ISO tolerances and fits

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November 3, 2026  ·  Indumor

The machinist's reference handbook · Chapter 3 of 25← Previous  |  Next →  |  Print

In this chapter

  1. Zero line, deviations and IT grades
  2. The letters: where the tolerance zone sits
  3. Clearance and interference: the recommended fits
  4. General tolerances: ISO 2768
  5. Roughness: N grades and the symbol

A Ø30 shaft with h6 along its whole length, including the stretches where nothing sits on it. That is the most repeated mistake on the drawings that reach a shop: the draftsman puts the tightest tolerance he knows on the whole part, and the machinist ends up grinding what could have come off the lathe in one pass, or quoting a price that scares the customer away. A tolerance is a design decision: tight where there is a fit, general where there is not. The tables in this chapter are for choosing it and for reading it.

Zero line, deviations and IT grades

Every toleranced dimension has three numbers: the nominal size, the upper deviation and the lower deviation, in microns and with sign, measured from the zero line that runs through the nominal. For holes they are written in capitals (ES, EI), for shafts in lower case (es, ei), and the tolerance is the difference between the two. ISO 286-1 defines twenty standard grades, IT01 to IT18, and each one fixes how many microns of tolerance go with each range of diameters. Grinding and precision machining work in IT5 and IT6; IT7 is fine turning and milling; IT8 to IT10 is ordinary machining; IT11 to IT13 covers roughing, cut plate and dimensions without a fit. Inch shops have the same idea in ASME B4.1, with its RC, LC, LT, LN and FN classes; ASME B4.2 is the metric version and uses the ISO letters. Which instrument checks each grade is in chapter 2.

T = ES − EI (hole)     T = es − ei (shaft)

ES, EI: upper and lower deviations of the hole; es, ei: those of the shaft; all in µm, with sign, from the zero line. Maximum size = nominal + upper deviation; minimum size = nominal + lower deviation.

Standard tolerance grades IT5 to IT13, in µm (ISO 286-1:2010, table 1)
Nominal size
mm		IT5
µm		IT6
µm		IT7
µm		IT8
µm		IT9
µm		IT10
µm		IT11
µm		IT12
µm		IT13
µm
up to 3461014254060100140
3 to 6581218304875120180
6 to 10691522365890150220
10 to 1881118274370110180270
18 to 3091321335284130210330
30 to 501116253962100160250390
50 to 801319304674120190300460
80 to 1201522355487140220350540
120 to 18018254063100160250400630
180 to 25020294672115185290460720
250 to 31523325281130210320520810
315 to 40025365789140230360570890
400 to 50027406397155250400630970

Each range runs from the previous limit, excluded, to the one shown, included: 30 mm falls in "18 to 30" and 30.01 in "30 to 50".

The table values are not arbitrary: they come from the tolerance unit i, which grows with the cube root of the diameter, and each grade is a fixed multiple of i. From IT6 up each grade is 1.6 times the one before, and every five grades the tolerance multiplies by ten. D is the geometric mean of the range, which is why the table moves in steps rather than millimeter by millimeter.

i = 0.45 · ∛D + 0.001 · D

i: tolerance unit (µm). D: geometric mean of the range, √(D₁ · D₂), in mm. IT5 = 7 i, IT6 = 10 i, IT7 = 16 i, IT8 = 25 i, IT9 = 40 i, IT10 = 64 i, IT11 = 100 i, IT12 = 160 i, IT13 = 250 i. For 18 to 30 mm: D = 23.2, i = 1.31 µm and IT7 = 16 × 1.31 = 21 µm, the value in the table.

The letters: where the tolerance zone sits

The letter says on which side of the zero line the zone sits and how far from it. For shafts, a to h sit below the nominal (clearance against an H hole), js straddles it, and k onward sit above it (transition and interference). Holes follow the same logic in capitals, mirrored: H has its lower deviation at zero and is the basic hole. Nearly every shop works on the hole-basis system, an H hole with the fit chosen on the shaft, because the hole comes from a reamer or a drill and the shaft is corrected on the lathe with one more pass.

