Diamond size · Measured, not estimated
How big is a diamond, actually?
A one carat round is not 6.5 mm. That is the figure every size chart on the internet repeats, and it describes an ideal stone rather than a real one. We measured 5 078 real graded diamonds out of our live inventory, and the market comes out about 4% smaller face up than the book says.
True size, actually true
See how big a diamond really is
Calibrate the screen with any bank card and every stone below is shown at genuine life size, on your device, in your hand. The measurements are the median of 5 078 real graded stones out of our live inventory, not the idealised figures every other chart repeats.
Step one: hold a bank card against the screen
Any bank card, loyalty card or driving licence. They are all fixed at 85.60 mm wide by international standard, which makes one the most accurate ruler most people own. Drag until the outline matches yours.
Not calibrated yet, so this is an estimate. Match a bank card above and it becomes exact.
Median of 5 078 graded stones, captured from our live inventory on 25 August 2026. Where a size is thin in the market we say so rather than inventing a number.
The book against the bench, at one carat
Left column is the standard trade approximation. Right column is the median of the real stones we can actually buy today. Every shape comes in under the book, and the step cuts come in worst.
| Shape | The usual chart | Real median | Stones | Face-up difference |
|---|---|---|---|---|
| Round | 6.5 × 6.5 mm | 6.37 × 6.35 mm | 27 | -4% |
| Oval | 7.7 × 5.7 mm | 7.67 × 5.53 mm | 48 | -3% |
| Marquise | 10 × 5 mm | 9.61 × 5.21 mm | 48 | +0% |
| Pear | 8.7 × 5.7 mm | 8.59 × 5.51 mm | 48 | -5% |
| Emerald | 7 × 5 mm | 6.52 × 4.7 mm | 48 | -12% |
| Radiant | 6.2 × 5.6 mm | 6.36 × 5.01 mm | 48 | -8% |
| Cushion | 5.75 × 5.75 mm | 5.78 × 5.37 mm | 48 | -6% |
| Asscher | 5.6 × 5.6 mm | 5.45 × 5.37 mm | 48 | -7% |
| Princess | 5.5 × 5.5 mm | 5.44 × 5.39 mm | 48 | -3% |
| Heart | 6.3 × 6.3 mm | 6.1 × 6.78 mm | 48 | +4% |
This is not a scandal and nobody is being cheated. Cutters keep weight rather than losing it, because weight is what the trade pays for, and a stone cut to the ideal spread would weigh less and cost the cutter money. It does mean that if you have been shopping off a size chart, the stone arriving is very likely a little smaller than the picture in your head. We would rather you knew that from us, in advance.
Questions people actually ask
How big is a 1 carat diamond?
A one carat round brilliant measures about 6.37 mm across, measured as the median of real graded stones rather than the idealised 6.5 mm usually quoted. Fancy shapes of the same weight differ: an oval is roughly 7.67 by 5.53 mm and a princess about 5.44 by 5.39 mm. Carat is weight, not size.
Does a 2 carat diamond look twice as big as a 1 carat?
No, and this is the most expensive misunderstanding in diamond buying. Weight grows with the cube of the dimensions while what you see from above grows with the square, so doubling the carat adds only about 60% to the face-up area and roughly 26% to the width. You pay double and you see about half as much extra as you expect.
Which diamond shape looks biggest for its carat weight?
Marquise, then pear and oval. Elongated shapes spread more of their weight across the top, so they read larger face up than a round of the same weight. Princess, asscher and cushion hold more weight in their depth and look smaller from above.
Why are real diamonds smaller than the size charts say?
Because the published charts describe a stone cut to perfect proportions, and most stones are not. Measuring 5 078 real graded stones out of live inventory, the median comes out about 4% smaller face up than the standard table, and emerald cuts are the worst offenders. Cutters keep weight rather than losing it, because weight is what gets paid for.
Can a website really show a diamond at actual size?
Only if it calibrates first. A CSS millimetre assumes 96 pixels per inch, which is wrong on nearly every phone and most laptops, so tools that claim actual size without calibrating can be out by up to 40%. Matching a bank card, fixed at 85.60 mm worldwide by ISO/IEC 7810, gives the exact pixels-per-millimetre of that specific screen.
Now go and look at real ones
Every measurement on this page came from stones that were genuinely for sale when we captured it. You can search the same inventory, filtered to the shape and weight you have just been looking at, with real prices.