Carat size, at true scale.
Calibrate your screen once, then see any diamond at its true physical size: eight shapes, 14 carat weights, on a hand or side-by-side. AI summaries can't replicate this: every render uses YOUR screen's pixel density.
Hold a credit card to the screen.
Drag the slider until the box matches your card's real-world width. Every diamond after this renders at its true physical size. We save it locally, once per device.
Showing approximate size · Calibrate for true scale
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How this works.
Every diamond face-up size in this tool comes from standard GIA / industry reference dimensions for well-cut stones. Real diamonds can vary ±5-10% from these tables depending on cut depth and proportions: a "spready" cut faces up larger; a deep cut faces up smaller at the same carat weight.
Calibration uses ISO/IEC 7810 ID-1. Every credit card, debit card, and driver's license worldwide is manufactured to this standard: 85.6mm × 53.98mm exactly. When you align our on-screen card outline to your physical card, we calculate your screen's pixels-per-millimeter and persist it to localStorage so you only calibrate once per device.
Finger-width references in "on a hand" mode correspond to typical US ring sizes: slim ≈ size 4 (13.5mm), average ≈ size 6 (15.7mm), wide ≈ size 9 (18.2mm). Your actual finger may be wider or narrower.
Sources: GIA Diamond Reference Charts · ISO/IEC 7810 ID-1 standard.
Common questions.
How big is a 1 carat diamond?+
A well-cut 1-carat round brilliant measures about 6.5mm in diameter, slightly larger than a standard pencil eraser. Fancy shapes the same weight face up larger because the weight is distributed differently: a 1-carat oval is roughly 7.7 × 5.7mm, a 1-carat marquise is 10 × 5mm. Use the visualizer above to see exactly how each shape compares at any carat weight.
How big is a 1.5 carat diamond?+
A 1.5-carat round brilliant measures about 7.4mm in diameter, visibly larger than 1 carat (6.5mm) on a finger. The actual face-up area is roughly 25% larger than a 1ct, while carat weight (which measures mass, not size) is 50% greater. This is why two stones with the same carat can look noticeably different sizes depending on cut quality and shape.
How big is a 2 carat diamond?+
A 2-carat round brilliant is roughly 8.2mm in diameter, a significant jump from 1.5 carat (7.4mm) and visually substantial on most fingers. Above 2 carats, diamonds start to read as "obviously large" rather than "nice size." Use the on-hand mode in the visualizer to see how 2 carats reads against your finger width.
Why does the carat-to-mm size depend on shape?+
Carat is a weight measurement (1 carat = 200mg). Different shapes distribute that weight differently. Marquise and oval cuts spread the weight along the length, making them face up larger than rounds at the same carat. Princess and asscher cuts have more weight in the depth (under the table), which is why they often face up smaller than rounds. This is also why "spread" and "depth" specs matter so much when comparing diamonds.
Is the visualizer accurate?+
Yes, once calibrated. The calibration step uses ISO/IEC 7810 ID-1, the international standard size for every credit card, debit card, and driver's license: 85.6mm × 53.98mm exactly. When you align our on-screen card outline to your physical card, we calculate your screen's pixels-per-millimeter and render every diamond at its true face-up dimensions from standard GIA / industry reference tables.
Why do I need to calibrate?+
Different devices have different screen densities. A 13-inch laptop, a 27-inch monitor, and a phone all show "1 inch" at different physical sizes. Without calibration, "1 carat = 6.5mm" is just a number on screen, not a real visual reference. The calibration converts our mm measurements into the right number of pixels for YOUR screen, making the visualization actually useful.