+−zero line = nominal sizeHoleShaftsHES = +ITEI = 0Tclarge clearancegsmall clearancehminimum clearance 0ktransitionpinterferencesheavy interferenceeseiNot to scale: the letter says on which side of the zero line the zone sits and how far from it; the IT grade sets its width
Figure 1. Tolerance zones relative to the zero line. Capitals are holes, lower case are shafts; combined with an H hole they give clearance (c, g, h), transition (k) or interference (p, s).
Deviations of holes H6, H7, H8, H9 and H11: upper over lower, in µm (ISO 286-2:2010)
Nominal size
mm		H6
ES / EI, µm		H7
ES / EI, µm		H8
ES / EI, µm		H9
ES / EI, µm		H11
ES / EI, µm
up to 3+6
0		+10
0		+14
0		+25
0		+60
0
3 to 6+8
0		+12
0		+18
0		+30
0		+75
0
6 to 10+9
0		+15
0		+22
0		+36
0		+90
0
10 to 18+11
0		+18
0		+27
0		+43
0		+110
0
18 to 30+13
0		+21
0		+33
0		+52
0		+130
0
30 to 50+16
0		+25
0		+39
0		+62
0		+160
0
50 to 80+19
0		+30
0		+46
0		+74
0		+190
0
80 to 120+22
0		+35
0		+54
0		+87
0		+220
0
120 to 180+25
0		+40
0		+63
0		+100
0		+250
0
180 to 250+29
0		+46
0		+72
0		+115
0		+290
0
250 to 315+32
0		+52
0		+81
0		+130
0		+320
0
315 to 400+36
0		+57
0		+89
0		+140
0		+360
0
400 to 500+40
0		+63
0		+97
0		+155
0		+400
0
Deviations of clearance shafts c11, d9, e8, f7, g6 and h6 to h11: upper over lower, in µm (ISO 286-2:2010)
Nominal size
mm		c11
es / ei, µm		d9
es / ei, µm		e8
es / ei, µm		f7
es / ei, µm		g6
es / ei, µm		h6
es / ei, µm		h7
es / ei, µm		h9
es / ei, µm		h11
es / ei, µm
up to 3−60
−120		−20
−45		−14
−28		−6
−16		−2
−8		0
−6		0
−10		0
−25		0
−60
3 to 6−70
−145		−30
−60		−20
−38		−10
−22		−4
−12		0
−8		0
−12		0
−30		0
−75
6 to 10−80
−170		−40
−76		−25
−47		−13
−28		−5
−14		0
−9		0
−15		0
−36		0
−90
10 to 18−95
−205		−50
−93		−32
−59		−16
−34		−6
−17		0
−11		0
−18		0
−43		0
−110
18 to 30−110
−240		−65
−117		−40
−73		−20
−41		−7
−20		0
−13		0
−21		0
−52		0
−130
30 to 40−120
−280		−80
−142		−50
−89		−25
−50		−9
−25		0
−16		0
−25		0
−62		0
−160
40 to 50−130
−290		−80
−142		−50
−89		−25
−50		−9
−25		0
−16		0
−25		0
−62		0
−160
50 to 65−140
−330		−100
−174		−60
−106		−30
−60		−10
−29		0
−19		0
−30		0
−74		0
−190
65 to 80−150
−340		−100
−174		−60
−106		−30
−60		−10
−29		0
−19		0
−30		0
−74		0
−190
80 to 100−170
−390		−120
−207		−72
−126		−36
−71		−12
−34		0
−22		0
−35		0
−87		0
−220
100 to 120−180
−400		−120
−207		−72
−126		−36
−71		−12
−34		0
−22		0
−35		0
−87		0
−220
120 to 140−200
−450		−145
−245		−85
−148		−43
−83		−14
−39		0
−25		0
−40		0
−100		0
−250
140 to 160−210
−460		−145
−245		−85
−148		−43
−83		−14
−39		0
−25		0
−40		0
−100		0
−250
160 to 180−230
−480		−145
−245		−85
