Fits – Metric (ISO System)

The ISO system of limits and fits (ISO 286, and ANSI/ASME B4.2 for the preferred fits) describes every hole and shaft with a short symbol such as 25H7 or 25g6. The symbol gives the basic size, a letter that places the tolerance zone, and a number that sets its width. Two symbols make a fit: 25H7/g6. This page explains how to read and use the system, with computed limit tables for the ten preferred fits across common sizes. All numbers come from the Machinery’s Handbook tables (BS 4500 and ANSI B4.2) and are checked against the Handbook’s own worked values. It is part of the Fits & Tolerances guide; the idea of a fit is on How Fits Work.

How the system works

Part of the symbol	Meaning	In 25H7
Basic size	The size the deviations are measured from; it is the same for the hole and the shaft	25 mm
Letter (fundamental deviation)	Places the tolerance zone relative to the basic size. Capital letters are holes (internal features); lower-case letters are shafts (external features)	H: the hole’s lower limit is the basic size
Number (IT grade)	Sets the width of the zone: the standard tolerance for that size. A higher number is a looser tolerance	7: IT7

The words hole and shaft mean any internal and external feature: the width of a slot is a hole, the thickness of a key is a shaft. The Machinery’s Handbook defines the deviations like this:

• Upper deviation: the algebraic difference between the maximum limit of size and the basic size. It is ES for a hole and es for a shaft.
• Lower deviation: the algebraic difference between the minimum limit of size and the basic size: EI for a hole, ei for a shaft.
• Fundamental deviation: whichever of the two deviations is nearer the zero line. It is what the letter fixes; the grade then fixes the other limit by adding or subtracting the tolerance.
• Zero line: the line of zero deviation, which represents the basic size.
• The Handbook’s ISO text lists 27 fundamental deviation positions for sizes up to 500 mm. The letter js has no fundamental deviation: its zone is split equally about the zero line, ±IT/2.

IT grades: how big the tolerance is

The IT grades (“International Tolerance”) set the width of the tolerance zone. The Handbook’s Table 4 lists 18 grades, IT01, IT0 and IT1 to IT16, for sizes to 500 mm. (ISO 286-1:2010 itself extends the series to IT18 and to larger sizes.) Each size range has its own values, because a larger part cannot be made as accurately as a small one. The grades used in the preferred fits are IT6, IT7, IT8, IT9 and IT11.

Nominal size (mm)	IT5	IT6	IT7	IT8	IT9	IT10	IT11	IT12
up to 3	4	6	10	14	25	40	60	100
over 3 to 6	5	8	12	18	30	48	75	120
over 6 to 10	6	9	15	22	36	58	90	150
over 10 to 18	8	11	18	27	43	70	110	180
over 18 to 30	9	13	21	33	52	84	130	210
over 30 to 50	11	16	25	39	62	100	160	250
over 50 to 80	13	19	30	46	74	120	190	300
over 80 to 120	15	22	35	54	87	140	220	350
over 120 to 180	18	25	40	63	100	160	250	400
over 180 to 250	20	29	46	72	115	185	290	460
over 250 to 315	23	32	52	81	130	210	320	520
over 315 to 400	25	36	57	89	140	230	360	570
over 400 to 500	27	40	63	97	155	250	400	630

Values in micrometers (0.001 mm), from the Handbook’s Table 4. A 25 mm feature falls in the “over 18 to 30” row: IT6 = 13 µm, IT7 = 21 µm, IT8 = 33 µm.

Where the numbers come from

The grades are built from a tolerance unit i that grows with the cube root of the size, in micrometers, with D the geometric mean of the size range in millimeters. Each grade is a multiple of i:

i = 0.45 × ∛D + 0.001 × D
IT5 = 7i, IT6 = 10i, IT7 = 16i, IT8 = 25i, IT9 = 40i, IT10 = 64i, IT11 = 100i, IT12 = 160i …

Size range (mm)	D (mm)	i (µm)	IT6: 10i → table	IT7: 16i → table	IT8: 25i → table
18 to 30	23.24	1.307	13.1 → 13	20.9 → 21	32.7 → 33
50 to 80	63.25	1.856	18.6 → 19	29.7 → 30	46.4 → 46
120 to 180	146.97	2.522	25.2 → 25	40.3 → 40	63.0 → 63

The calculated value, rounded to the standard’s own rules, gives the tabulated value. You never need the formula to use a fit (use the table), but it shows why tolerances grow with size and why each grade step is about 60% larger than the one before it.

