British Standard Limits and Fits BS 4500:1969
IT

The specific Tolerance for a particular IT grade is calculated via the following formula:

T=10^{0.2 \times (ITG-1)} \cdot (0.45 \times \sqrt[3]{D}+0.001\times D)

T is the tolerance in micrometres [µm]
D is the geometric mean dimension in millimeters [mm]
ITG is the IT Grade, a positive integer.


Not applicable to sizes below 1 mm.
The dimensions are given in 0.001 mm, except for the nominal sizes which are in millimeters.

NOMINAL( BASIC) SIZES (INCHES)

INTERNATIONAL TOLERANCE GRADESOVERUP TO INCL.
OVER  UP TO INCL. IT4 IT5 IT6 IT7 IT8 IT9 IT10 IT11 IT12 IT13
0 0.12 0.12 0.15 0.25 0.4 0.6 1.0 1.6 2.5 4 6
0.12 0.24 0.15 0.20 0.3 0.5 0.7 1.2 1.8 3.0 5 7
0.24 0.40 0.15 0.25 0.4 0.6 0.9 1.4 2.2 3.5 6 9
0.40 0.71 0.2 0.3 0.4 0.7 1.0 1.6 2.8 4.0 7 10
0.71 1.19 0.25 0.4 0.5 0.8 1.2 2.0 3.5 5.0 8 12
1.19 1.97 0.3 0.4 0.6 1.0 1.6 2.5 4.0 6 10 16
1.97 3.15 0.3 0.5 0.7 1.2 1.8 3.0 4.5 7 12 18
3.15 4.73 0.4 0.6 0.9 1.4 2.2 3.5 5 9 14 22
4.73 7.09 0.5 0.7 1.0 1.6 2.5 4.0 6 10 16 25
7.09 9.85 0.6 0.8 1.2 1.8 2.8 4.5 7 12 18 28
9.85 12.41 0.6 0.9 1.2 2.0 3.0 5.0 8 12 20 30
12.41 15.75 0.7 1.0 1.4 2.2 3.5 6 9 14 22 35
15.75 19.69 0.8 1.0 1.63 2.5 4 6 10 16 25 40
19.69 30.09 0.9 1.2 2.0 3 5 8 12 20 30 50
30.09 41.49 1.0 1.6 2.5 4 6 10 16 25 40 60
41.49 56.19 1.2 2.0 3 5 8 12 20 30 50 80
56.19 76.39 1.6 2.5 4 6 10 16 25 40 60 100
76.39 100.9 2.0 3 5 8 12 20 30 50 80 125
100.9 131.9 2.5 4 6 10 16 25 40 60 100 160
131.9 171.9 3 5 8 12 20 30 50 80 125 200
131.9 200 4 6 10 16 25 40 60 100 160 250
Tolerances in Thousandths of an Inch (0.001)