ISO 286 defines the international tolerance and fit system for linear dimensions up to 3150 mm (DIN ISO 286-1:2010). H7 designates the bore tolerance, k6 the shaft tolerance — H7/k6 is the standard transition fit for rolling bearing inner rings under normal load. Before this system existed, manufacturers in different countries worked according to national tolerance standards that differed in numerical values, letter designations and basic philosophy. ISO 286, adopted in Germany as DIN ISO 286, created a uniform language: the same specification — such as H7/k6 — means the same dimensional ratio worldwide.
The current edition ISO 286-1:2010 defines the entire tolerance system for linear dimensions up to 3150 mm. Part 2 contains the standard tolerances and fundamental deviations in tabular form. This guide provides the numerical values: which IT tolerance results in which dimension, which fit belongs to which application? Conceptual principles (fit types, assembly instructions) are explained in the companion article Shafts and bearings: Understanding fits.
Key takeaway: H7/k6 is the standard transition fit for rolling bearing inner rings under normal load. H7/g6 is the typical clearance fit for plain bearings. H7/s6 stands for fixed interference fits (thermal joining). The exact µm values for your size can be found in the tables below.
Basics ISO 286: Four key terms
The ISO 286 system is based on four terms that build on each other:
- Nominal dimension: The theoretically exact dimension from the drawing, e.g. ⌀50 mm. It is the reference point for all dimensions, not the actual manufactured dimension.
- Fundamental deviation: Determines the position of the tolerance zone in relation to the zero line — whether the dimension starts above or below the nominal dimension. It is determined by the letter.
- Tolerance grade (IT grade): Describes the width of the permissible dimensional range. IT1 is extremely narrow (precision metrology), IT18 is very wide (raw casting dimensions). The number in the abbreviation determines the IT grade.
- Tolerance zone: The combination of fundamental deviation and IT grade. It is the zone between the largest and smallest permissible dimension.
The abbreviation clearly separates bore and shaft: Capital letter = Bore (inside dimension, A–ZC), Lowercase letter = Shaft (outside dimension, a–zc). The specification H7/k6 means: Bore with tolerance zone H, IT grade 7; shaft with tolerance zone k, IT grade 6.
Letters A to G (bores) / a to g (shafts): Fundamental deviation on the minus side of the zero line → create clearance. Letter H: lower deviation exactly on the zero line (EI = 0) → hole-basis. Letters K to ZC / k to zc: fundamental deviation on the plus side → create interference.
Basic tolerances IT01-IT18
The table shows the basic tolerance values in micrometers (µm) in accordance with ISO 286-1:2010 for four nominal dimension ranges. ⌀10 = range >6-10 mm, ⌀30 = >18-30 mm, ⌀50 = >30-50 mm, ⌀100 = >80-120 mm.
| IT grade | Typical use | ⌀10 | ⌀30 | ⌀50 | ⌀100 |
|---|---|---|---|---|---|
| IT01 | Gauge blocks, gauges of the highest quality | 0.4 | 0.6 | 0.6 | 1 |
| IT0 | Gauge blocks, precision gauges | 0.6 | 1 | 1 | 1.5 |
| IT1 | Precision gauges | 1 | 1.5 | 1.5 | 2.5 |
| IT2 | Finest fits | 1.5 | 2.5 | 2.5 | 4 |
| IT3 | Fine fits | 2.5 | 4 | 4 | 6 |
| IT4 | Fine fits (e.g. rolling bearing P5) | 4 | 6 | 7 | 10 |
| IT5 | Precise fits, rolling bearings (k5, m5) | 6 | 9 | 11 | 15 |
| IT6 | Standard mechanical engineering fits (k6) | 9 | 13 | 16 | 22 |
| IT7 | Standard fits, H7 bore | 15 | 21 | 25 | 35 |
| IT8 | Rough fits, H8 bore | 22 | 33 | 39 | 54 |
| IT9 | General fits, h9 shafts | 36 | 52 | 62 | 87 |
| IT10 | Rough tolerances | 58 | 84 | 100 | 140 |
| IT11 | H11 bores, rough fits | 90 | 130 | 160 | 220 |
| IT12 | Rough press fits | 150 | 210 | 250 | 350 |
| IT13 | Sheet metal parts, stamped parts | 220 | 330 | 390 | 540 |
| IT14 | General tolerances coarse | 360 | 520 | 620 | 870 |
| IT15 | Free-size tolerances | 580 | 840 | 1,000 | 1,400 |
| IT16 | Free-size tolerances, wide | 900 | 1,300 | 1,600 | 2,200 |
| IT17 | Raw castings, forgings | 1,500 | 2,100 | 2,500 | 3,500 |
| IT18 | Raw castings, widest tolerance | 2,200 | 3,300 | 3,900 | 5,400 |
All values in µm. Bold indicates IT5–IT7, which are most relevant for fitting applications in mechanical engineering.