−148		−43
−83		−14
−39		0
−25		0
−40		0
−100		0
−250
180 to 200−240
−530		−170
−285		−100
−172		−50
−96		−15
−44		0
−29		0
−46		0
−115		0
−290
200 to 225−260
−550		−170
−285		−100
−172		−50
−96		−15
−44		0
−29		0
−46		0
−115		0
−290
225 to 250−280
−570		−170
−285		−100
−172		−50
−96		−15
−44		0
−29		0
−46		0
−115		0
−290
250 to 280−300
−620		−190
−320		−110
−191		−56
−108		−17
−49		0
−32		0
−52		0
−130		0
−320
280 to 315−330
−650		−190
−320		−110
−191		−56
−108		−17
−49		0
−32		0
−52		0
−130		0
−320
315 to 355−360
−720		−210
−350		−125
−214		−62
−119		−18
−54		0
−36		0
−57		0
−140		0
−360
355 to 400−400
−760		−210
−350		−125
−214		−62
−119		−18
−54		0
−36		0
−57		0
−140		0
−360
400 to 450−440
−840		−230
−385		−135
−232		−68
−131		−20
−60		0
−40		0
−63		0
−155		0
−400
450 to 500−480
−880		−230
−385		−135
−232		−68
−131		−20
−60		0
−40		0
−63		0
−155		0
−400
Deviations of transition and interference shafts js6, k6, m6, n6, p6, r6 and s6, in µm (ISO 286-2:2010)
Nominal size
mm		js6
es / ei, µm		k6
es / ei, µm		m6
es / ei, µm		n6
es / ei, µm		p6
es / ei, µm		r6
es / ei, µm		s6
es / ei, µm
up to 3±3+6
0		+8
+2		+10
+4		+12
+6		+16
+10		+20
+14
3 to 6±4+9
+1		+12
+4		+16
+8		+20
+12		+23
+15		+27
+19
6 to 10±4.5+10
+1		+15
+6		+19
+10		+24
+15		+28
+19		+32
+23
10 to 18±5.5+12
+1		+18
+7		+23
+12		+29
+18		+34
+23		+39
+28
18 to 30±6.5+15
+2		+21
+8		+28
+15		+35
+22		+41
+28		+48
+35
30 to 40±8+18
+2		+25
+9		+33
+17		+42
+26		+50
+34		+59
+43
40 to 50±8+18
+2		+25
+9		+33
+17		+42
+26		+50
+34		+59
+43
50 to 65±9.5+21
+2		+30
+11		+39
+20		+51
+32		+60
+41		+72
+53
65 to 80±9.5+21
+2		+30
+11		+39
+20		+51
+32		+62
+43		+78
+59
80 to 100±11+25
+3		+35
+13		+45
+23		+59
+37		+73
+51		+93
+71
100 to 120±11+25
+3		+35
+13		+45
+23		+59
+37		+76
+54		+101
+79
120 to 140±12.5+28
+3		+40
+15		+52
+27		+68
+43		+88
+63		+117
+92
140 to 160±12.5+28
+3		+40
+15		+52
+27		+68
+43		+90
+65		+125
+100
160 to 180±12.5+28
+3		+40
+15		+52
+27		+68
+43		+93
+68		+133
+108
180 to 200±14.5+33
+4		+46
+17		+60
+31		+79
+50		+106
+77		+151
+122
200 to 225±14.5+33
+4		+46
+17		+60
+31		+79
+50		+109
+80		+159
+130
225 to 250±14.5+33
+4		+46
+17		+60
+31		+79
+50		+113
+84		+169
+140
250 to 280±16+36
+4		+52
+20		+66
+34		+88
+56		+126
+94		+190
+158
280 to 315±16+36
+4		+52
+20		+66
+34		+88
+56		+130
+98		+202
+170
315 to 355±18+40
+4		+57
+21		+73
+37		+98
+62		+144
+108		+226
+190
355 to 400±18+40
+4		+57
+21		+73
+37		+98
+62		+150
+114		+244
+208
400 to 450±20+45
+5		+63
+23		+80
+40		+108
+68		+166
+126		+272
+232
450 to 500±20+45
+5		+63
+23		+80
+40		+108
+68		+172
+132		+292
+252

