Carat
The unit of weight for diamonds and gemstones. One carat equals exactly 0.2 grams — it measures mass, not how large a stone appears.
The word comes from the carob seed, which ancient traders used as balance weights for their consistency. The modern metric carat was standardised in 1907 at exactly 0.2 grams. One carat divides into 100 points, so a 0.75 ct stone is also called a "seventy-five pointer".
Carat weight and face-up size are related but not the same thing. A well-proportioned round brilliant at 1.00 ct is approximately 6.4–6.5 mm in diameter. A poorly cut stone of the same weight can measure less; an elongated shape like an oval will measure more across its longest axis.
Diamond prices are not linear with carat weight. They follow a stepped schedule with sharp premium jumps at round-number thresholds: 0.50, 1.00, 1.50 and 2.00 ct. Buying just below a threshold is one of the most reliable value strategies available to diamond buyers.
- DEFINITION
- 0.2 g
- POINTS
- 100 pts = 1 ct
- PRICE
- Non-linear
0.50 ct
A half-carat round measures roughly 5.2 mm face-up. Popular for side stones and budget-conscious solitaires. The price-per-carat is meaningfully lower than 1.00 ct.
Illustration. Circle diameter is proportional to face-up size at this weight.
0.75 ct
A three-quarter carat sits between the two magic thresholds. Good value: you get noticeably more stone than 0.50 ct without the price jump at 1.00 ct.
Illustration. Circle diameter is proportional to face-up size at this weight.
1.00 ct
The most recognised milestone. A round brilliant at 1.00 ct is 6.4–6.5 mm face-up. Crossing the 1.00 ct threshold carries a real price premium — sometimes 15–20% over 0.95 ct.
Illustration. Circle diameter is proportional to face-up size at this weight.
1.50 ct
A 1.50 ct round is about 7.4 mm — noticeably larger than a 1.00 ct. Another price-step threshold, though less sharp than at 1.00 ct.
Illustration. Circle diameter is proportional to face-up size at this weight.
2.00 ct
Two carats command a significant premium. Price-per-carat jumps again at this threshold. A well-cut 2.00 ct round is 8.0–8.2 mm — prominently visible on most finger sizes.
Illustration. Circle diameter is proportional to face-up size at this weight.
3.00 ct
Three carats is considered a statement stone. Price-per-carat rises steeply. Availability narrows considerably, especially for high colour and clarity grades.
Illustration. Circle diameter is proportional to face-up size at this weight.
Common carat weights
Round brilliant · face-up diameter
Weight is not size
Two diamonds can weigh exactly 1.00 ct and look dramatically different face-up. Cut proportions determine how much of the weight appears as visible diameter — and how much is "hidden" in depth.
Shape matters too. Elongated cuts like oval, marquise, and pear distribute the same weight over a larger face-up footprint. A 1.00 ct oval appears notably larger than a 1.00 ct round, even though they weigh the same.
Deep cut
1.00 ct · ~6.0 mm
Weight hidden in pavilion depth. Smaller face-up, lower brilliance.
Ideal cut
1.00 ct · ~6.5 mm
Weight correctly distributed. Expected face-up size and maximum brilliance.
Shallow cut
1.00 ct · ~7.0 mm
Weight spread wide but shallow. Larger face-up, glassy, less brilliant.
Price thresholds
Diamond price-per-carat rises with weight, but not smoothly. At round-number milestones, price-per-carat jumps sharply — because demand concentrates at these weights and cutters preserve rough to reach them.
The most dramatic threshold is 1.00 ct. A 0.99 ct stone is visually identical to a 1.00 ct stone but can cost 15–25% less per carat. The practical size difference is under 0.05 mm — invisible to the eye.
Buying at 0.90–0.99 ct instead of 1.00 ct is one of the highest-impact value moves available — more than stepping down one colour or clarity grade.
Bar heights show relative price-per-carat index. Steps at 1.00, 1.50 and 2.00 ct reflect real threshold premiums. Live values appear once the data feed is connected.
Same weight, different footprint
At 1.00 ct, face-up area varies significantly by shape. Elongated cuts tend to look larger; deep cuts like princess can look smaller.
Round
6.5 mm ∅
33.2 mm²
Baseline
Oval
8.0 × 5.5
34.6 mm²
+4% area
Marquise
10.0 × 5.0
39.3 mm²
+18% area
Pear
8.5 × 5.5
36.7 mm²
+11% area
Princess
5.5 × 5.5
30.3 mm²
−9% area
Emerald
7.0 × 5.0
27.5 mm²
−17% area
Typical measurements for well-proportioned stones. Actual dimensions vary by cut quality and specific proportions.
Questions about carat
Is a bigger carat always better?
Not necessarily. A larger but poorly cut stone can appear smaller face-up and far less brilliant than a smaller, excellently cut stone. Cut quality determines how much of the carat weight translates into visible size. Many buyers find that a 0.90 ct Excellent cut looks identical to a 1.00 ct Fair cut — and costs significantly less.
Why does the price jump so sharply at 1.00 ct?
Diamond prices are not linear with weight — they follow a stepped schedule. These are called magic sizes. Cutters face strong financial pressure to bring rough diamonds in at or above round-number thresholds (0.50, 1.00, 1.50, 2.00 ct) because stones at those weights command a premium. This creates a cliff: a 1.00 ct stone can cost 15–25% more per carat than a 0.98 ct stone of identical quality. Buying just below a threshold (e.g. 0.95–0.99 ct) is one of the most reliable ways to get more stone per dollar.
Does carat weight affect apparent size differently by shape?
Yes, substantially. An elongated shape like oval or marquise distributes the same weight over a larger face-up area than a round brilliant. A 1.00 ct oval typically measures 8.0 × 5.5 mm, giving a larger apparent size than a 1.00 ct round at 6.5 mm. Emerald and radiant cuts also tend to look larger for their weight. This is why face-up area (in mm²) is often a more useful comparison than carat weight alone.
What is the difference between carat and karat?
"Carat" (ct) is a unit of gemstone weight: 1 carat = 0.2 grams. "Karat" (k or kt) is a measure of gold purity: 24k is pure gold, 18k is 75% gold. They sound similar but measure completely different things and are not interchangeable.
How does carat affect a diamond's look on a finger?
Finger size and width matter considerably. On a slender finger, a 0.75 ct can look generous; on wider fingers, 1.50 ct may look proportionally right. Setting style also plays a role — a solitaire with a simple band emphasises the stone; a halo setting makes the centre stone appear larger by surrounding it with a ring of smaller diamonds. Elongated shapes like oval and pear also create an illusion of greater size.
Can two diamonds weigh exactly 1.00 ct but look different sizes?
Yes. A deep-cut stone hides weight in the pavilion and has a smaller face-up diameter. A shallow-cut stone spreads weight into a larger face-up area but loses brilliance. Only a well-proportioned Excellent or Very Good cut stone converts its carat weight into the expected face-up diameter. DiamondTheGuide shows both carat weight and face-up dimensions so you can compare stones on both axes.