Find the units before you read a value
A drawing's units are stated once, in the title block or a general note, for example UNLESS OTHERWISE SPECIFIED, DIMENSIONS ARE IN MILLIMETERS. Individual dimensions carry no unit symbol. A value of 25 means 25 mm on one drawing and 25 in on another, and nothing else on the sheet tells them apart. Read the note first, then the projection symbol and the general tolerance, before any dimension.
When the note is missing, the values themselves are evidence but not proof. A 300 mm plate dimensioned 11.81 is suspicious, and so is a part whose stated sizes do not fit the scale of the views. Either one is reason to ask the drawing's owner, not to decide for them.
How inch and millimeter values are written
ASME Y14.5 writes the two units differently, and the difference is a useful check on which unit a sheet uses:
| Convention | Millimeters | Decimal inches |
|---|---|---|
| A value below one | Leading zero: 0.5 | No leading zero: .50 |
| Whole numbers | No decimal point or trailing zero: 25 | Decimals to the precision: 1.000 |
| Tolerance digits | The limits need not match the nominal's decimals: 25 ±0.1 | Limits and tolerances written to the same number of decimals: 1.000 ±.005 |
In inch practice, the number of decimals often signals the general tolerance. A title block may assign ±.01 to two decimals and ±.005 to three, so 1.00 and 1.000 are different requirements. ISO drawings are usually in millimeters and often use a decimal comma, as in 6,6. A comma read as a thousands separator turns 6,6 into 66.
Dual-dimensioned drawings
A dual-dimensioned drawing gives each value in both units, one as the primary (controlling) value and the other for reference. The primary is commonly written first or above, with the secondary in brackets, and a note on the sheet explains the arrangement. Only the primary value is a requirement. The secondary is a rounded conversion, and a part inspected to it can be rejected or accepted wrongly.
If a dual-dimensioned drawing does not say which unit governs, or the two values disagree by more than their rounding explains, the drawing has a defect. Raise it with the drawing's owner. The guide to drawing interpretation errors lists unit confusion among the common misreadings.
Converting toleranced dimensions
The conversion factor is exact: 1 in = 25.4 mm, so a converted value is exact before it is rounded. The risk is in the rounding. Convert a toleranced size as its two limits, not as a nominal value plus a converted tolerance, and round each limit toward the middle of the band, so the converted band lies inside the original. ISO 370 gives rules for converting toleranced dimensions between inches and millimeters.
01 Write the limits
1.000 ±.005 in has limits of 1.005 and 0.995 in.
02 Convert each limit exactly
1.005 × 25.4 = 25.527 mm and 0.995 × 25.4 = 25.273 mm.
03 Round inward
To two decimals, round the upper limit down to 25.52 and the lower limit up to 25.28. The band shrinks from 0.254 to 0.24 mm and never grows.
04 Record the governing value
Note that the inch value governs, and that the millimeter limits are a conversion for convenience.
The unit converter converts lengths exactly, and the fractional inch converter handles sizes given as 17/64 or 1 3/8.
Fractional inches
Older inch drawings, and commercial sizes such as drills and stock, use fractions. A fraction is an exact value, so 17/64 in is exactly 0.265625 in and 6.746875 mm, but a fraction on a drawing usually carries a looser general tolerance than a decimal does. Read the title block for the tolerance that applies to fractions before you treat 1/4 as 0.2500.
Never convert by measuring
Scaling a printed drawing does not reveal its units, and it never supplies a missing value. Read the stated value and the stated unit, and ask when either is missing.
What TruePart reads today
In the Technical Preview, TruePart reads drawings dimensioned in millimeters only, and reads the units from the title block.
Questions
How do I know if a drawing is in inches or millimeters?
Read the title block or the general notes, which state the units. If neither does, treat the units as unknown and ask the drawing's owner.
Which value governs on a dual-dimensioned drawing?
The primary value, which the drawing's note identifies. The other value is a rounded conversion for reference.
Is 1 inch exactly 25.4 mm?
Yes, by definition. Converted values are exact until they are rounded, and the rounding is where tolerances can be lost.
Why round converted limits inward?
Rounding outward would accept parts the original tolerance rejects. Rounding each limit toward the middle keeps every accepted part inside the original band.
Sources
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI) · NISTAppendix B: the inch is exactly 25.4 mm.
- ASME Y14.5-2018, Dimensioning and Tolerancing · ASMEMillimeter and decimal inch dimensioning conventions, including leading zeros and tolerance decimals.
- ISO 370, Toleranced dimensions, Conversion from inches into millimetres and vice versa · ISORounding rules for converted toleranced dimensions.
- ISO 129-1, Technical product documentation (TPD), Presentation of dimensions and tolerances, Part 1: General principles · ISOISO presentation of dimension values on drawings.
What TruePart does today
- LimitIn the Technical Preview, TruePart reads drawings dimensioned in millimeters only, and reads the units from the title block.