How dense is platinum? Pure platinum has a density of about 21.45 g/cm3 (21,450 kg/m3) at room temperature, often rounded to 21.4 or 21.5. In US units, that is about 0.775 pounds per cubic inch or 1,339 lb/ft3. So platinum is about 11 percent denser than gold and roughly twice as dense as silver.
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Convert Platinum Density to Any Unit
For example, lab sheets list density in g/cm3, engineers often use kg/m3, and US machine shops think in pounds per cubic inch. So enter 21.45 for pure platinum, or your own measured value, and the converter shows it in each unit at once.
To begin, platinum is element 78, a silvery-white metal with the symbol Pt. Also, its name comes from the Spanish “platina”, meaning little silver. At first, Spanish settlers in South America treated the grains as a nuisance mixed in with gold. Today, however, platinum is one of the most valuable metals on Earth, and its weight in the hand is part of its appeal.
The Royal Society of Chemistry gives the density as 21.5 g/cm3 on its platinum element page. However, more precise handbooks list 21.45 g/cm3 at 20 degrees C (68 degrees F), and both figures describe the same metal at different rounding. In addition, you can check the number from the crystal itself, as the section on why platinum is so dense shows.
For example, a 1-inch cube of platinum weighs about 351.5 grams, or 12.4 ounces. Thus it is about as heavy as a full can of soda, packed into a cube you could hide in your fist.
How Dense Is Platinum Next to Other Metals?
Overall, platinum sits near the very top of the density chart. In fact, only osmium and iridium, its sister platinum-group metals, are denser among the stable elements. For comparison, the table below uses common handbook values at room temperature, and the last column shows how platinum compares with each one.
| Metal | g/cm3 | lb/ft3 | lb/in3 | Platinum compared |
|---|---|---|---|---|
| Osmium | 22.59 | 1,410 | 0.816 | 5% lighter |
| Iridium | 22.56 | 1,408 | 0.815 | 5% lighter |
| Platinum | 21.45 | 1,339 | 0.775 | – |
| Gold | 19.32 | 1,206 | 0.698 | 11% denser |
| Tungsten | 19.25 | 1,202 | 0.695 | 11% denser |
| Mercury (liquid) | 13.53 | 845 | 0.489 | 59% denser |
| Rhodium | 12.41 | 775 | 0.448 | 73% denser |
| Palladium | 12.02 | 750 | 0.434 | 78% denser |
| Lead | 11.34 | 708 | 0.410 | 89% denser |
| Silver | 10.49 | 655 | 0.379 | About 2 times |
| Copper | 8.96 | 559 | 0.324 | 2.4 times |
| Carbon steel | about 7.85 | about 490 | about 0.284 | 2.7 times |
| Aluminum | 2.70 | 169 | 0.098 | 7.9 times |
Two comparisons, however, come up most often. First, platinum against gold: for the same volume, platinum is about 11 percent heavier. Our guide on how dense gold is covers gold by karat, which matters because 14K and 18K gold are far lighter than pure gold. Second, gold and tungsten are almost twins at about 19.3 g/cm3, and the post on how dense tungsten is explains why that near match helps counterfeiters. Platinum, by contrast, has no common cheap metal of the same density, which makes a density test more useful for it.
How Dense Is Platinum in Weight Terms? Cubes, Bars and Troy Ounces
The question “how dense is platinum” becomes practical once you turn the answer into weight. Therefore, this chart uses 21.45 g/cm3 with exact NIST factors: 1 inch = 2.54 cm, 1 pound = 453.59237 g and 1 troy ounce = 31.1034768 g. Because precious metals trade in troy ounces, that column is included as well.
| Size of pure platinum | Volume | Metric weight | US weight | Troy ounces |
|---|---|---|---|---|
| 1 cm cube | 1 cm3 | 21.45 g | 0.76 oz | 0.69 ozt |
| 1-inch cube | 16.39 cm3 | 351.5 g | 12.40 oz | 11.30 ozt |
| 2-inch cube | 131.1 cm3 | 2.81 kg | 6.20 lb | 90.4 ozt |
| 10 cm cube (1 liter) | 1,000 cm3 | 21.45 kg | 47.29 lb | 689.6 ozt |
| 1 US gallon | 3,785 cm3 | 81.20 kg | 179.0 lb | 2,611 ozt |
| 1 cubic foot | 28,317 cm3 | 607.4 kg | 1,339 lb | 19,528 ozt |
| 1 troy ounce of platinum | 1.45 cm3 (a 1.13 cm cube) | 31.10 g | 1.10 oz | 1 ozt |
| 1 kilogram of platinum | 46.62 cm3 (2.84 in3) | 1 kg | 2.20 lb | 32.15 ozt |
Printable: save or print this chart and keep it with your scale or bench notes.
