sample on any scale, in any unit.
- Hang it from a thread so it is fully under water and touching nothing.
- Weigh it again and enter both numbers.
- Compare the result against the minerals listed below it.
Frequently asked questions
What is specific gravity?
How many times heavier something is than the same volume of water. Quartz at 2.65 means a lump of quartz weighs 2.65 times what an identical lump of water would. It is a ratio rather than a weight, which is why it has no units and why the number is the same everywhere.
What is the formula?
Specific gravity equals the weight in air divided by the weight in air minus the weight in water. The bottom of that fraction is the weight of the water the sample pushed out of the way, which is Archimedes' insight and has needed no revision since.
Does it matter which unit I weigh in?
No, and that surprises people. Both weights appear in the same fraction, so the units cancel — grams, ounces and grains all give exactly the same number. The only requirement is using the same unit for both readings, which is why this page does not ask.
Why is my number impossibly high?
Almost always because the sample is resting on the bottom of the container rather than hanging free. That transfers weight to the container, makes the submerged reading far too low, and inflates the answer. The sample must hang from a thread and touch nothing.
What about air bubbles?
They lift the sample and make the submerged weight read light, which also inflates the result. Wet the specimen first and tap it once it is under, especially anything rough or pitted where bubbles cling in the hollows.
Does porous rock work?
Poorly, unless you soak it first. Anything that absorbs water gains weight while you are measuring, so the reading drifts and comes out high. Soak it until it stops taking water, or treat the result as approximate and lean on other tests.
Does water temperature matter?
Barely. Warm water is slightly less dense than cold, but across any temperature you would put your hands in the difference is a fraction of a per cent — far smaller than the error from one bubble. It is not worth correcting for outside a laboratory.
Why is this better than measuring volume directly?
Because reading a water level in a measuring cylinder is hard to do accurately for a small sample — a millilitre either way is a large error on a 20 millilitre rock. Weighing is precise on cheap equipment, and the displacement method turns a hard volume measurement into two easy weight measurements.
What do common values look like?
Ordinary rock-forming minerals sit between about 2.5 and 3.0 — quartz 2.65, feldspar about 2.57, calcite 2.71. Anything above 4 is unusual: pyrite is about 5, hematite 5.3, galena 7.4, cinnabar 8.1. That spread is precisely why the measurement identifies so well.
How does this help identify a mineral?
Enormously, because the values are spread out while hardness is crammed into ten steps. Across the mineral key on this site, a scratch test alone identifies 2 minerals of 39 outright; hardness together with specific gravity identifies 21. One extra measurement, ten times the result.
Is specific gravity the same as density?
They are numerically the same in grams per cubic centimetre, since water is one gram per cubic centimetre, but they are not the same idea. Density is a measurement with units; specific gravity is a ratio without any. That is why specific gravity survives being measured in ounces and density does not.
Can I use this on something other than rock?
Yes, on anything solid that sinks and does not absorb water — metal, glass, ceramic, dense plastic. Anything that floats gives a negative submerged weight and the method needs adjusting, and anything that dissolves is a poor candidate for being hung in a cup of water at all.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.