IT grades ISO system explained

Definition and scope

The IT grade system (International Tolerance) is an internationally standardized code for defining tolerances on linear dimensions, established in ISO 286-1 and ISO 286-2. An IT grade represents a tolerance band of fixed magnitude for a range of nominal size, identified by the letters IT followed by a number. The higher the number, the larger the tolerance; for example, IT14 allows a variation of up to 0,36 mm / 0.014 in for a dimension of 10 mm. This system is fundamental for the interchangeability of manufactured parts and is applied in all branches of mechanical engineering, from micro-mechanics to heavy machinery construction.

Fundamental tolerance table

Tolerances are defined in micrometers (µm) or millimeters (mm) for each IT grade and nominal size range. The following table collects the standard values for the most commonly used grades (IT6 to IT18) up to 500 mm. The finer grades (IT01 to IT5) are used in gauges, instruments and high-precision parts.

Nominal range (mm)	Nominal range (in)	IT6 (µm / µin)	IT7 (µm / µin)	IT8 (µm / µin)	IT9 (µm / µin)	IT10 (µm / µin)	IT11 (µm / µin)	IT12	IT13	IT14	IT15	IT16	IT17	IT18
≤ 3	≤ 0.118	6 µm / 236 µin	10 µm / 394 µin	14 µm / 551 µin	25 µm / 984 µin	40 µm / 1575 µin	60 µm / 2362 µin	0,10 mm / 0.0039 in	0,14 mm / 0.0055 in	0,25 mm / 0.0098 in	0,40 mm / 0.0157 in	0,60 mm / 0.0236 in	1,00 mm / 0.0394 in	1,40 mm / 0.0551 in
> 3 … 6	> 0.118 … 0.236	8 µm / 315 µin	12 µm / 472 µin	18 µm / 709 µin	30 µm / 1181 µin	48 µm / 1890 µin	75 µm / 2953 µin	0,12 mm / 0.0047 in	0,18 mm / 0.0071 in	0,30 mm / 0.0118 in	0,48 mm / 0.0189 in	0,75 mm / 0.0295 in	1,20 mm / 0.0472 in	1,80 mm / 0.0709 in
> 6 … 10	> 0.236 … 0.394	9 µm / 354 µin	15 µm / 591 µin	22 µm / 866 µin	36 µm / 1417 µin	58 µm / 2283 µin	90 µm / 3543 µin	0,15 mm / 0.0059 in	0,22 mm / 0.0087 in	0,36 mm / 0.0142 in	0,58 mm / 0.0228 in	0,90 mm / 0.0354 in	1,50 mm / 0.0591 in	2,20 mm / 0.0866 in
> 10 … 18	> 0.394 … 0.709	11 µm / 433 µin	18 µm / 709 µin	27 µm / 1063 µin	43 µm / 1693 µin	70 µm / 2756 µin	110 µm / 4331 µin	0,18 mm / 0.0071 in	0,27 mm / 0.0106 in	0,43 mm / 0.0169 in	0,70 mm / 0.0276 in	1,10 mm / 0.0433 in	1,80 mm / 0.0709 in	2,70 mm / 0.1063 in
> 18 … 30	> 0.709 … 1.181	13 µm / 512 µin	21 µm / 827 µin	33 µm / 1299 µin	52 µm / 2047 µin	84 µm / 3307 µin	130 µm / 5118 µin	0,21 mm / 0.0083 in	0,33 mm / 0.0130 in	0,52 mm / 0.0205 in	0,84 mm / 0.0331 in	1,30 mm / 0.0512 in	2,10 mm / 0.0827 in	3,30 mm / 0.1299 in
> 30 … 50	> 1.181 … 1.969	16 µm / 630 µin	25 µm / 984 µin	39 µm / 1535 µin	62 µm / 2441 µin	100 µm / 3937 µin	160 µm / 6299 µin	0,25 mm / 0.0098 in	0,39 mm / 0.0154 in	0,62 mm / 0.0244 in	1,00 mm / 0.0394 in	1,60 mm / 0.0630 in	2,50 mm / 0.0984 in	3,90 mm / 0.1535 in
> 50 … 80	> 1.969 … 3.150	19 µm / 748 µin	30 µm / 1181 µin	46 µm / 1811 µin	74 µm / 2913 µin	120 µm / 4724 µin	190 µm / 7480 µin	0,30 mm / 0.0118 in	0,46 mm / 0.0181 in	0,74 mm / 0.0291 in	1,20 mm / 0.0472 in	1,90 mm / 0.0748 in	3,00 mm / 0.1181 in	4,60 mm / 0.1811 in
> 80 … 120	> 3.150 … 4.724	22 µm / 866 µin	35 µm / 1378 µin	54 µm / 2126 µin	87 µm / 3425 µin	140 µm / 5512 µin	220 µm / 8661 µin	0,35 mm / 0.0138 in	0,54 mm / 0.0212 in	0,87 mm / 0.0343 in	1,40 mm / 0.0551 in	2,20 mm / 0.0866 in	3,50 mm / 0.1378 in	5,40 mm / 0.2126 in
> 120 … 180	> 4.724 … 7.087	25 µm / 984 µin	40 µm / 1575 µin	63 µm / 2480 µin	100 µm / 3937 µin	160 µm / 6299 µin	250 µm / 9843 µin	0,40 mm / 0.0157 in	0,63 mm / 0.0248 in	1,00 mm / 0.0394 in	1,60 mm / 0.0630 in	2,50 mm / 0.0984 in	4,00 mm / 0.1575 in	6,30 mm / 0.2480 in
> 180 … 250	> 7.087 … 9.843	29 µm / 1142 µin	46 µm / 1811 µin	72 µm / 2835 µin	115 µm / 4528 µin	185 µm / 7283 µin	290 µm / 11417 µin	0,46 mm / 0.0181 in	0,72 mm / 0.0283 in	1,15 mm / 0.0453 in	1,85 mm / 0.0728 in	2,90 mm / 0.1142 in	4,60 mm / 0.1811 in	7,20 mm / 0.2835 in
> 250 … 315	> 9.843 … 12.402	32 µm / 1260 µin	52 µm / 2047 µin	81 µm / 3189 µin	130 µm / 5118 µin	210 µm / 8268 µin	320 µm / 12598 µin	0,52 mm / 0.0205 in	0,81 mm / 0.0319 in	1,30 mm / 0.0512 in	2,10 mm / 0.0827 in	3,20 mm / 0.1260 in	5,20 mm / 0.2047 in	8,10 mm / 0.3189 in
> 315 … 400	> 12.402 … 15.748	36 µm / 1417 µin	57 µm / 2244 µin	89 µm / 3504 µin	140 µm / 5512 µin	230 µm / 9055 µin	360 µm / 14173 µin	0,57 mm / 0.0224 in	0,89 mm / 0.0350 in	1,40 mm / 0.0551 in	2,30 mm / 0.0906 in	3,60 mm / 0.1417 in	5,70 mm / 0.2244 in	8,90 mm / 0.3504 in
> 400 … 500	> 15.748 … 19.685	40 µm / 1575 µin	63 µm / 2480 µin	97 µm / 3819 µin	155 µm / 6102 µin	250 µm / 9843 µin	400 µm / 15748 µin	0,63 mm / 0.0248 in	0,97 mm / 0.0382 in	1,55 mm / 0.0610 in	2,50 mm / 0.0984 in	4,00 mm / 0.1575 in	6,30 mm / 0.2480 in	9,70 mm / 0.3819 in

