One and a half carats is the weight people move to when one carat feels close but not quite enough. It is a visible step up rather than a marginal one, and it is where a few of the choices that did not matter much at a carat start to matter a great deal.
How big is a 1.5 carat diamond?
A well proportioned round brilliant at 1.50ct measures roughly 7.3 to 7.4mm across, against about 6.4mm at one carat.
That is not a large number on paper, and it is worth understanding why it looks like more than it sounds. Diameter grows with the cube root of weight, so half again the weight buys about 14% more width. But the stone covers area, not width, and the face-up area grows by roughly a third. That is why the step from one carat to one and a half reads clearly on the hand while the numbers look modest.
See diamond carat weight for how spread and weight diverge.
Buy just under the mark
The same principle that applies at one carat applies here, and the saving is larger because the stone is more expensive.
Prices step at 1.50ct. A 1.40ct or 1.45ct of identical cut, colour and clarity sits below that step while measuring around a tenth of a millimetre less. Nobody has ever noticed a tenth of a millimetre on a hand. If the certificate figure matters to you, buy the 1.50. If the look matters, the money is better spent elsewhere.
Colour and clarity start to show
This is the real difference between shopping at one carat and at one and a half.
Body colour becomes easier to see as a stone gets larger, because there is more depth for light to travel through. A J colour that reads white at 0.70ct can show a hint of warmth at 1.50ct, particularly in platinum or white gold where there is nothing to mask it. We would generally suggest not going below H or I in white metal at this weight, though yellow and rose gold remain far more forgiving.
Inclusions follow the same logic. A larger table gives the eye more to look at, so an SI that would pass unnoticed at three quarters of a carat may be visible here. VS1 or VS2 is the sensible band. Our guides to diamond colour and diamond clarity cover where each grade stops mattering.
Cut still comes first
A larger stone punishes a poor cut more, not less. There is more light to mishandle, and a deep or shallow 1.50ct can look duller than a well cut carat while costing considerably more. Hold Excellent or Ideal on a round brilliant, and on fancy shapes look at proportion, symmetry and polish rather than a single headline grade. See diamond cut.
Shape at one and a half carats
The extra weight gives you more freedom, and shape choice changes the impression a great deal.
Oval at this weight is striking, spreading well across the finger and reading larger still. Round brilliant gives maximum brilliance and is the benchmark. Emerald comes into its own here, because the hall of mirrors effect needs size to be appreciated, though it demands the better clarity discussed above. Cushion and radiant both carry the weight comfortably.
Settings that carry the weight
At 1.50ct the setting has more to hold and slightly more to protect.
A six claw solitaire is more secure than four at this size and still lets light through. A bezel is the most protective option and suits an active wearer. A halo is less often needed here, because the stone already has presence, though a hidden halo adds lift without adding apparent width. Band proportion matters more too: a very fine band under a substantial stone can look precarious and is harder to resize later.
What it costs
Stone prices at this weight vary widely with the other three Cs, and more so than at a carat, because every grade is multiplied across more stone. Our configurator shows live prices as you filter, which is more reliable than any figure printed here.
Settings start at £910 and a complete bespoke ring starts at £2,000. A one and a half carat commission generally sits toward the upper half of our usual £3,000 to £12,000 range, or beyond it with premium grades. The price guide sets out what moves it.
This is also the weight at which laboratory grown diamonds make the largest absolute difference, since the saving scales with the stone.
Frequently asked questions
How many millimetres is a 1.5 carat diamond?
A well cut round brilliant measures roughly 7.3 to 7.4mm across, compared with about 6.4mm at one carat.
Is 1.5 carats noticeably bigger than 1 carat?
Yes. The width increases by about 14% but the face-up area by roughly a third, and it is area the eye reads. The step is clearly visible on the hand.
What colour grade should I choose at 1.5 carats?
H or I and above in platinum or white gold, where there is nothing to disguise warmth. Yellow and rose gold are more forgiving and a J can sit very comfortably.
Is a 1.40ct much smaller than a 1.50ct?
About a tenth of a millimetre, which is invisible. The price difference is not, because 1.50ct is a price step.
Do I need a stronger setting at this weight?
Not stronger exactly, but better judged. Six claws rather than four, and a band with enough substance to look right beneath the stone and to resize cleanly later.
To see certified stones at this weight with live prices, start with a diamond. To understand the commission first, read how designing your own ring works, or book a consultation.
