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Dalloway Precision Reference chart · ISO 286

ISO 286 tolerance grade (IT) chart

The width of each tolerance grade in microns, by nominal size — colour-zoned by process and cost. This is the table behind every H7/g6 callout on a drawing.

Tolerance **width** in microns (µm). 1 µm = 0.001 mm. Nominal size in mm, over–up to. Zones: IT5–6 ground IT7 precision IT8–9 general IT10–11 non-critical
Size range (mm)	IT5	IT6	IT7	IT8	IT9	IT10	IT11
1–3	4	6	10	14	25	40	60
3–6	5	8	12	18	30	48	75
6–10	6	9	15	22	36	58	90
10–18	8	11	18	27	43	70	110
18–30	9	13	21	33	52	84	130
30–50	11	16	25	39	62	100	160
50–80	13	19	30	46	74	120	190
80–120	15	22	35	54	87	140	220
120–180	18	25	40	63	100	160	250
180–250	20	29	46	72	115	185	290
250–315	23	32	52	81	130	210	320
315–400	25	36	57	89	140	230	360
400–500	27	40	63	97	155	250	400

What the grades mean on the floor

IT5–IT6 ground or precision-bored — bearing seats. IT7 is the everyday precision grade and where most H7 holes land. IT8–IT9 general machining. IT10–IT11 non-critical, where a general tolerance would have done the job.

The cost curve is brutally steep

Each step down roughly doubles the difficulty. The jump from IT7 to IT6 often means a second operation or a different process entirely. If a feature does not need it, relaxing one grade is one of the largest savings available on any drawing.

Reading a fit callout

The letter sets where the tolerance zone sits relative to nominal; the number sets how wide it is. In Ø25 H7/g6, H7 is the hole and g6 the shaft — the numbers come from this table, the letters decide clearance or interference.

Tolerance widens with size

A grade is not a fixed number: IT7 is far tighter at Ø3 than at Ø150. The grade describes relative precision, which is why the table has to be read across as well as down. Quoting "IT7" without a size says less than people think.

Temperature is not academic

Standard metrology temperature is 20 °C. Steel moves roughly 12 µm per metre per °C, so a large part measured warm off the machine can fail on paper and pass once it has normalised. Let tight work settle before inspecting.

Work out the actual limits

This chart gives the width; the deviation depends on the letter. Get real upper and lower limits, clearance and interference from the ISO 286 limits & fits calculator at dalloways.com/tools.

Dalloway Precision Engineering Ltd — ISO 286 tolerance grade chart. UK subcontract CNC machining, Lye, West Midlands.
Typical reference values: the governing standard or your material certificate takes precedence.

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