The torus is the diameter of the thickness of an O-ring. We will always need two measurements to define it: the inner and outer diameters of the seal. The formula is very simple: you subtract the inner diameter from the outer diameter and divide the result by 2. The table below illustrates the tolerances on the torus of standard O-rings:
| Cross Section Diameter d2 (mm) | Tolerances (mm) |
| ≤1,80 | ±0,08 |
| 1,80 – 2,65 | ±0,09 |
| 2,65 – 3,55 | ±0,10 |
| 3,55 – 5,30 | ±0,13 |
| 5,30 – 7,00 | ±0,15 |
| 7,00 – 8,00 | ±0,18 |
| 8,00 – 10,00 | ±0,21 |
| 10,00 – 12,00 | ±0,25 |
| ≥ 12,00 | – |