How Fits Work
Clearance, transition and interference fits, hole and shaft basis, allowance, and how to choose a fit by function.
A fit is what happens when two parts go together: loose, snug, or tight. It is set by the limits of size you put on the two parts, and it decides whether a shaft turns, slides, locates, or stays put. This page explains the three kinds of fit, how to compute the range of any fit, the two ways of fixing the basic size (hole basis and shaft basis), and how to choose a fit for a job. The detailed tables are on the Fits – Metric (ISO System) and Fits – Inch (ANSI B4.1) pages. It is part of the Fits & Tolerances guide.
What a fit is
The Machinery’s Handbook puts it plainly: fits between cylindrical parts govern the proper assembly and performance of many mechanisms. Clearance fits permit relative freedom of motion between a shaft and a hole, axially, radially, or both. Interference fits secure a certain amount of tightness between parts, whether they are meant to stay assembled or to be taken apart from time to time. Or two parts may need to fit together snugly, without apparent tightness or looseness. The designer’s job is to specify these different types of fits in such a way that the shop can produce them.
That requires two limits of size for the hole and two for the shaft, which is why a fit is always a pair of tolerances, never one.
Clearance, transition and interference
| Type | Definition (ANSI B4.1) | What it does | Typical use |
|---|---|---|---|
| Clearance fit | The limits of size are so specified that a clearance always results when the parts are assembled | The shaft is always smaller than the hole; the parts can move, or be assembled by hand | Shafts in bearings, sliding parts, loose locating features |
| Transition fit | The limits are so specified that either a clearance or an interference may result | The shaft may be slightly smaller or slightly larger than the hole, depending on where the two parts fall in their tolerances | Accurate location of parts that must still be taken apart: gears, pulleys, couplings on shafts |
| Interference fit | The limits are so specified that an interference always results when the parts are assembled | The shaft is always larger than the hole; assembly needs force, heat or cold | Permanent or heavily loaded joints: hubs, wheels, bushings, bearing races |
Computing a fit
Minimum clearance = smallest hole − largest shaft
Maximum clearance = largest hole − smallest shaft
Interference = −clearance (a negative clearance is an interference)
Fit range = hole tolerance + shaft tolerance
The smallest hole minus the largest shaft is the allowance: the tightest the fit can be. A positive allowance is a minimum clearance; a negative allowance is a maximum interference. The spread between the loosest and tightest assembly is the sum of the two tolerances, which is why making either part tighter tightens the whole fit.
Worked example: the Handbook’s loose running fit
A 60 mm loose running fit, 60H11/c11 (the Handbook’s Table 2 values): hole 60.000 to 60.190, shaft 59.670 to 59.860.
Minimum clearance = 60.000 − 59.860 = 0.140
Maximum clearance = 60.190 − 59.670 = 0.520
Fit range = 0.190 + 0.190 = 0.380 = 0.520 − 0.140
The shaft is always smaller than the hole, so this is a clearance fit with 0.140 to 0.520 mm of play. The Handbook shows the same fit written the other way, shaft basis (C11/h11), with the same clearances; see the next section.
Hole basis and shaft basis
| System | The fixed part | The variable part | ISO symbol |
|---|---|---|---|
| Basic hole system (hole basis) | The hole: its smallest size is the basic size (lower deviation zero). The hole letter is H | The shaft takes the tolerance zone that gives the fit | H7/g6, H7/p6 |
| Basic shaft system (shaft basis) | The shaft: its largest size is the basic size (upper deviation zero). The shaft letter is h | The hole takes the tolerance zone that gives the fit | G7/h6, P7/h6 |
- Hole basis is the default in both standards. Holes are made with fixed-size tools (drills, reamers, broaches) and checked with plug gauges, so it is cheaper to have one hole size and vary the shaft, which is easy to turn or grind to any size.
- Shaft basis is better when a single shaft, such as drawn or ground bar stock, has to take several different fits along its length (several bearings, collars and gears on one shaft), because the shaft can be left at one size.
- The two are equivalent. The Handbook notes that each hole-basis fit has a shaft-basis equivalent with the same clearances or interferences. To convert, swap the fundamental deviations between hole and shaft and keep the same IT grade on each part.
