How to measure the size of a gem: length, width and depth in mm

By HeyGema™ · Updated

In short

Measure the stone from above in millimeters: the diameter of a round stone, or the longest length and widest width of any other shape. Use a sliding caliper that reads to 0.01 mm for a loose stone. Depth, from the flat top to the point at the bottom, needs the stone out of its setting.

What you need

  • A digital sliding caliper that reads to 0.01 mm (a ruler works for a rough size)
  • A soft cloth, since a clean stone measures true
  • The stone out of its setting if you want the depth

How to measure a loose stone

  1. 1

    Round stones: measure two diameters

    Hold the stone by its edges, measure the narrowest and the widest diameter across the girdle, the thin edge around the stone, and average them.

  2. 2

    Other shapes: length and width

    Length is the longest distance across the top, width the widest distance at a right angle to it. For a pear, marquise or heart the length runs from tip to the far end.

  3. 3

    Depth

    Stand the stone on its flat top and measure from the table to the point (culet), or lay it on its side and measure from the table to the culet.

  4. 4

    Write the numbers down

    Give length x width x depth in mm, for example 7.52 x 5.48 x 3.41 mm. The ratio of length to width tells the shape of an oval or emerald cut.

How to measure a stone in a ring

Measure across the top of the stone between the prongs or the bezel, not across the metal. Use calipers with fine tips, or measure on a ruler and photograph the stone close up. You cannot measure depth in a setting, so estimate weight from the diameter alone, or ask a jeweler to measure the stone with a gauge.

From millimeters to carats

With length, width and depth, use the diamond weight calculator. With length and width only, use the mm to carat calculator. Both estimate the weight of a well-cut stone; a real weight needs a scale.

Where to measure each shape

Round
Measure: Diameter at two angles, averaged
Oval
Measure: Longest length and widest width
Cushion
Measure: Longest length and widest width
Princess
Measure: Side to side across the top
Emerald cut
Measure: Longest length and widest width
Asscher
Measure: Side to side across the top
Radiant
Measure: Longest length and widest width
Pear
Measure: Longest length and widest width
Marquise
Measure: Longest length and widest width
Heart
Measure: Side to side across the top
Trillion
Measure: Side to side across the top
Baguette
Measure: Longest length and widest width

Questions

In millimeters: the diameter of a round stone, or the length and width of other shapes, across the top. Depth is measured from the flat top to the bottom point.
Measure across the top of the stone between the prongs. Depth cannot be measured in a setting, so a diameter-only estimate or a jeweler's gauge is used.
A digital sliding caliper that reads to 0.01 mm is the usual tool. A ruler gives a rough size for larger stones.
Enter the measurements in the mm to carat calculator for a quick estimate. A scale or a grading report gives the exact weight.

More size charts