For example, 1/4 inch is exactly 0.25 inch and exactly 6.35 mm. A displayed nearby metric drill is an alternative size, not an exact conversion unless its diameter is 6.35 mm.
Precision policy
The calculation engine represents decimal inputs as rational numbers and rounds only for display. Inch-to-millimeter conversion uses the exact international-inch definition.
Choosing a nearby drill
When an exact equivalent is unavailable, compare the diameter difference and decide whether larger or smaller is acceptable for the operation. Tap drilling, reaming and clearance holes have different constraints.
Important limitation
Do not round a converted value and then use the rounded result as if it were the original nominal size on a tolerance-critical drawing.
Do not round a converted value and then use the rounded result as if it were the original nominal size on a tolerance-critical drawing.
Technical references
- SI Units – Length — National Institute of Standards and Technology. Accessed 2026-11-09.
Standards references identify scope and terminology; protected dimensional tables are not reproduced.