Tolerances and limits
Work out the size limits from the dimension. Get 10 right to finish. ASME practice is in inches, ISO in millimetres.
What is the largest size this part may be?
Four ways to write a tolerance
| Type | Example | Largest | Smallest | Total |
|---|---|---|---|---|
| Equal bilateral | 2.000 ±.002 | 2.002 | 1.998 | .004 |
| Unequal bilateral | 25 +0.2 / -0.1 | 25.2 | 24.9 | 0.3 |
| Unilateral | .750 +.000 / -.005 | .750 | .745 | .005 |
| Limit dimension | 2.002 over 1.998 | 2.002 | 1.998 | .004 |
On US inch drawings a dimension and its tolerance have the same number of decimal places, and values under one inch have no zero before the point: .750, not 0.750. A dimension with no tolerance of its own takes the general tolerance from the title block. See how to read a title block.
Common mistakes
- Adding the tolerance twice. ±.002 gives a total of .004, not .002.
- Reading a unilateral tolerance as both ways. +.000 means it may not get any bigger.
Questions
- What is the difference between bilateral and unilateral tolerance?
- A bilateral tolerance lets the size vary both ways, such as 2.000 ±.002 or 25 +0.2/-0.1. A unilateral tolerance lets it vary one way only, such as .750 +.000/-.005.
- What is total tolerance?
- The difference between the largest and the smallest size allowed. 2.000 ±.002 runs from 1.998 to 2.002, so the total tolerance is .004.
- How do I read a limit dimension?
- Two values are stacked: the larger one on top is the largest size, the smaller one below is the smallest size. No working out is needed.
Explained in our own words and checked against several textbooks and published tables. For the full rules, see ASME Y14.5, ASME Y14.36, ISO 286, ISO 965, ISO 1302 and ISO 21920-1.