In other words, a kilogram of platinum fits in a cube only about 3.6 cm (1.4 inches) on a side. By comparison, a kilogram of water needs a cube 10 cm on a side.
Why Is Platinum So Dense?
In short, density comes down to two things: how heavy each atom is, and how tightly the atoms pack. As it happens, platinum scores high on both counts.
To begin with, each platinum atom is heavy, with an atomic weight of 195.084. In other words, that is about 3.5 times the mass of an iron atom. Next, the atom is small for its mass. This is because, in heavy elements, the inner electrons sit close to the large nuclear charge, so the atom does not grow in step with its weight. Finally, platinum atoms stack in a face-centered cubic (FCC) crystal, the tightest way to pack equal spheres, which fills about 74 percent of space.
Better still, you can check the handbook figure with simple arithmetic. The FCC unit cell of platinum is about 0.3924 nm on a side and holds 4 atoms. So 4 atoms times 195.084 g/mol, divided by Avogadro’s number and the cell volume, gives about 21.44 g/cm3. As a result, the crystal math and the measured value agree to within a tenth of a percent.
Gold uses the same FCC packing, so why is it lighter? In fact, gold atoms are about 1 percent heavier. However, gold atoms are larger, so fewer of them fit in each cubic centimeter. In short, platinum wins on compactness, not on atom weight.
How Dense Is Platinum Jewelry? Pure Metal vs Pt 950 and Pt 900
In practice, jewelry is rarely pure platinum. Instead, US jewelers usually stamp it “950 Plat” or “Pt950”, which means 95 percent platinum by weight plus a hardening metal. As a result, the second metal changes the density, sometimes by several percent.
| Alloy | Mix by weight | Estimated density | Typical use |
|---|---|---|---|
| Pure platinum | 99.95% Pt | 21.45 g/cm3 | Lab crucibles, bullion bars and coins |
| Pt 900 iridium | 90% Pt, 10% Ir | About 21.5 g/cm3 | Jewelry, the old kilogram prototype |
| Pt 950 iridium | 95% Pt, 5% Ir | About 21.5 g/cm3 | Soft, easy-to-cast jewelry |
| Pt 950 ruthenium | 95% Pt, 5% Ru | About 20.7 g/cm3 | Wedding bands, machined rings |
| Pt 950 cobalt | 95% Pt, 5% Co | About 20.0 g/cm3 | Detailed castings |
The alloy values are estimates from a simple mixing rule (volumes add) using handbook densities for each metal. Even so, real castings can differ slightly because of porosity and exact composition.
Because iridium is denser than platinum, it barely moves the number. By contrast, ruthenium and cobalt are much lighter, so they pull it down. That is why two “950” rings of the same size can weigh slightly different amounts.
Meanwhile, platinum-iridium also has a famous measuring role. According to the BIPM, the international bureau that keeps the SI units, the International Prototype of the Kilogram is a cylinder of 90 percent platinum and 10 percent iridium, about 39 mm tall and wide. In fact, it defined the kilogram until May 20, 2019. Moreover, its tiny size, about 46.6 cm3 for a full kilogram, works only because the alloy is so dense.
Recommended Tools for Measuring Platinum Density
Luckily, a home density check needs only a few tools. First, a 0.01 g pocket scale weighs rings, coins and small bars precisely. Next, a set of glass graduated cylinders measures displaced water for larger pieces. In addition, a digital caliper lets you measure regular shapes such as bars and coins, so you can work out the volume without water.
How Dense Is Platinum in Your Piece? Weigh It in Water
For small items, however, weighing in water is more accurate than reading a cylinder. Specifically, this method follows Archimedes’ principle: a submerged object is pushed up by the weight of the water it displaces. Here is the method step by step:
- Clean and dry the piece. Also, remove stones if you can, since gems and settings change the result.
- Weigh it in air. First, write down the mass in grams to two decimals, for example 10.00 g.
- Set a cup of water on the scale and tare it. Then use room-temperature water and zero the display.
- Hang the piece in the water on a thin thread. Next, keep it fully under the surface without touching the cup’s sides or bottom.
- Read the new number. Because this reading equals the mass of displaced water in grams, it also matches the volume in cm3. For example, a 10.00 g platinum ring reads about 0.47 g.
- Divide the air weight by that reading. For instance, 10.00 / 0.47 gives about 21.3 g/cm3. Then enter your result in the converter above.
For bigger bars or coins, you can also measure length, width and thickness with a caliper. Then multiply them to get the volume in cm3, and finally divide the weight by that volume.
How Dense Is Platinum Compared With Common Fakes?