r and s change value inside a main range from 50 mm up, and c from 30 mm up; that is why these tables follow the sub-ranges of the standard.

Clearance and interference: the recommended fits

With the hole and shaft limits you get the clearance or the interference of the assembly. Maximum clearance occurs with the largest hole and the smallest shaft; minimum, the other way round. If the minimum comes out negative, there is interference.

Max clearance = ES − ei     Min clearance = EI − es     Max interference = es − EI     Min interference = ei − ES

If the minimum clearance comes out negative, its absolute value is the maximum interference. Clearance fit: both clearances positive; interference fit: both negative; transition fit: one of each sign.

Ø30 H7/g6Hole (housing)Ø30 H7ShaftØ30 g6ES = +0.021 → max 30.021EI = 0 → min 30.000es = −0.007 → max 29.993ei = −0.020 → min 29.980Min clearance = EI − es = 0.007 mm · Max clearance = ES − ei = 0.041 mm
Figure 2. Ø30 H7/g6 on the hole-basis system: the hole runs from 30.000 to 30.021 and the shaft from 29.980 to 29.993; the clearance lands between 7 and 41 microns.

ISO 286-1 recommends a handful of fits so nobody has to pick among the thousand possible combinations. The table shows the most used ones worked out for Ø30, with what you feel when assembling them: by hand, with a soft hammer, in the press or by heating the hole. Bearings have rules of their own for the shaft and the housing, covered in chapter 17.

Recommended hole-basis fits, worked out for Ø30 mm, in µm (ISO 286 deviations)
FitTypeHole
ES / EI		Shaft
es / ei		Max clearance
µm		Min clearance
µm		Max interference
µm		Min interference
µm		Assembly and typical use
H7/g6Clearance+21
0		−7
−20		417––Close running: shafts that turn or slide
with lubrication and little play
H7/h6Clearance+21
0		0
−13		340––Locational clearance: hand-fitted, does not turn
(pilots, posts, guide bushes)
H7/js6Transition+21
0		±6.527.5–6.5–By hand or rubber mallet
(pulleys and gears that come off again)
H7/k6Transition+21
0		+15
+2		19–15–Soft hammer (bearings on shafts,
couplings, keyed gears)
H7/m6Transition+21
0		+21
+8		13–21–Light press; does not turn
or come apart often
H7/n6Transition+21
0		+28
+15		6–28–Press; gear rims and bushes that must
not turn, keyed
H7/p6Interference+21
0		+35
+22		––351Light press fit
(bushes in housings, bronze rims on hubs)
H7/s6Interference+21
0		+48
+35		––4814Heavy press or shrink fit
(rims, hubs that carry torque without a key)
H8/f7Clearance+33
0		−20
−41		7420––Normal running: plain bearings,
shafts in bushes, pumps
H8/e8Clearance+33
0		−40
−73		10640––Loose running: long bearings,
several supports, hot service
H9/d9Clearance+52
0		−65
−117		16965––Very loose: idler pulleys, farm machinery,
dirty bushes
H11/c11Clearance+130
0		−110
−240		370110––Large clearance: goes in dirty or hot
(hinge pins, brake shafts)

Worked example · a bronze bush pressed into a housing

A bronze bush, Ø50 outside and Ø40 inside, is pressed into a cast iron housing, and a Ø40 steel shaft turns inside it. H7/p6 is chosen for the bush in the housing and H8/f7 for the shaft in the bush. With the tables in this chapter:

