Measure Ring Size

Method

Measure ring size with a ruler

A ruler measures a ring you already own — it does not measure a finger. Lay the ring flat, read straight across the inside of the band, and you have the one number every size system is built from: the inner diameter in millimetres.

What a ruler can and cannot do

It can: read a ring's inner diameter

The inside edge of a round ring is a circle, and its diameter is a straight line through the centre. A ruler laid across that circle reads it directly. With care you can read to the nearest 0.5 mm, which is about one half-size of error at worst — good enough for most everyday rings.

It cannot: measure a finger

A ruler is straight and a finger is round. Holding a ruler against a finger and estimating its width measures the finger's width at one angle, not its circumference, and the width changes as you press. For a finger, use a tape, a strip, or string — never a ruler held against the skin.

Step by step

  1. Pick a ring that fits, and check it is round

    Use a ring that fits the exact finger on the exact hand. Roll it on a flat table: a round ring rolls smoothly, a bent or dented one wobbles and has no true diameter — measure a different ring if yours wobbles.

  2. Lay it flat, ruler across the centre

    Lay the ring flat on a table. Lay the ruler across it so the ruler's edge passes through the centre of the ring — the widest point of the hole. If the ruler sits off-centre, you are measuring a chord, which is always shorter than the diameter.

  3. Read inside edge to inside edge

    Read from the inner edge of the band on one side to the inner edge on the other side — not outer edge to outer edge, which adds twice the band thickness and can be 3–4 mm too large. Note the reading in millimetres.

  4. Mind the zero and the angle

    Start from the ruler's zero mark, not from the end of the ruler, which is often worn or rounded. Look straight down at the ruler: reading at an angle (parallax) shifts the apparent position by a millimetre or more on a thick ruler.

  5. Repeat at three angles

    Rotate the ring a little and read again, three times in total. A round ring gives the same reading each time; if the readings differ, the ring is slightly oval and the average is the honest number to use.

  6. Convert the diameter

    Type the diameter into the calculator below. It gives the circumference (diameter × π) and every system's size from the published table.

Three steps: check the ring is round, lay a ruler across its centre, read inside edge to inside edge
Ruler method, step by step. Read inside edge to inside edge on a real ruler — never use the artwork in this diagram as a ruler.

Convert your ruler measurement

Decimal numbers are welcome. Use a dot for decimals, for example 16.5.

What you measured

Ring inner diameter

Changing the unit reinterprets the number you typed in the new unit — it does not convert it. Re-enter the number if you measured in a different unit.

Note: Measure straight across the inside of the ring, from inner edge to inner edge — not the outside, and not the band thickness. A ruler laid across the ring, read in millimetres, is more accurate than estimating from a photo.

Try an example

Highlight my country's size

Enter a measurement to see your size.

Formulas: circumference = π × diameter; diameter = circumference ÷ π. Matching uses your full, unrounded value; displayed diameter is rounded to 2 decimal places and circumference to 1 decimal place.
Note: A ruler reading to 0.5 mm is good, but a screen or printed match is better: it compares the whole circle at once instead of one straight line. If your ruler reading lands between two sizes, check it against the online sizer or the printable circles before ordering.

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