What does an IT grade mean?

ISO 286 divides tolerance magnitude into grades (IT01–IT18); the smaller the number, the tighter the tolerance. IT is only the tolerance magnitude—the actual upper and lower deviations are set together with the fundamental deviation letter such as H, g or f.

What IT grade can general CNC machining reach?

General milling stably reaches IT8–IT10, and IT7 with good process control; IT6 and finer usually require finishing such as grinding or honing.

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ISO Tolerance Lookup (IT5–IT13)

Q: What is the tolerance for φ25 H7?

25mm falls in the 18–30mm size range, so IT7 = 21μm; an H hole has a lower deviation of 0, so φ25H7 = +0.021 / 0.

Enter a size, pick a grade, and look up the ISO 286 standard tolerance value instantly; a complete reference table is provided below.

Nominal size (mm, ≤250)

IT grade

Standard tolerance value—μm

Converted—mm

H hole reference deviation—lower deviation 0

IT is the tolerance MAGNITUDE, not the upper/lower deviations. The actual deviations depend on the fundamental deviation letter: for example, an H7 hole = lower deviation 0, upper deviation +IT7; shafts such as g6 and f7 each carry a negative deviation. For a specific fit, consult ISO 286-2 or confirm with the customer.

ISO 286 standard tolerance table (IT5–IT13, in μm)

Size range (mm)	IT5	IT6	IT7	IT8	IT9	IT10	IT11	IT12	IT13
≤3	4	6	10	14	25	40	60	100	140
3–6	5	8	12	18	30	48	75	120	180
6–10	6	9	15	22	36	58	90	150	220
10–18	8	11	18	27	43	70	110	180	270
18–30	9	13	21	33	52	84	130	210	330
30–50	11	16	25	39	62	100	160	250	390
50–80	13	19	30	46	74	120	190	300	460
80–120	15	22	35	54	87	140	220	350	540
120–180	18	25	40	63	100	160	250	400	630
180–250	20	29	46	72	115	185	290	460	720

FAQ

What is the tolerance for φ25 H7?

25mm falls in the 18–30mm range, so IT7 = 21μm. An H hole has a lower deviation of 0, so φ25H7 = φ25 +0.021 / 0.

How much do IT7 and IT9 differ?

About 2.4× within the same size range: for example, in the 18–30mm range IT7 = 21μm and IT9 = 52μm. Each grade step multiplies the tolerance by roughly 1.6.

What IT grade can general milling reach?

General CNC milling stably reaches IT8–IT10; IT7 requires a good machine and process control; IT6 and finer usually call for finishing such as grinding or honing.

Further reading: A complete guide to machining accuracy and IT grades · Tolerance and GD&T · A tolerance annotation guide for designers | More tools: Tools hub

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