Fortunately, density is a good first screen for platinum, because few cheap metals come close. For example, silver-colored fakes are usually white gold, palladium, silver or plated steel, and all of them are far lighter. So a ring that measures 10 to 16 g/cm3 is almost certainly not platinum.
| Your result | What it likely means |
|---|---|
| 20.0 to 21.6 g/cm3 | Consistent with pure platinum or a 950/900 alloy |
| About 14.5 to 16 g/cm3 | Could be 18K white gold |
| About 12 g/cm3 | Could be palladium or a palladium alloy |
| About 10 to 11 g/cm3 | Could be silver or a silver alloy |
| Below 9 g/cm3 | Likely plated base metal, such as steel or brass |
Still, density alone cannot prove a piece is genuine. For example, a gold-tungsten mix or a hollow item could land in a misleading range. Jewelers and refiners therefore confirm with an X-ray fluorescence (XRF) test, a hallmark check, or both.
Do and Don’t When You Test Platinum
Do
- Use 21.45 g/cm3 for pure platinum.
- Expect about 20 to 21.5 g/cm3 for 950 and 900 jewelry.
- Also, weigh in grams to two decimal places.
- Remove air bubbles from the piece before reading.
- Finally, repeat the test and average the results.
Don’t
- Test pieces that still hold stones or hollow parts.
- Read a 100 mL cylinder for a ring that displaces half a milliliter.
- Confuse platinum with palladium, because palladium is far lighter.
- Treat density as proof without a hallmark or XRF check.
- Also, never scrub or file a piece just to test it.
Honest Limits of These Platinum Numbers
The 21.45 g/cm3 figure describes pure, fully dense platinum at about 20 degrees C. However, real objects rarely match it exactly. For one thing, cast jewelry can hold tiny pores that lower the result, and alloys have their own values, as the table above shows. On the other hand, temperature matters very little, since platinum expands only slightly when heated. Instead, measurement error is the bigger issue. For instance, a 10 g ring displaces under half a milliliter, which a kitchen or school cylinder cannot read well. In short, treat a home result within about 3 percent of the expected alloy value as a match.
When Accuracy Really Matters
Of course, a rough figure is fine for curiosity. Some jobs, however, need certified numbers:
- Buying or selling bullion. Instead, dealers verify bars and coins with calibrated scales, XRF analyzers or ultrasonic checks, not a kitchen test.
- Jewelry manufacturing. For example, casters use the alloy supplier’s density to estimate how much metal a wax model will need.
- Lab and industrial parts. Similarly, crucibles, thermocouples and electrodes come with material certificates that state composition and purity.
- Catalysts and recycling. Likewise, refiners assay the metal content because value depends on exact purity.
Industry is also where most platinum goes. The US Geological Survey’s platinum-group metals summary names catalytic converters as the leading US use. In addition, it lists South Africa as the largest producer of mined platinum by far, ahead of Russia and Zimbabwe.
Frequently Asked Questions
How dense is platinum in g/cm3?
Pure platinum is about 21.45 g/cm3 at room temperature. However, many periodic tables round this to 21.4 or 21.5 g/cm3.
How dense is platinum in pounds per cubic inch?
Pure platinum is about 0.775 pounds per cubic inch, which also equals about 1,339 pounds per cubic foot.
How dense is platinum compared with gold?
Platinum is about 21.45 g/cm3 and gold is about 19.32 g/cm3, so platinum is roughly 11 percent denser, not 60 percent.
Is platinum the densest metal?
No. In fact, osmium is the densest at about 22.59 g/cm3, followed closely by iridium at 22.56. Platinum comes third at about 21.45 g/cm3.
How dense is platinum compared with silver?
Platinum is about twice as dense as silver. For example, silver is about 10.49 g/cm3, while platinum is about 21.45 g/cm3.
What is the density of 950 platinum?
It depends on the second metal. For example, Pt 950 with iridium is about 21.5 g/cm3, with ruthenium about 20.7, and with cobalt about 20.0 g/cm3, based on estimates from handbook values.
How much does a 1-inch cube of platinum weigh?
A 1-inch cube of pure platinum weighs about 351.5 grams, which is 12.4 ounces or 11.3 troy ounces.
How big is a troy ounce of platinum?
One troy ounce of platinum takes up only about 1.45 cm3. In other words, that is a cube about 1.13 cm, or less than half an inch, on each side.
Why is platinum so heavy for its size?
First, each platinum atom is heavy, with an atomic weight of 195.084. In addition, the atoms are compact and pack in a tight face-centered cubic crystal.
Is palladium as dense as platinum?
No. In fact, palladium is about 12.02 g/cm3, so platinum is roughly 78 percent denser. That difference makes density a quick way to tell them apart.
So, How Dense Is Platinum? The Bottom Line
To sum up, pure platinum has a density of about 21.45 g/cm3, or 0.775 lb/in3. As a result, it ranks third among the metals, behind osmium and iridium. It is also about 11 percent denser than gold and about twice as dense as silver.
For real jewelry, though, check the alloy stamp first, because 950 platinum runs from about 20 to 21.5 g/cm3. Then weigh your piece in water, enter the result in the converter at the top of the page, and compare it with the tables above.
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