  • Housing Ø50 H7: IT7 in the 30 to 50 range is 25 µm, so the bore runs from 50.000 to 50.025. Bush Ø50 p6: +42/+26, from 50.026 to 50.042. Interference at least 26 − 25 = 1 µm, at most 42 µm.
  • Bush Ø40 H8: IT8 is 39 µm, from 40.000 to 40.039. Shaft Ø40 f7: −25/−50, from 39.950 to 39.975. Clearance at least 25 µm, at most 89 µm.
  • When pressed in, the bush closes on the inside by roughly what it is squeezed on the outside, up to about 40 µm, and the Ø40 would go out of tolerance: the bore of the bush is reamed to H8 after assembly, not before.

ISO fit calculator

Hole and shaft limits and the resulting clearance or interference, with the ISO 286-2 deviations by diameter range (1 to 500 mm).

General tolerances: ISO 2768

Dimensions without a tolerance are not free: they are covered by the general tolerance written in the title block, almost always "ISO 2768-m" or "ISO 2768-mK". The lower-case letter is the class for linear and angular dimensions (f fine, m medium, c coarse, v very coarse) and the capital is the ISO 2768-2 class for straightness, flatness, perpendicularity and symmetry (H, K, L). An ordinary machined part goes in m; a welded or plasma-cut structure in c or v. If the title block says nothing, ask before cutting.

General tolerances for linear dimensions, ± mm (ISO 2768-1:1989, table 1)
Nominal size
mm		f (fine)
± mm		m (medium)
± mm		c (coarse)
± mm		v (very coarse)
± mm
0.5 to 30.050.10.2–
3 to 60.050.10.30.5
6 to 300.10.20.51
30 to 1200.150.30.81.5
120 to 4000.20.51.22.5
400 to 10000.30.824
1000 to 20000.51.236
2000 to 4000–248
General tolerances for external radii and chamfer heights, ± mm (ISO 2768-1, table 2)
Nominal size
mm		f
± mm		m
± mm		c
± mm		v
± mm
0.5 to 30.20.20.40.4
3 to 60.50.511
over 61122
General tolerances for angular dimensions, by length of the shorter side of the angle (ISO 2768-1, table 3)
Shorter side
mm		fmcv
up to 10±1°±1°±1°30′±3°
10 to 50±0°30′±0°30′±1°±2°
50 to 120±0°20′±0°20′±0°30′±1°
120 to 400±0°10′±0°10′±0°15′±0°30′
over 400±0°5′±0°5′±0°10′±0°20′

The capital letter comes with a caveat. ISO 2768-2 was withdrawn in 2021 and replaced by ISO 22081, which no longer has the H, K and L classes. They still turn up on plenty of drawings, especially old ones and spare parts, and that is where the tables below apply. On a new drawing, it is better not to call for them.

Straightness and flatness, in mm, by nominal length of the feature (ISO 2768-2:1989, table 1)
Nominal length
mm		H
mm		K
mm		L
mm
up to 100.020.050.1
10 to 300.050.10.2
30 to 1000.10.20.4
100 to 3000.20.40.8
300 to 10000.30.61.2
1000 to 30000.40.81.6
Perpendicularity, in mm, by length of the shorter side (ISO 2768-2:1989, table 2)
Shorter side
mm		H
mm		K
mm		L
mm
up to 1000.20.40.6
100 to 3000.30.61
300 to 10000.40.81.5
1000 to 30000.512
Symmetry, in mm, by nominal length (ISO 2768-2:1989, table 3)
Nominal length
mm		H
mm		K
mm		L
mm
up to 1000.50.60.6
100 to 3000.50.61
300 to 10000.50.81.5
1000 to 30000.512

Circular run-out from the same standard, table 4: H 0.1 mm, K 0.2 mm and L 0.5 mm.

