| North American Standard | Internal Diameter | British Standard |
|---|---|---|
| US & Canada | in mm | Equivalent |
| ¼ | 12.04 | A |
| ¾ | 12.24 | A½ |
| 1 | 12.45 | B |
| 1¼ | 12.65 | B½ |
| 1½ | 12.90 | C |
| 1¾ | 13.06 | C½ |
| 2 | 13.26 | D |
| 2¼ | 13.46 | D½ |
| 2½ | 13.66 | E |
| 2¾ | 13.87 | E½ |
| 3 | 14.07 | F |
| 3⅛ | 14.17 | F½ |
| 3⅜ | 14.38 | G |
| 3⅝ | 14.58 | G½ |
| 3¾ | 14.80 | H |
| 4 | 15.00 | H½ |
| 4¼ | 15.20 | I |
| 4½ | 15.40 | I½ |
| 4¾ | 15.60 | J |
| 5 | 15.80 | J½ |
| 5¼ | 16.00 | K |
| 5½ | 16.20 | K½ |
| 5¾ | 16.40 | L |
| 6 | 16.60 | L½ |
| 6¼ | 16.80 | M |
| North American Standard | Internal Diameter | British Standard |
|---|---|---|
| US & Canada | in mm | Equivalent |
| 6½ | 17.00 | M½ |
| 6¾ | 17.20 | N |
| 7 | 17.40 | N½ |
| 7¼ | 17.60 | O |
| 7½ | 17.80 | O½ |
| 7¾ | 18.00 | P |
| 8 | 18.20 | P½ |
| 8¼ | 18.40 | Q |
| 8½ | 18.60 | Q½ |
| 8¾ | 18.80 | R |
| 9 | 19.00 | R½ |
| 9¼ | 19.20 | S |
| 9½ | 19.40 | S½ |
| 9¾ | 19.60 | T |
| 10 | 19.80 | T½ |
| 10¼ | 20.00 | U |
| 10½ | 20.20 | U½ |
| 10¾ | 20.40 | V |
| 11 | 20.60 | V½ |
| 11¼ | 20.80 | W |
| 11½ | 21.00 | W½ |
| 11¾ | 21.20 | X |
| 12 | 21.40 | X½ |
| 12¼ | 21.60 | Y |
| 12½ | 21.80 | Y½ |
| 12¾ | 22.00 | Z |