Hole-basis system (H) vs. shaft-basis system (h)
ISO 286 recognizes two system approaches: The hole-basis system keeps the bore tolerance constant (always H) and varies the shaft tolerances (g6, h6, k6, p6 …). The shaft-basis system keeps the shaft tolerance constant (always h) and varies the bore tolerances (F7, H7, K7, P7 …).
In practice, the hole-basis system dominates: standard drilling tools (drills, reamers) automatically provide an H tolerance. A single reamer for H7⌀30 covers all fit types — from clearance fit with g6 to interference fit with s6 — because the adjustment is made exclusively on the shaft by turning or grinding.
The shaft-basis system is recommended if a shaft is sourced off the shelf in a fixed size (e.g. drawn round steel h11) and several mating partners (bearing, pulley, gear) require different fit types. Then you can vary with different bore tolerances without changing the shaft.
Fit types
Clearance fit
The tolerance zone of the bore is completely above that of the shaft — there is always positive clearance. Typical pairings: H7/g6 (plain bearings, guide pins), H7/f7 (running shaft in oil bearing), H8/f7 (loosely running hub). The clearance enables relative movement; sufficient lubrication is a prerequisite.
Transition fit
The tolerance zones overlap. Depending on the actual individual dimension, there is either a slight clearance or a slight interference. Typical pairings: H7/k6 (gear wheel, rolling bearing inner ring normal), H7/m6 (coupling hub), H7/n6 (pin connection). Transition fits are always combined with a feather key or pin, as the frictional connection alone is not sufficient.
Press fit
The tolerance zone of the shaft is completely above that of the bore — there is always interference and therefore a force-fit connection. Typical pairings: H7/p6 (bearing inner ring with rotating load, slight interference fit), H7/r6 (bushing, tight fit, medium interference), H7/s6 (fixed hub, large interference — thermal joining often necessary). Press fits transmit torque without additional form-fit elements.
Top 15 standard fits: Clearance and interference in µm
All fits in the hole-basis system. Format: Minimum … Maximum in µm. Positive = clearance (bore > shaft), negative = interference (shaft > bore).
¹ H7/p6 at ⌀10: Maximum +2 µm clearance possible - standard-compliant, to be designed as a press fit.
| Fit | Type | ⌀10 | ⌀30 | ⌀50 | ⌀100 | Typical application |
|---|---|---|---|---|---|---|
| H7/g6 | Clearance | +5 … +29 | +7 … +41 | +9 … +50 | +12 … +69 | Plain bearing, guide pin |
| H7/h6 | Clearance | 0 … +24 | 0 … +34 | 0 … +41 | 0 … +57 | Standard shaft, detachable key hub |
| H7/f7 | Clearance | +13 … +43 | +20 … +62 | +25 … +75 | +36 … +106 | Running fit, oil bearing |
| H8/f7 | Clearance | +13 … +50 | +20 … +74 | +25 … +89 | +36 … +125 | Loose running shaft |
| H8/h9 | Clearance | 0 … +58 | 0 … +85 | 0 … +101 | 0 … +141 | Loose standard fit, spacer sleeve |
| H9/h9 | Clearance | 0 … +72 | 0 … +104 | 0 … +124 | 0 … +174 | Rough fit, covers |
| H11/h11 | Clearance | 0 … +180 | 0 … +260 | 0 … +320 | 0 … +440 | Very rough assembly fit |
| H7/js6 | Transition | −5 … +19 | −7 … +27 | −8 … +33 | −11 … +46 | Easily detachable hub, symmetrical transition |
| H6/k5 | Transition | −7 … +8 | −11 … +11 | −13 … +14 | −18 … +19 | Precision rolling bearings (P5) |
| H7/k6 | Transition | −10 … +14 | −15 … +19 | −18 … +23 | −25 … +32 | Gear, rolling bearing inner ring — standard |
| H7/m6 | Transition | −13 … +11 | −21 … +13 | −25 … +16 | −35 … +22 | Coupling hub, fixed hub with key |
| H7/n6 | Transition | −19 … +5 | −28 … +6 | −33 … +8 | −45 … +12 | Pin connection, ring fit |
| H7/p6 | Press | −22 … +2 ¹ | −35 … −1 | −42 … −1 | −59 … −2 | Bearing inner ring, rotating load |
| H7/r6 | Press | −25 … −1 | −41 … −7 | −50 … −9 | −76 … −19 | Bushing, tight fit, medium interference |
| H7/s6 | Press | −32 … −8 | −48 … −14 | −59 … −18 | −93 … −36 | Hub fixed, thermal joining |
Selection matrix: Application → recommended fit
The following matrix translates common design tasks directly into a fit recommendation (hole-basis system).
