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Formulas International Tolerance (IT) Grades
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Formulas for Standard International Tolerance (IT) Grades per.ISO 2861 1993 and ISO 9191993
See: International Tolerance (IT) Grades Table
The IS0 system of limits and fits provides for 20 standard tolerance grades designated IT0l, IT0, ITl, , IT18 in the size range from 0 up to 500 mm (incl.), and 18 standard tolerance grades in the size range from 500 mm up to 3,150 mm (incl.), designated IT1 to IT18.
The values for standard tolerances for basic sizes from 500 mm up to 3 150 mm (incl.) were subsequently developed for experimental purposes, and since they have proved acceptable to industry they are now given as a part of the IS0 system.
Formula for standard tolerances in grades IT01, IT0 and IT1 for basic sizes up to and including 500 mm.
Standard Tolerance Grade 
Formula for calculation where D is the geometric mean of the basic size in millimeters 
IT01 
0,3 + 0,008D 
IT0 
0,5 + 0,012D 
IT1 
0,8 + 0,020D 
Formula for Standard Tolerance Grade IT2. IT3 and IT4
None: The table values for tolerances in these grades have been approximately scaled in geometrical progression between the values for IT1 and IT5.
Formula for Standard Tolerance grade IT5 to IT18
The values for standard tolerances in grades IT5 to IT18 for basic sizes up to and including 500 mm are determined as a function of the standard tolerance factor, i.
i = 0,45 D^{1/3} + 0.001D
where D is the geometric mean of the basic size step in mm
D = (Low Range * High Range)^{(1/2)}
This formula was empirically derived, being based on various national practices and on the premise that, for the same manufacturing process, the relationship between the magnitude of the manufacturing errors and the basic size approximates a parabolic function.
The values of the standard tolerances are calculated in terms of the standard tolerance factor, i, as shown in table below.
It should be noted that from IT6 upwards, the standard tolerances are multiplied by a factor of 10 at each fifth step. This rule applies to all standard tolerances and may be used to extrapolate values for IT grades above IT18.
Basic Size 
Standard Tolerance Grades 

IT1 
IT2 
IT3 
IT4 
IT5 
1T6 
1T7 
1T8 
1T9 

Above 
Up to and including 
Formulae for standard tolerances (Results in micrometers) 

 
500 
 
 
 
 
7 i 
10 i 
16 i 
25 i 
40i 
500 
3,150 
2 
2,7 I 
3,7 I 
5 I 
7 I 
10 I 
16 I 
25 I 
40 I 
Basic
Size 
Standard Tolerance Grades 

1T10

IT11

IT12

IT13

IT14

IT15

IT16

IT17

IT18


Above

Up to
and including 
Formulae for standard tolerances (Results in micrometers) 



500

64 i

100 i

160 i

250 i

400 i

640 i

1000 i

1600 i

2500 i

500

3,150

64 I

100 I

160 I

250 I

400 I

640 I

1000 I

1600 I

2500 I

Derivation of standard tolerances (IT) for basic sizes from 500 mm up to and including 3,150mm
The standard tolerance factor, I, in micrometers, is calculated from the following formula :
I = 0,004D + 2,1
where D is the geometric mean of the basic size step in millimeters
Related:
 Engineering, Manufacturing Tolerance Limits Fits Charts  ISO 286 Tolerances  Metric Tolerance
 Machinist Drilling Mechanical Tolerance Capabilities Chart  ANSI Size Drilled Hole Tolerance, ISO Metric Drill Sizes
 Design for Manufacturability and Assembly DFM DFMA Premium Resources
 Sheet Metal Design, Mechanical Tolerances and Overview
 Casting Tolerances per. NADCA and ISO
 Aluminum Extrusion Dimensions and Tolerances Specification
 ISO Tolerances For Bolt Fastener and Holes Table Chart ISO 286
Reference:
ISO 2861 1988
Current ISO standard has removed formulas