Fundamental deviations: where it sits

The letter places the zone. For shafts, a to g put the whole zone below the zero line (clearance), h puts its top on the zero line, j and js straddle it, and k to zc put it above (interference). For holes the letters mirror this: A to G above, H on the zero line, K to ZC below. The table gives the fundamental deviations of the shaft letters used in the preferred fits, for sample sizes (the values for other sizes are in the Handbook’s Tables 5a and 5b).

Size	c (es)	d (es)	f (es)	g (es)	h (es)	k (ei)	n (ei)	p (ei)	s (ei)	u (ei)
6 mm	−70	−30	−10	−4	0	+1	+8	+12	+19	+23
10 mm	−80	−40	−13	−5	0	+1	+10	+15	+23	+28
16 mm	−95	−50	−16	−6	0	+1	+12	+18	+28	+33
25 mm	−110	−65	−20	−7	0	+2	+15	+22	+35	+48
40 mm	−120	−80	−25	−9	0	+2	+17	+26	+43	+60
60 mm	−140	−100	−30	−10	0	+2	+20	+32	+53	+87
100 mm	−170	−120	−36	−12	0	+3	+23	+37	+71	+124

Micrometers. For c, d, f, g and h the table gives the upper deviation es; for k, n, p, s and u it gives the lower deviation ei. The other limit is found by adding or subtracting the IT value (for k, the table value applies to IT4 to IT7 only).

The ten preferred fits

ANSI/ASME B4.2 (the Handbook’s Table 1) selects ten hole-basis fits, each with an equivalent shaft-basis fit that has the same clearances or interferences. This is all most designs need.

Type	Hole basis	Shaft basis	Handbook description
Clearance	H11/c11	C11/h11	Loose running fit for wide commercial tolerances or allowances on external members
Clearance	H9/d9	D9/h9	Free running fit not for use where accuracy is essential, but good for large temperature variations, high running speeds or heavy journal pressures
Clearance	H8/f7	F8/h7	Close running fit for running on accurate machines and for accurate moderate speeds and journal pressures
Clearance	H7/g6	G7/h6	Sliding fit not intended to run freely, but to move and turn freely and locate accurately
Clearance	H7/h6	H7/h6	Locational clearance fit: a snug fit for locating stationary parts, freely assembled and disassembled
Transition	H7/k6	K7/h6	Locational transition fit for accurate location: a compromise between clearance and interference
Transition	H7/n6	N7/h6	Locational transition fit for more accurate location where greater interference is permissible
Interference	H7/p6	P7/h6	Locational interference fit for rigidity and alignment of prime accuracy of location, without special bore-pressure requirements (a transition fit for sizes up to 3 mm)
Interference	H7/s6	S7/h6	Medium drive fit for ordinary steel parts or shrink fits on light sections; the tightest fit usable with cast iron
Interference	H7/u6	U7/h6	Force fit for parts that can be highly stressed, or shrink fits where the heavy pressing forces required are impractical

Limits at 25 mm

The full limits of size for a 25 mm feature, computed from the tables above. The “fit” column gives the smallest and largest clearance; a negative number is an interference. These values match the 25 mm row of the Handbook’s ANSI B4.2 Table 3 for the transition and interference fits.

Fit	Hole limits (mm)	Shaft limits (mm)	Smallest to largest clearance (mm)
H11/c11	25.000 to 25.130	24.760 to 24.890	0.110 to 0.370
H9/d9	25.000 to 25.052	24.883 to 24.935	0.065 to 0.169
H8/f7	25.000 to 25.033	24.959 to 24.980	0.020 to 0.074
H7/g6	25.000 to 25.021	24.980 to 24.993	0.007 to 0.041
H7/h6	25.000 to 25.021	24.987 to 25.000	0.000 to 0.034
H7/k6	25.000 to 25.021	25.002 to 25.015	-0.015 to 0.019
H7/n6	25.000 to 25.021	25.015 to 25.028	-0.028 to 0.006
H7/p6	25.000 to 25.021	25.022 to 25.035	-0.035 to -0.001
H7/s6	25.000 to 25.021	25.035 to 25.048	-0.048 to -0.014
H7/u6	25.000 to 25.021	25.048 to 25.061	-0.061 to -0.027

Fits across sizes

The same ten fits at seven sizes: smallest to largest clearance, in micrometers (a minus sign means interference). For a size not listed, use the Handbook’s tables or the Fit Calculator.