| Use case | Recommended fit | Type | Note |
|---|---|---|---|
| Rolling bearing inner ring, light/normal rotating load | H7/k6 or H6/k5 | Transition | Circumferential load → Seat must remain firm; k5 for P5 bearing |
| Rolling bearing inner ring, heavy/impact load | H7/m6 or H7/p6 | Transition / Press | Larger interference prevents the inner ring from creeping |
| Rolling bearing inner ring, vertical (point load) | H7/h6 or H7/g6 | Clearance | Can be dismantled; inner ring does not rotate |
| Pulley, detachable (maintenance) | H7/h6 or H7/js6 | Clearance / Transition | With feather key, pulley must be removable |
| Pulley, fixed (no removal) | H7/k6 or H7/m6 | Transition | With feather key; pulley seats without play |
| Gear wheel on shaft, backlash-free | H7/k6 or H7/m6 | Transition | Torque transmission via feather key DIN 6885 or custom gear profile |
| Coupling hub (detachable coupling) | H7/k6 | Transition | Standard for detachable couplings; with feather key |
| Cylindrical pin (DIN 7) | H7/p6 or H7/r6 | Press | Dowel pin can only be released by pressing or driving out |
| Sliding bushing, replaceable | H7/f7 or H7/g6 | Clearance | Easy to insert and replace, no tools required |
Practical Tip from TEA:
From our procurement experience: complaints about fitted parts arise far less often from the wrong choice of fit than from unclear or missing tolerance specifications on the drawing. Always state the basis system (hole-basis H is the standard) plus the complete designation combination — for example ⌀30 H7/s6 for a true press fit or ⌀30 H7/k6 for a firm transition fit, instead of just writing „press fit“ on the drawing. And choose the coarsest fit that still reliably fulfils the function: a tighter tolerance than necessary (e.g. IT5 instead of IT6/IT7) drives manufacturing cost and lead time up unnecessarily.
Calculating press fits: joint pressure, torque and joining temperature
With a press fit (interference fit, e.g. H7/s6 or H7/u6), the interference generates a joint pressure between shaft and hub — from it follow the transmissible torque and the required joining temperature. The design of interference assemblies is standardised in DIN 7190-1. The following approximation formulas apply to the most common case: solid shaft and hub made of the same material (steel).