For finer grades (IT01 to IT5) used in gauges and masters, the values up to the 30 mm range are:

Nominal range (mm)	IT01 (µm / µin)	IT0 (µm / µin)	IT1 (µm / µin)	IT2 (µm / µin)	IT3 (µm / µin)	IT4 (µm / µin)	IT5 (µm / µin)
≤ 3	0,3 µm / 11,8 µin	0,5 µm / 19,7 µin	0,8 µm / 31,5 µin	1,2 µm / 47,2 µin	2 µm / 78,7 µin	3 µm / 118,1 µin	4 µm / 157,5 µin
> 3 … 6	0,4 µm / 15,7 µin	0,6 µm / 23,6 µin	1,0 µm / 39,4 µin	1,5 µm / 59,1 µin	2,5 µm / 98,4 µin	4 µm / 157,5 µin	5 µm / 196,9 µin
> 6 … 10	0,4 µm / 15,7 µin	0,6 µm / 23,6 µin	1,0 µm / 39,4 µin	1,5 µm / 59,1 µin	2,5 µm / 98,4 µin	4 µm / 157,5 µin	6 µm / 236,2 µin
> 10 … 18	0,5 µm / 19,7 µin	0,8 µm / 31,5 µin	1,2 µm / 47,2 µin	2 µm / 78,7 µin	3 µm / 118,1 µin	5 µm / 196,9 µin	8 µm / 315,0 µin
> 18 … 30	0,6 µm / 23,6 µin	1,0 µm / 39,4 µin	1,5 µm / 59,1 µin	2,5 µm / 98,4 µin	4 µm / 157,5 µin	6 µm / 236,2 µin	9 µm / 354,3 µin

Real application examples

A hole of 30 mm diameter specified as 30 H7 (IT7 quality, position H) has a minimum dimension of 30,000 mm / 1.1811 in and a maximum of 30,021 mm / 1.1819 in. If combined with a shaft 30 g6 (IT6, position g), the shaft limits are 29,980 mm / 1.1803 in and 29,993 mm / 1.1808 in, obtaining a controlled clearance fit between 7 µm / 276 µin and 41 µm / 1614 µin.