Choosing a fit by function
Start from what the joint has to do, then pick the lightest fit that does it. The table gives the Handbook’s descriptions of the preferred metric fits (ISO symbols, hole basis shown first, shaft basis equivalent beside it) and the closest inch fit classes (ANSI B4.1).
| The joint must… | ISO fit (hole basis / shaft basis) | ANSI inch class | Handbook description |
|---|---|---|---|
| run loosely, with wide commercial tolerances | H11/c11 C11/h11 | RC 8, RC 9 | Loose running: wide commercial tolerances or allowances on external members |
| run freely, with large temperature changes, high speed or heavy pressures | H9/d9 D9/h9 | RC 7 | Free running: not for use where accuracy is essential |
| run on accurate machines with moderate speed and pressure | H8/f7 F8/h7 | RC 4 to RC 6 | Close running |
| move and turn freely and locate accurately, but not run freely | H7/g6 G7/h6 | RC 1 to RC 3 | Sliding fit |
| locate stationary parts that can be freely assembled and taken apart | H7/h6 H7/h6 | LC | Locational clearance: a snug fit |
| locate accurately, but with some interference allowed | H7/k6 K7/h6 | LT | Locational transition: a compromise between clearance and interference |
| locate more accurately, with greater interference | H7/n6 N7/h6 | LT | Locational transition, more interference |
| be rigid and aligned, without special bore-pressure needs | H7/p6 P7/h6 | LN | Locational interference: rigidity and alignment of prime accuracy of location |
| drive on, for ordinary steel parts or light shrink fits | H7/s6 S7/h6 | FN 1, FN 2 | Medium drive: the tightest fit usable with cast iron |
| be force fitted, or a heavy shrink fit | H7/u6 U7/h6 | FN 3 to FN 5 | Force fit for highly stressed parts, or shrink fits where heavy pressing forces are impractical |
Two cautions. First, the pairing of ISO and inch classes is approximate: the two systems give different clearance patterns, so check the limits against the tables for the size you actually have. Second, H7/p6 is a transition fit for basic sizes up to 3 mm, and an interference fit above that (the Handbook’s footnote to Table 1).
Questions that decide the fit
- Does the joint move? If it has to rotate or slide, it needs clearance, and enough of it for the lubricant film, temperature changes and speed. Running fits get looser with size, speed and temperature range.
- Must it be taken apart? Transition and light interference fits can be disassembled; heavy interference fits cannot without damage.
- How accurate must the location be? The tighter the fit range, the better the location. A clearance fit always allows some play, which is lost accuracy.
- Does it carry torque or load by friction alone? That needs interference; the pressure it creates and the stress it puts in the hub are on the Press, Shrink and Expansion Fits page.
- What will the temperature do? Parts of different materials change size by different amounts; see Temperature, Coatings and Fit Problems.
- Can the shop make it? Tight fits need fine processes (grinding, honing, reaming); see Tolerances by Process and Surface Finish.
Two systems: inch and metric
| Inch (ANSI/ASME B4.1) | Metric (ISO 286, ANSI/ASME B4.2) | |
|---|---|---|
| Designation | Class letters and number: RC 4, LC 6, LT 2, LN 3, FN 2 | Tolerance symbols: H7/g6 (letter = position, number = IT grade) |
| Tolerance grades | Standard tolerance grades 4 to 13 (in thousandths of an inch) over size ranges to 200 inches | IT01, IT0, IT1 up to IT16 in the Handbook’s ISO table, with 27 fundamental deviation positions for sizes up to 500 mm |
| How fits are given | Each class has ready-made limits of clearance or interference for each size range | You pick the hole zone and the shaft zone, and calculate or look up the limits |
| Basis | Hole basis (modified fits for shaft basis, symbol S, and bilateral hole, symbol B) | Hole basis (H) or shaft basis (h), freely combined |
| Where used | US inch drawings | Most of the rest of the world, and metric work in the US |
Worked examples
Example 1: the Handbook’s Class RC 1 fit
An RC 1 close sliding fit is required to assemble a hole and a 2-inch shaft with no perceptible play. For that size the Handbook gives hole limits 2.0000 to 2.0005 and shaft limits 1.9993 to 1.9996 inch.
- Minimum clearance = 2.0000 − 1.9996 = 0.0004 inch.
- Maximum clearance = 2.0005 − 1.9993 = 0.0012 inch.
- Fit range = 0.0005 + 0.0003 = 0.0008 inch, the same as 0.0012 − 0.0004.