Joint pressure from the interference
What matters is not the measured interference but the effective interference ξ: during joining the roughness peaks are flattened, which reduces the interference by approx. 0.8·(Rzshaft + Rzhub). With the effective (diametral) interference ξ, the joint diameter d, the hub outside diameter da and the modulus of elasticity E (steel ≈ 210 000 N/mm²):
p = (E · ξ / d) · (da² − d²) / (2 · da²)
Transmissible torque and axial force
Through the friction at the joint seat (coefficient of friction μ ≈ 0.1 for steel/steel, dry) and the joint length l, the assembly transmits:
Torque: T = μ · p · π · d² · l / 2
Axial force: F = μ · p · π · d · l
Worked example
Shaft ⌀30 mm in a hub with da = 60 mm, joint length l = 40 mm, effective interference ξ = 20 µm, steel/steel (E = 210 000 N/mm², μ = 0.1):
- Joint pressure: p = (210 000 · 0.020 / 30) · (60² − 30²)/(2 · 60²) ≈ 52 N/mm²
- Torque: T = 0.1 · 52.5 · π · 30² · 40 / 2 ≈ 297 N·m
- Press-in force: F = 0.1 · 52.5 · π · 30 · 40 ≈ 19.8 kN
Joining temperature (thermal joining)
Instead of pressing, the hub is heated (or the shaft cooled) until the interference plus a small assembly clearance is bridged. With the coefficient of thermal expansion α (steel ≈ 11·10⁻⁶ 1/K):
ΔT = (ξmax + assembly clearance) / (α · d)
For ξmax = 30 µm plus 20 µm assembly clearance at d = 30 mm: ΔT = 0.050 / (11·10⁻⁶ · 30) ≈ 150 K — the hub therefore needs to be heated to around 150 °C above room temperature. Stay below about 300 °C so that the quenched-and-tempered microstructure is not affected.
These formulas are approximations for a solid shaft and a hub made of the same steel. For hollow shafts, mixed materials, high rotational speeds (centrifugal force) or safety-relevant connections, the full design to DIN 7190-1 is authoritative — we are happy to support you with the design.
Bearing fits according to SKF/INA convention
Rolling bearing manufacturers distinguish between two load cases. Circumferential load (rotating ring): The ring rotates relative to the load — it must have a tight fit, otherwise it will creep and widen the seat. Point load (stationary ring): The load always acts at the same point — slight clearance is permissible and even desirable so that the ring rotates slowly and wear is distributed evenly.
| Component / load case | Shaft tol. (inner ring) | Housing tol. (outer ring) |
|---|---|---|
| Inner ring, normal circumferential load | k5, k6, m5, m6 | H7, J7 |
| Inner ring, heavy / impact-type circumferential load | n6, p6 | H7 |
| Inner ring, point load (stationary) | g6, h5, h6, js5, js6 | H7, H8 |
| Outer ring, normal point load (located in the housing) | – | H7, J7 |
| Outer ring, circumferential load (rotates in the housing) | – | K7, M7, N7 |
Detailed calculations and installation instructions can be found in the article Shafts and bearings: Understanding fits.
Common errors with ISO 286 fits
Error 1: Tolerance and surface roughness mixed up
IT6 at ⌀50 is 16 µm. If the surface roughness Rz = 16 µm (Ra ≈ 3.2 µm), roughness peaks fill the entire tolerance zone — the fit will not function. Rule of thumb: Rz ≤ 25 % of the IT value. For IT6 (16 µm) therefore Rz ≤ 4 µm (Ra ≤ 0.8 µm). Also note for interference fits: roughness peaks are flattened during assembly — the effective interference decreases by approx. 0.6 × Rz.
Error 2: Thermal effects on interference fits ignored
Steel expands by approx. 11–12 µm/(m·K), aluminium by approx. 23 µm/(m·K). With a steel shaft in an aluminium hub (H7/s6, ⌀50) and +80 °C operating temperature, the hub expands by approx. 48 µm more than the shaft. This can completely eliminate the nominal minimum interference of 18 µm and loosen the joint. With a material mix, the interference must be thermally designed or the joint secured by form fit.
Error 3: ISO 2768 instead of ISO 286 for fitting dimensions
ISO 2768 (general tolerances) roughly corresponds to IT12–IT14. If you do not enter an explicit tolerance for a rolling bearing pairing and rely on "ISO 2768 medium", you will get around IT13 ≈ 390 µm at ⌀50 — 15 times as much as IT7. Every mating surface requires an explicit ISO 286 tolerance in the drawing.
Error 4: Transition fit without form fit
H7/k6 at ⌀30 has a possible clearance of up to +19 µm. If the most unfavourable tolerance combination occurs, the hub is seated with minimal frictional grip — or with clearance. A transition fit must never be relied upon as the sole means of torque transmission. A feather key, clamping sleeve or clamping element is always required.
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