Fit	6 mm	10 mm	16 mm	25 mm	40 mm	60 mm	100 mm
H11/c11	70 to 220	80 to 260	95 to 315	110 to 370	120 to 440	140 to 520	170 to 610
H9/d9	30 to 90	40 to 112	50 to 136	65 to 169	80 to 204	100 to 248	120 to 294
H8/f7	10 to 40	13 to 50	16 to 61	20 to 74	25 to 89	30 to 106	36 to 125
H7/g6	4 to 24	5 to 29	6 to 35	7 to 41	9 to 50	10 to 59	12 to 69
H7/h6	0 to 20	0 to 24	0 to 29	0 to 34	0 to 41	0 to 49	0 to 57
H7/k6	−9 to 11	−10 to 14	−12 to 17	−15 to 19	−18 to 23	−21 to 28	−25 to 32
H7/n6	−16 to 4	−19 to 5	−23 to 6	−28 to 6	−33 to 8	−39 to 10	−45 to 12
H7/p6	−20 to 0	−24 to 0	−29 to 0	−35 to −1	−42 to −1	−51 to −2	−59 to −2
H7/s6	−27 to −7	−32 to −8	−39 to −10	−48 to −14	−59 to −18	−72 to −23	−93 to −36
H7/u6	−31 to −11	−37 to −13	−44 to −15	−61 to −27	−76 to −35	−106 to −57	−146 to −89

Read it across a row to see how a fit gets looser as the part gets bigger (clearance fits) or tighter (interference fits). Read it down a column for the size you are designing.

Shaft-basis fits

In the basic shaft system the shaft is h (its upper limit is the basic size) and the hole takes the letter. To get the shaft-basis equivalent of a hole-basis fit, swap the fundamental deviations between hole and shaft and keep each part’s grade. The 25 mm results (computed):

Hole basis	Shaft basis	Hole (mm)	Shaft (mm)	Clearance (mm)	Same fit?
H11/c11	C11/h11	25.110 to 25.240	24.870 to 25.000	0.110 to 0.370	same
H9/d9	D9/h9	25.065 to 25.117	24.948 to 25.000	0.065 to 0.169	same
H8/f7	F8/h7	25.020 to 25.053	24.979 to 25.000	0.020 to 0.074	same
H7/g6	G7/h6	25.007 to 25.028	24.987 to 25.000	0.007 to 0.041	same
H7/h6	H7/h6	25.000 to 25.021	24.987 to 25.000	0.000 to 0.034	same
H7/k6	K7/h6	24.985 to 25.006	24.987 to 25.000	-0.015 to 0.019	same
H7/n6	N7/h6	24.972 to 24.993	24.987 to 25.000	-0.028 to 0.006	same
H7/p6	P7/h6	24.965 to 24.986	24.987 to 25.000	-0.035 to -0.001	same
H7/s6	S7/h6	24.952 to 24.973	24.987 to 25.000	-0.048 to -0.014	same
H7/u6	U7/h6	24.939 to 24.960	24.987 to 25.000	-0.061 to -0.027	same

The clearances are the same as the hole-basis fits, as the Handbook notes; only the individual zones differ. This is also how the Handbook’s example works: 60H11/c11 (hole 60.000 to 60.190, shaft 59.670 to 59.860) and 60C11/h11 (hole 60.140 to 60.330, shaft 59.810 to 60.000) both give 0.140 to 0.520 mm of clearance.

Worked examples

Example 1: limits of 40H7/g6 from the tables

• Size 40 mm: IT7 (30 to 50 mm row) = 25 µm; IT6 = 16 µm.
• Hole H7: EI = 0, ES = +25 µm, so the hole is 40.000 to 40.025 mm.
• Shaft g6: the table gives es = -9 µm (30 to 40 mm row), so ei = -9 − 16 = -25 µm. The shaft is 39.975 to 39.991 mm.
• Clearance: smallest = 40.000 − 39.991 = 0.009 mm; largest = 40.025 − 39.975 = 0.050 mm.

Example 2: a press fit and what it asks of the process

A 60 mm gear hub is to be located and driven with H7/p6.

• Hole 60H7: 60.000 to 60.030; shaft 60p6: 60.032 to 60.051 mm.
• Interference: 2 to 51 µm (0.002 to 0.051 mm).
• The fit range is 0.030 + 0.019 = 0.049 mm. Both parts must be ground, honed or reamed: IT6 and IT7 are beyond ordinary turning and milling (see Tolerances by Process and Surface Finish).
• How much force and stress this interference makes is on the Press, Shrink and Expansion Fits page.