Roughness: N grades and the symbol

Dimensional tolerance goes hand in hand with surface finish, and the two have to agree: an H7 bore with Ra 12.5 can neither be measured nor work. The usual pairing is Ra 0.8 to 1.6 for an H7/g6 fit, Ra 3.2 for general machining and Ra 6.3 to 12.5 for roughing and thermal cutting; inch drawings carry the same values in microinches, 32, 63, 125 and 250 µin, per ASME B46.1. The N grades of the old ISO 1302 are still on drawings and on the comparator blocks in the shop.

Roughness grades N1 to N12 with Ra in µm and µin, and the process that reaches them (ISO 1302:1992; the triangles are those of old drawings)
GradeRa
µm		Ra
µin		TrianglesTypical process
N10.0251∇∇∇∇Fine lapping, superfinishing
N20.052∇∇∇∇Lapping, mirror polishing
N30.14∇∇∇∇Fine honing, lapping
N40.28∇∇∇Honing, fine grinding
N50.416∇∇∇Fine grinding, diamond turning
N60.832∇∇∇Grinding, fine reaming
N71.663∇∇Finish turning and milling, reaming, broaching
N83.2125∇∇Normal turning and milling, fine drilling
N96.3250∇∇Rough turning and milling, drilling
N1012.5500∇Heavy roughing, sawing, plasma cutting
N11251000∇Oxy-fuel cutting, forging, die casting
N12502000∇Sand casting, hot rolling

The match with the triangles is approximate: DIN 3141, now withdrawn, defined them by roughness height, not by N grade.

turnedcRa 1.6aRz 6.3b⊥d2ea · Required roughness: Ra 1.6 µm (N7, 63 µin)b · Second requirement: Rz 6.3 µmc · Process, treatment or coating: turnedd · Lay direction: ⊥ (perpendicular to the view)e · Machining allowance: 2 mmOld drawings (ISO 1302 of 1992 and DIN):the Ra value sat in the opening of the V;earlier, triangles: ∇∇ ≈ N7-N9 and ∇∇∇ ≈ N4-N6any processremoval requiredremoval not permitted
Figure 3. Surface texture symbol per ISO 1302:2002 (ISO 21920-1 keeps the same layout) and the three basic forms of the symbol.

What roughness each process leaves, and how it is measured with a profilometer, is in chapter 14; geometric tolerance symbols are in chapter 25. Threads carry tolerances too (6H for the nut, 6g for the screw, with the same logic of letters and numbers), and that is where the book goes next: the metric thread chart of the following chapter.

Frequently asked questions

What does H7 mean on a drawing?

H is the basic hole: its lower deviation is zero and the upper one is +IT7. For a Ø30 bore IT7 is 21 µm, so the hole may measure from 30.000 to 30.021 mm.

What is the difference between H7/g6 and H7/k6?

H7/g6 is a clearance fit: at Ø30 the shaft is 7 to 41 µm smaller than the hole and turns or slides. H7/k6 is a transition fit, from 19 µm of clearance to 15 µm of interference, assembled with a soft hammer.

What tolerance applies to a dimension with no tolerance on the drawing?

The general tolerance in the title block, usually ISO 2768-m: ±0.1 mm up to 6 mm, ±0.2 up to 30, ±0.3 up to 120, ±0.5 up to 400 and ±0.8 up to 1000 mm.

Sources: ISO 286-1:2010 (tolerance system, IT grades and fundamental deviations); ISO 286-2:2010 (tables of shaft and hole deviations); ISO 2768-1:1989 (general tolerances); ISO 2768-2:1989 (classes H, K and L, withdrawn in 2021) and ISO 22081:2021, which replaces it; ISO 1302:1992 (N grades); ISO 1302:2002 and ISO 21920-1:2021 (indication of surface texture); ISO 4287 (Ra and Rz parameters); ASME B4.1 and B4.2 (preferred limits and fits); ASME B46.1 (surface texture).

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Chapter 2: caliper and micrometer

Which instrument checks each tolerance and how to read it.

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CNC bending tolerances

What to spec on the drawing of a bent part.

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Handbook index

The twenty-five chapters and the appendices.

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