Formulas International Tolerance (IT) Grades

Related Resources: mechanical-tolerances

Formulas International Tolerance (IT) Grades

Formulas for Standard International Tolerance (IT) Grades per.ISO 286-1 1993 and ISO 919-1993

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
Basic Size		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
Basic Size		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

Reference:

ISO 286-1 1988

Current ISO standard has removed formulas