Example 3: the same fit from either basis

• Hole basis, 25H7/k6: hole 25.000 to 25.021, shaft 25.002 to 25.015: -15 to +19 µm.
• Shaft basis, 25K7/h6: the K7 hole is ES = 6 and EI = -15 µm (K with its grade correction: the lower limit of k6 is +2, and IT7 − IT6 = 8 µm is added), so the hole is 24.985 to 25.006 and the h6 shaft is 24.987 to 25.000: -15 to +19 µm.
• The same transition fit, reached two ways. Use whichever basis suits the stock and tooling.

Troubleshooting

Symptom	Likely cause	Try
A press fit goes in by hand	Hole at the loose end of H7 and shaft at the loose end of p6; p6 is only a transition fit below 3 mm	Check the size range; use the next tighter letter, or check the actual measured sizes
A sliding fit seizes	Shaft at the large end of g6, hole at the small end of H7, plus heat or dirt: the minimum clearance is only a few micrometers	Use H8/f7 for more clearance, or check temperature and surface finish
Fit limits do not match a colleague’s	Different size range (the IT row or the deviation sub-range), or a different edition of the standard	Check which table and size ranges were used; sizes at a boundary belong to the lower range
K7/h6 and H7/k6 give different numbers	The grade correction (delta) was left out of the hole deviation	For K to ZC holes up to IT7 or IT8 add IT(n) − IT(n−1) to the upper deviation
Tolerance grade cannot be made	IT6 and IT7 need grinding, honing or reaming	Loosen the grade or change the process

Related

• Fits & Tolerances: the overview and quick reference.
• How Fits Work: clearance, transition and interference; hole and shaft basis.
• Fits – Inch (ANSI B4.1): the inch fit classes.
• Press, Shrink and Expansion Fits: what an interference fit does to the parts.
• Tolerances by Process and Surface Finish: which process holds which IT grade.
• Fit Calculator (ISO 286): limits for any size and fit.

FAQ

What is the difference between the IT grade and the fundamental deviation?

The IT grade sets how wide the tolerance zone is. The fundamental deviation (the letter) sets where the zone sits relative to the basic size. You need both to know the limits.

Which sizes belong to which row?

A size on the boundary belongs to the lower range: 30 mm is in the “over 18 to 30” row, and 30.1 mm is in “over 30 to 50”. The Handbook’s tables use “over” and “to” in this way.

Why does the Handbook give IT01 to IT16 and not IT18?

It reproduces the older BS 4500:1969 and ANSI B4.2 tables. ISO 286-1:2010 extends the series to IT18 and to larger sizes; the grades up to IT16 and the sizes up to 500 mm cover almost all ordinary work.

Can I use any letter and grade together?

Yes. The system is built so any hole zone can be combined with any shaft zone. The ten preferred fits are the ones to try first; the Handbook’s Table 1 gives the first-choice zones, and the standard lists second and third choices.

What does js mean?

A symmetric zone: ±IT/2 about the basic size. For example 25js6 is 25 ± 0.0065 mm (IT6 = 13 µm). In grades IT7 to IT11 the Handbook’s rule is that an odd IT value is replaced by the even value just below before halving, so 25js7 (IT7 = 21 µm) is ± 0.010 mm.

Is ANSI B4.2 the same as ISO 286?

The preferred metric fits in ANSI/ASME B4.2 use the ISO system. B4.2 selects first-choice preferred sizes and zones from it for use in the United States; the underlying tolerances and deviations come from ISO 286.

References

• Machinery’s Handbook, 31st edition, Industrial Press: “ANSI/ASME Preferred Metric Limits and Fits” (Table 1, description of preferred fits; Tables 2 to 5, preferred fits) and “British Standard Metric ISO Limits and Fits” (Table 4, standard tolerances; Tables 5a and 5b, fundamental deviations for shafts; Tables 6a and 6b, holes).
• ANSI/ASME B4.2-1978 (R2009), Preferred Metric Limits and Fits; BS 4500:1969, ISO limits and fits.
• ISO 286-1:2010 and ISO 286-2:2010, Geometrical product specifications (GPS): ISO code system for tolerances on linear sizes.

Last reviewed: October 2026. The limit tables on this page are computed from the Handbook’s Table 4 and Tables 5a and 5b and agree with its printed fit tables at the 10 mm, 25 mm and 60 mm checks.