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ISO 286 IT Tolerance Grade Table (IT5 to IT9)

Drawings often specify a hole or shaft with a letter and a number, such as H7 or g6. The number is an international tolerance grade whose width depends on size. This table gives the IT5 to IT9 widths used for most machined features.

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Nominal size range (mm)	IT5 (um)	IT6 (um)	IT7 (um)	IT8 (um)	IT9 (um)
Over 1 to 3	4	6	10	14	25
Over 3 to 6	5	8	12	18	30
Over 6 to 10	6	9	15	22	36
Over 10 to 18	8	11	18	27	43
Over 18 to 30	9	13	21	33	52
Over 30 to 50	11	16	25	39	62
Over 50 to 80	13	19	30	46	74
Over 80 to 120	15	22	35	54	87
Over 120 to 180	18	25	40	63	100
Over 180 to 250	20	29	46	72	115

Values are the width of the tolerance zone in micrometres (1 um = 0.001 mm) from ISO 286-1 for the listed nominal size ranges. A fit such as H7 on a 25 mm bore means the hole may be 0 to +21 um from nominal. Check the current standard for sizes above 250 mm and for grades outside IT5 to IT9.

Tolerance data in your quality system

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How to read the table

Find the row that contains your nominal size, then read across to the grade. For a 25 mm bore with an H7 tolerance, the nominal size range is over 18 to 30, and the IT7 width is 21 micrometres. Since H means the lower deviation is zero, the hole is allowed from 25.000 to 25.021 mm.

What IT grades mean

The IT number sets the width of the tolerance zone and the letter sets its position relative to nominal. Smaller grades are tighter: IT5 and IT6 usually need grinding or fine boring, IT7 is achievable with good boring, reaming or finish turning, and IT9 is typical for general milled features. Tolerance width also grows with size.

IT5 to IT6: ground or honed features
IT7: reamed and finish bored holes
IT8 to IT9: general machining
Wider range means a wider tolerance zone

Using it in manufacturing

Use the table to check whether a drawing tolerance is realistic for the machine and process, to set inspection gauge limits, and to quote. A tolerance step down by one grade increases cost noticeably because it affects process choice, tool wear, thermal control and inspection time.

Common mistakes

Confusing the tolerance width with the deviation, using the wrong size range for a size on a boundary (a size of exactly 30 mm belongs to the range up to 30), and forgetting that micrometres are not millimetres. Also remember the fit letter position still decides whether the part is at nominal or offset.

Frequently Asked Questions

What is the tolerance for H7 on a 20 mm hole?

Twenty millimetres falls in the over 18 to 30 range, where IT7 is 21 micrometres. H7 means the hole runs from nominal to plus 21 micrometres, so 20.000 to 20.021 mm.

What is the difference between IT7 and H7?

IT7 is the grade that sets the tolerance width, which is 21 micrometres for 18 to 30 mm. H7 combines that width with the H position, which starts at zero deviation and goes plus. IT is width; the letter is position.

Can a CNC mill hold IT7?

Often yes, on a rigid, thermally stable machine with a finish boring head or reamer and in-process probing. IT6 or better usually needs grinding, honing, or very controlled finish boring.

Is IT the same as a general tolerance?

No. General tolerances such as ISO 2768 apply to dimensions without a specific tolerance on the drawing. IT grades are applied to individual toleranced features, mostly for fits.

If your tolerance requirements are spread across drawings and quality systems, we can look at how they could be searched and checked against inspection plans.

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