囲碁将棋盤 榧寿堂 ／ Go Stone Sizes Explained: Why the Number Is Thickness, and Why Black Is Wider Than White
Go Stone Sizes Explained: Why the Number Is Thickness, and Why Black Is Wider Than White
Go stones are sold by a size number (go, 号) that refers to thickness, not diameter. This trips up most first-time buyers: a “size 32” stone is not wider than a “size 28” stone — it is taller. Below is what each number means, and why black and white stones are deliberately made to different diameters.
Diameter is fixed. The size number is thickness.
Japanese go stones use a standard diameter that does not change with the size number:
| Stone | Standard diameter |
|---|---|
| White (clamshell) | 21.9 mm |
| Black (Nachiguro slate) | 22.2 mm |
Black stones are made about 0.3 mm larger on purpose. White is an expanding colour to the human eye, so a white stone of the same physical size looks bigger than a black one. Making the black slightly wider is what allows the two colours to appear equal on the board. It is a correction for perception, not a manufacturing tolerance.
What the size numbers mean
The number is the thickness. Thicker stones sit more solidly and make a heavier sound when placed, and they cost more — not because they play better, but because a thick stone requires a thick shell, and thick shells are rare.
| Size (号) | Thickness |
|---|---|
| 25 | 7.0 mm |
| 28 | 7.5 mm |
| 31 | 8.4 mm |
| 32 | 8.8 mm |
| 33 | 9.2 mm |
| 34 | 9.5 mm |
| 40 | 11.3 mm |
| 50 | 14.3 mm |
Clamshell stones are commonly sold in even-numbered sizes from 28 (7.5 mm) up to 50 (14.3 mm). Sizes up to roughly 36 are generally considered the easiest to handle in normal play.
Thickness and grade are two different things
A common misreading of price lists: people assume an expensive stone is a “better grade”. For clamshell stones, grade is decided by the stripe pattern of the shell (yuki, tsuki, hana), while thickness is decided by how deep the shell was. They move independently. A thin stone can be top grade; a thick stone can be a lower grade. Both raise the price, for different reasons.
See Clam-Shell Go Stone Grades (Yuki, Tsuki, Hana) for how the grades are separated.
Matching stones to your board
Thickness should be chosen against the board, not on its own. A very thick stone on a thin tabletop board looks and sounds wrong; a thin stone disappears on a deep legged board. If you are choosing both at once, decide the board first and let it set the range for the stones.
- How to Choose Go Stones: Clam-Shell Grades, Thickness and Nachiguro Black Stones
- Go & Shogi Board Thickness: Tabletop 2-Sun vs Legged 5–6 Sun
- How to Choose Go Bowls (Goke): Material, Form, and Matching Stone Size
Sources. Stone thickness by size number: the Nihon Ki-in (Japan Go Association). Diameters, the even-numbered 28–50 range and the grade/thickness distinction: Kuroki Goishi Ten, a shell-stone maker founded in 1917 (per its official site, checked 2026-10-06). Figures checked 2026-08-18. Specifications can differ between makers — confirm with the seller before ordering.