IT Grade Values — Fundamental Deviations
IT grade values define the magnitude of the tolerance zone. Lower IT number = tighter tolerance. The tolerance value increases with both the IT grade number and the nominal dimension size. Values below in µm (micrometres).
Values are total tolerance zone width in µm (not ± values). For example, IT6 on a 25mm shaft = 13 µm total, meaning ±6.5 µm if symmetrically disposed. The actual disposition (upper/lower deviation) depends on the fundamental deviation letter (see Notation section).
| IT Grade | 1–3 | 3–6 | 6–10 | 10–18 | 18–30 | 30–50 | 50–80 | 80–120 | 120–180 | 180–250 | 250–315 | 315–400 | 400–500 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| IT01 | 0.3 | 0.4 | 0.4 | 0.5 | 0.6 | 0.6 | 0.8 | 1 | 1.2 | 2 | 2.5 | 3 | 4 |
| IT0 | 0.5 | 0.6 | 0.6 | 0.8 | 1 | 1 | 1.2 | 1.5 | 2 | 3 | 4 | 5 | 6 |
| IT1 | 0.8 | 1 | 1 | 1.2 | 1.5 | 1.5 | 2 | 2.5 | 3.5 | 4.5 | 6 | 7 | 8 |
| IT2 | 1.2 | 1.5 | 1.5 | 2 | 2.5 | 2.5 | 3 | 4 | 5 | 7 | 8 | 9 | 10 |
| IT3 | 2 | 2.5 | 2.5 | 3 | 4 | 4 | 5 | 6 | 8 | 10 | 12 | 13 | 15 |
| IT4 | 3 | 4 | 4 | 5 | 6 | 7 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |
| IT5 | 4 | 5 | 6 | 8 | 9 | 11 | 13 | 15 | 18 | 20 | 23 | 25 | 27 |
| IT6 | 6 | 8 | 9 | 11 | 13 | 16 | 19 | 22 | 25 | 29 | 32 | 36 | 40 |
| IT7 | 10 | 12 | 15 | 18 | 21 | 25 | 30 | 35 | 40 | 46 | 52 | 57 | 63 |
| IT8 | 14 | 18 | 22 | 27 | 33 | 39 | 46 | 54 | 63 | 72 | 81 | 89 | 97 |
| IT9 | 25 | 30 | 36 | 43 | 52 | 62 | 74 | 87 | 100 | 115 | 130 | 140 | 155 |
| IT10 | 40 | 48 | 58 | 70 | 84 | 100 | 120 | 140 | 160 | 185 | 210 | 230 | 250 |
| IT11 | 60 | 75 | 90 | 110 | 130 | 160 | 190 | 220 | 250 | 290 | 320 | 360 | 400 |
| IT12 | 100 | 120 | 150 | 180 | 210 | 250 | 300 | 350 | 400 | 460 | 520 | 570 | 630 |
| IT13 | 140 | 180 | 220 | 270 | 330 | 390 | 460 | 540 | 630 | 720 | 810 | 890 | 970 |
| IT14 | 250 | 300 | 360 | 430 | 520 | 620 | 740 | 870 | 1000 | 1150 | 1300 | 1400 | 1550 |
| IT15 | 400 | 480 | 580 | 700 | 840 | 1000 | 1200 | 1400 | 1600 | 1850 | 2100 | 2300 | 2500 |
| IT16 | 600 | 750 | 900 | 1100 | 1300 | 1600 | 1900 | 2200 | 2500 | 2900 | 3200 | 3600 | 4000 |
Highlighted rows (IT5–IT8) cover the most common engineering fit applications. All values in µm.
IT Grade by Manufacturing Process
Each manufacturing process has an inherent capability range. Specifying a tolerance tighter than the process can reliably achieve drives up cost or requires a process change. Use this as a first-pass check during design.
Every tightening of one IT grade roughly doubles manufacturing cost. IT6 costs approximately 4× more than IT8 for the same feature. Always ask: what is the functional requirement? — not what is the tightest tolerance achievable. The cost-optimal approach is to specify IT grades at the lowest number that satisfies function.
Types of Fits
A fit is the relationship between two mating parts. ISO 286 defines fits using a letter (fundamental deviation) + number (IT grade). Uppercase = hole, lowercase = shaft.
