Diamond Price Calculator

Each day we collect listings for natural, GIA-certified round diamonds across the major online retailers, normalize the grades, and fit a per-carat price surface across color and clarity. Fancy shapes use the round price as a base and apply a length-to-width adjustment learned from the same data.

When you change a grade or weight on the calculator above, the page interpolates between the nearest reference points in the surface and returns the result instantly. The figure is a retail estimate. Wholesale prices typically run 30 to 40 percent lower.

The full pipeline is small enough to fit on a postcard. For an input (shape, color, clarity, carat, ratio) we compute:

1.   P_round  =  exp( logP[color, clarity, carat] )
2.   P_ct     =  P_round  ·  M(shape, carat, ratio)
3.   P_lab    =  P_ct  ·  f_lab          (only if lab-grown is on)
4.   Total    =  P_ct  ·  carat

logP is a lookup of log-prices indexed by color, clarity, and a carat band. M is the shape multiplier covered below. f_lab is a single discount factor for lab-grown stones. Everything else is interpolation.

The trade prices most diamonds against four variables, often called the four Cs. Three of them are quality grades, the fourth is weight.

A round brilliant cut wastes the most rough during faceting, often 60 percent or more, which is why round commands the highest per-carat price at any given grade. Cushion, oval, emerald, pear, marquise, heart, and radiant cuts retain more weight from the rough, so cutters offer them at a lower rate for the same color and clarity.

In our model, the round price acts as the base. Each fancy shape carries its own multiplier curve that adjusts the round figure for the shape and for the length-to-width ratio you pick.

The multiplier M takes one of two forms depending on how the trade quotes that shape. Most fancy shapes are quoted as a percentage of the round price:

M(shape, c, r)  =  v(shape, c, r) / 100

Cushion is quoted as a delta from round, so its multiplier shape is slightly different:

M(cushion, c, r)  =  1  +  v(cushion, c, r) / 100

In both cases, v is a small empirical curve fit separately for each shape and each carat band, with the ratio r as the input.

Two ovals at the same color, clarity, and carat can trade at very different prices depending on how elongated each one looks. The market has settled on a narrow ideal range for each shape, and ratios outside that range carry a discount.

• Oval1.30 – 1.50Ideal 1.40
• Cushion1.00 – 1.30Ideal 1.05 (square) or 1.20 (rectangular)
• Emerald1.30 – 1.50Ideal 1.45
• Pear1.45 – 1.75Ideal 1.55
• Marquise1.75 – 2.25Ideal 2.00
• Radiant1.00 – 1.50Ideal 1.35
• Heart0.90 – 1.05Ideal 0.95

The slider above the calculator's ratio input is bounded by the practical market range for the chosen shape. Push it toward either end and the price drops; sit close to the ideal and the price holds.

For ratios between two anchor points r_a and r_b, the multiplier is a straight linear interpolation:

v(r)  =  v_a  +  ( r − r_a ) / ( r_b − r_a )  ·  ( v_b − v_a )

Lab-grown diamonds have the same crystal structure, optical behavior, and certification framework as natural stones. The price gap is the result of supply. Lab production now scales quickly, and the wholesale cost has fallen most years since 2016.

As of 2026, the retail market prices a lab-grown stone at roughly 18 percent of the natural reference for the same shape, color, clarity, carat, and ratio. We apply that single discount factor when the calculator's lab-grown toggle is on. The figure is reviewed against retail data and adjusted when the market moves.

P_lab  =  P_natural  ·  f_lab,    where    f_lab = 0.18

A 2.00 ct diamond is not twice the price of a 1.00 ct stone of the same grade. It is closer to three or four times the total, because rough at that size is far rarer. The curve is steepest at the round-number thresholds the trade has agreed on: 0.30, 0.50, 0.90, 1.00, 1.50, 2.00, and 3.00 carats.

Buyers can take advantage of this by stopping a few points short of a threshold. A 0.92 ct stone often looks identical to a 1.00 ct on the finger and trades meaningfully lower. The calculator exposes this by interpolating between threshold weights rather than rounding to the nearest one.

The interpolation runs in log-space, which is the right geometry for a price that grows multiplicatively with weight. Between the two threshold weights c_1 and c_2 that bracket your stone:

λ          =  ( c − c_1 )  /  ( c_2 − c_1 )

log P(c)   =  ( 1 − λ )  ·  log P(c_1)  +  λ  ·  log P(c_2)

P(c)       =  exp( log P(c) )

A worked example pulls the math together. For a heart-shaped 1.00 ct, color I, clarity VVS2, ratio 0.89:

P_round   =  exp( 7.977 )                    =  $2,914 / ct
v(0.89)   =  62.625              (interpolated for 1ct heart)
M(0.89)   =  62.625 / 100                    =  0.626
P_ct      =  $2,914  ·  0.626                =  $1,825 / ct
Total     =  $1,825  ·  1.00                 =  $1,825

A real diamond carries detail the calculator cannot see from three letter grades and a weight.

• Cut grade beneath the standard tier. A poorly proportioned stone at the same color and clarity can trade 20 percent lower than the model expects.
• Fluorescence. Strong blue fluorescence pulls the price down on colorless grades and can lift it on warmer grades.
• Certification lab. GIA carries a premium over IGI and HRD on equivalent grades. Stones without major-lab papers are heavily discounted.
• Polish and symmetry. Reported alongside cut on the certificate, and the trade rewards Excellent on both.
• Fancy color. Yellow, pink, blue, and green stones are priced on a different ladder this calculator does not cover.
• Individual variance. Two stones with identical paperwork can look noticeably different. Eye appeal still moves price at the margin.

How accurate is this diamond price calculator?

Each value reflects the daily median retail asking price for a diamond at the chosen grade. Real stones routinely trade 15 to 30 percent above or below this number based on cut grade, fluorescence, certification lab, and individual stone character. Treat the figure as a reference point, not a quote.

Why is the per-carat price higher for a 2-carat stone than a 1-carat stone?

Diamond pricing scales exponentially with weight because larger rough is much rarer. A 2-carat stone of the same color and clarity as a 1-carat stone is typically priced at three to four times the total, not two times.

How much cheaper are lab-grown diamonds than natural?

As of 2026 the retail market prices lab-grown diamonds at roughly 80 percent below the equivalent natural stone, and the discount has been widening over the past three years as production capacity has grown.

Why does a round diamond cost more than a fancy shape with the same grade?

Cutting a round brilliant from rough loses around 60 percent of the original weight. Fancy shapes such as oval, cushion, and emerald preserve more of the rough, so cutters can sell them at a lower per-carat price for the same color and clarity.

What is a length-to-width ratio?

For non-round shapes, the ratio describes how elongated the stone looks from above. A 1.40 oval is moderately elongated, a 2.00 marquise is sharply elongated, and a 1.05 cushion is almost square. Each shape has a narrow band of ratios that the market considers ideal, and stones outside that band trade at a discount.

Does this calculator cover lab-grown diamonds?

Yes. The lab-grown estimate applies a single market discount on top of the natural reference price for the same shape, color, clarity, carat, and ratio. The discount is reviewed against current retail data and updated as the market shifts.

Why don't I see a cut grade input?

The reference price already assumes a standard 'Excellent' cut for round and the most common cut for fancy shapes. Stones graded below this trade lower; stones with elite proportions and finish trade higher. We aim to surface a typical-quality reference rather than every cut tier.