In a bearing assembly on a 50 mm shaft, the typical specification is 50 k6 for the shaft (IT6, zone k) and 50 H7 for the housing. The shaft k6 varies between 50,002 mm / 1.9686 in and 50,018 mm / 1.9692 in, achieving a light interference suitable to prevent rotation of the inner race.

Interpretation of symbols

An IT grade alone does not define the position of the tolerance zone relative to the zero line; it only indicates the magnitude of the band. To fully specify a dimension, a letter (or two) is prefixed indicating the fundamental deviation: uppercase for holes (A, B, C, … H, JS, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC) and lowercase for shafts (a, b, c, … h, js, k, m, n, p, r, s, t, u, v, x, y, z, za, zb, zc).

• Position H/h: the tolerance is located completely above (H, hole) or below (h, shaft) the nominal dimension. For example, 4 H7 goes from 4,000 to 4,012 mm; 4 h7 goes from 3,988 to 4,000 mm.
• Position JS/js: symmetric distribution. 4 js7 is equivalent to 4 ± 0,006 mm, since IT7 in that range is 0,012 mm.
• For other letters, the fundamental deviation is looked up in standard tables and the variation extends inward into the material respecting the IT band width.

Machining processes and associated IT grades

Each manufacturing process can economically achieve a range of IT grades. The following table, based on industrial practice compiled by Roymech, relates the tolerance grade to the most common processes.

Process	IT4	IT5	IT6	IT7	IT8	IT9	IT10	IT11	IT12	IT13	IT14	IT15	IT16
Lapping	X	X	X
Honing		X	X	X
Cylindrical grinding		X	X	X	X
Diamond turning			X	X	X	X
Broaching			X	X	X
Reaming				X	X	X
Turning / Boring				X	X	X	X	X	X
Milling					X	X	X	X	X	X
Planing / Shaping							X	X	X	X	X
Drilling						X	X	X	X	X
Die casting									X	X	X	X	X
Forging									X	X	X	X
Sand casting										X	X	X	X
Hot rolling / Flame cutting											X	X	X

Comparison between ISO and ANSI systems

The ANSI B4.1 system defines fit classes (RC = running/sliding, LC = locational clearance, LT = transition, LN = locational interference, FN = force/shrink) and does not use IT grades. However, a practical correspondence based on tolerance magnitude is possible. For example, an ISO H7/f7 fit (hole basis) resembles ANSI RC3 or RC4 fit for medium diameters, while H7/h6 aligns with a transition class LC. In general, IT7 quality is equivalent to an ANSI tolerance class around 7‑8, and IT8‑IT9 correspond to more open classes common in general manufacturing.

Frequently Asked Questions (FAQ)

What does the number after the letters IT represent in a geometric specification?

The number indicates the magnitude of the tolerance band according to ISO 286. For example, IT7 for a diameter of 10 mm has a value of 15 µm / 0,00059 in, while IT14 for the same diameter reaches 0,36 mm / 0,014 in.

What is the tolerance of a 45 mm shaft specified as 45 h7?

A shaft 45 h7 has a tolerance band of 25 µm / 0,00098 in (IT7 for range 30‑50 mm) located below the nominal dimension, resulting in limits of 44,975 mm / 1.7707 in to 45,000 mm / 1.7717 in.

How is the tolerance obtained for an IT grade not directly listed in the tables?

From IT6 to IT18, tolerances are multiplied by 10 every five grades. For example, for the range 120‑180 mm, IT15 = 1,6 mm / 0.063 in; then IT20 = 1,6 mm / 0.063 in × 10 = 16 mm / 0.63 in. For intermediate grades, geometric interpolation is used.

How does a H7/h6 fit differ from a H7/g6 fit?

In the H7/h6 fit (hole and shaft in H/h position) the tolerance of 21 µm / 0.00083 in for the hole and 13 µm / 0.00051 in for the shaft in the 18‑30 mm range allows a minimum clearance of 0 mm and maximum of 34 µm / 0.00134 in. In H7/g6, the g deviation of the shaft shifts the band downward, guaranteeing a minimum clearance of 9 µm / 0.00035 in and maximum of 43 µm / 0.00169 in, ideal for precision guides.

What IT grade is realistic for turned parts in a conventional workshop?

Normal finishing turning consistently allows reaching IT8, which for a diameter of 60 mm represents a tolerance of 46 µm / 0.0018 in. With subsequent grinding, IT6 is achieved, reducing the variation to 19 µm / 0.00075 in on the same diameter.

How are IT tolerance values converted to imperial units?

The conversion is done by multiplying the value in mm by 0,03937 to obtain inches, or the value in µm by 39,37 to obtain microinches (µin). For example, IT9 at 200 mm is 0,115 mm / 0.00453 in and the same 115 µm are equivalent to 4528 µin.

References

• roymech.org: https://roymech.org/Useful_Tables/ISO_Tolerances