The shaft is always smaller than the hole. A positive clearance always exists, even at maximum material condition. Used for free movement, sliding, and assembly ease.
H7/f7 — easy running
H8/e8 — loose running
H11/c11 — very loose
May result in either a small clearance or a small interference depending on actual sizes. Used for accurate location with some restraint. Requires press or mallet for assembly.
H7/n6 — drive fit
H7/m6 — snug fit
The shaft is always larger than the hole. Material must elastically (or plastically) deform for assembly. Transmits torque and axial force without fasteners.
H7/r6 — force fit
H7/s6 — shrink fit
H7/u6 — heavy drive
Common Fits — Application Guide
The most frequently used hole-basis fits. Hole basis (H) is preferred as it requires fewer reamers, broaches, and gauges than shaft basis. All hole tolerances are H (fundamental deviation = zero, tolerance opens upward).
| Fit Designation | Type | Character | Typical Application |
|---|---|---|---|
| H11/c11 | Clearance | Very loose | Rough bearing, wide clearance for contaminated environments |
| H9/d9 | Clearance | Loose running | Loose pulleys, agricultural machinery, non-precision shafts |
| H8/f7 | Clearance | Easy running | General purpose running fits, journal bearings, gearboxes |
| H7/g6 | Clearance | Close sliding | Precision sliding parts, pistons, accurate guides |
| H7/h6 | Clearance | Sliding | Parts that must assemble without play — spigots, mandrels |
| H7/k6 | Transition | Push fit | Location fit, easily assembled with mallet — gear hubs, couplings |
| H7/n6 | Transition | Drive fit | Light press, driven with press — pulleys, hubs, permanent location |
| H7/p6 | Interference | Press fit | Standard press fit — bearing housings, keyed assemblies |
| H7/r6 | Interference | Force fit | Permanent assemblies — sleeve bearings, collar on shaft |
| H7/s6 | Interference | Shrink fit | Heated assembly — ring gears, heavy-duty transmissions |
| H7/u6 | Interference | Heavy drive | Large interference, hydraulic press required — high-torque flanges |
Fundamental Deviation Letters
The fundamental deviation defines the position of the tolerance zone relative to the nominal dimension. Letter codes are standardised by ISO 286.
| Letter | Applies to | Position of Tolerance Zone | Typical Use |
|---|---|---|---|
| A–H (uppercase) | Holes | Below nominal (clearance when paired with shaft) | A=largest clearance → H=zero deviation |
| H (uppercase) | Holes | Deviation = 0, tolerance opens upward | Standard hole-basis reference hole |
| J–ZC (uppercase) | Holes | Above nominal (transition/interference) | J, K = small interference; N,P...= increasing interference |
| a–h (lowercase) | Shafts | Below nominal (clearance fits) | a=large clearance → h=zero deviation |
| h (lowercase) | Shafts | Deviation = 0, tolerance opens downward | Standard shaft-basis reference shaft |
| j–zc (lowercase) | Shafts | Above nominal (transition/interference) | j, k = small interference; n,p...= increasing interference |
How to Read and Write Fit Notations
Fits are expressed as a fraction: hole tolerance over shaft tolerance. The complete designation includes nominal size, deviation letter, and IT grade number.
| Notation | Meaning | Interpretation |
|---|---|---|
| 25 H7/g6 | 25mm hole H7, shaft g6 | Clearance fit — precision sliding fit. Hole: +0/+21µm. Shaft: −7/−20µm |
| Ø50 H7 | 50mm hole, H7 only | Hole tolerance only specified; shaft tolerance TBD or individual |
| Ø30 k6 | 30mm shaft, k6 only | Shaft tolerance only; transition range. Use with H7 hole for push fit |
| 40 H7/p6 | 40mm hole H7, shaft p6 | Interference fit — standard press fit for bearing housing |
| H7/h6 | No nominal size shown | Applies to the feature size shown in the dimension. Sliding fit. |
Hole basis (H_ / _x): The hole is the constant member (H deviation = 0). The shaft is varied to achieve the desired fit. Preferred in most industries because standard reamers and broaches produce H-tolerance holes economically.
Shaft basis (_h / x_): The shaft is the constant member (h deviation = 0). Used when a single shaft must carry multiple components with different fits — for example, a motor shaft with a bearing (interference) and a pulley (clearance) on the same diameter.