Example 2: three fits on one hole (25 mm)
| Fit | Hole (H7) | Shaft | Result |
|---|---|---|---|
| H7/g6 | 25.000 to 25.021 | 24.980 to 24.993 | Clearance 7 to 41 µm |
| H7/k6 | 25.000 to 25.021 | 25.002 to 25.015 | Transition: up to 19 µm clearance or 15 µm interference |
| H7/p6 | 25.000 to 25.021 | 25.022 to 25.035 | Interference 1 to 35 µm |
The hole is the same in all three; only the shaft changes. H7/g6 is the sliding fit, H7/k6 can be assembled by hand but with no play, and H7/p6 always needs force. This is what “hole basis” buys: one reamer, three fits.
Example 3: converting hole basis to shaft basis
The Handbook converts a 60 mm loose running fit from hole basis (60H11/c11) to shaft basis (60C11/h11) by swapping the deviation letters and keeping the grades.
- Hole basis: hole 60.000 to 60.190, shaft 59.670 to 59.860: clearance 0.140 to 0.520.
- Shaft basis: hole 60.140 to 60.330, shaft 59.810 to 60.000: minimum clearance 60.140 − 60.000 = 0.140, maximum 60.330 − 59.810 = 0.520.
- The fits are identical; the individual tolerance zones are different. Pick the basis for what is cheapest to make.
Troubleshooting
| Symptom | Likely cause | Try |
|---|---|---|
| The shaft seizes when warm | The clearance was set for room temperature; the shaft grows more than the housing | Use a looser fit, or choose materials with closer expansion |
| A press fit is looser than expected | Tolerances at the loose end, or the hub is thin or soft | Check the fit range, not just the nominal interference; see the press fit page |
| Parts assemble by hand when they should be pressed | A transition fit was used where an interference fit was needed | Use the next tighter fit; check the minimum interference |
| A sliding fit is too loose and rattles | The fit range is too wide for the accuracy needed | Tighten one part’s tolerance, or choose RC 1 / H7/g6 |
| One shaft needs different fits along its length | Hole basis forces several shaft sizes | Consider a basic shaft system |
Related
- Fits & Tolerances: the overview and quick reference.
- Dimensional Tolerancing Basics: limits of size, allowance and tolerance.
- Fits – Metric (ISO System): IT grades, deviations and preferred fits.
- Fits – Inch (ANSI B4.1): RC, LC, LT, LN and FN tables.
- Modifiers and Material Conditions: how fits relate to MMC and virtual condition.
FAQ
What is the difference between allowance and clearance?
Allowance is the prescribed minimum clearance (or maximum interference) between mating parts at their maximum-material limits. Clearance is whatever you actually get between two real parts, anywhere in the range.
Is “snug” a clearance fit?
Yes, if there is always some clearance (the Handbook’s locational clearance fits are snug but can be freely assembled and disassembled). A fit that may be either clearance or interference is a transition fit.
Why is hole basis preferred?
Because holes are made with fixed-size tools and gauged with plug gauges, so fewer hole sizes means fewer tools. Shafts are easy to turn or grind to any size.
Can I mix inch classes and ISO symbols?
Not on the same fit. Use one system for a given pair of parts. Converting between them means recalculating limits, not just renaming the symbols.
What does “H7/g6” actually mean?
The hole has fundamental deviation H and tolerance grade IT7; the shaft has fundamental deviation g and grade IT6. The sizes come from the tables, for the basic size in question.
Do fits apply to threaded parts?
Threaded parts have their own class-of-fit systems (see the Fits in Practice page), but the idea is the same: tolerances on the mating parts set how tight they are.
References
- Machinery’s Handbook, 31st edition, Industrial Press: “Limits and Fits” (definitions, description of fits), “ANSI/ASME Standard Limits and Fits”, and “ANSI/ASME Preferred Metric Limits and Fits”, including Table 1, Description of Preferred Fits.
- ANSI/ASME B4.1, Preferred Limits and Fits for Cylindrical Parts; ANSI/ASME B4.2, Preferred Metric Limits and Fits.
- ISO 286-1 and ISO 286-2, Geometrical product specifications (GPS): ISO code system for tolerances on linear sizes.
Last reviewed: October 2026. The 25 mm deviations are the standard ISO 286 values for H7, g6, k6 and p6; the 60 mm and 2 inch examples are the Handbook’s.