Fits & Tolerances

Two questions decide whether parts work together. How much may each part vary? That is tolerancing. How tight or loose is the joint when the parts meet? That is a fit. This guide covers both, in plain language, with worked numbers, tables and diagrams. It leans on the Machinery’s Handbook and the standards behind it (ANSI/ASME B4.1 and B4.2, ISO 286 and ISO 2768), and it links to the GD&T guide for the geometric side of the same drawing.

The two halves of this guide

Fits: how parts go together

• How Fits Work: Clearance, transition and interference fits, hole and shaft basis, allowance, and how to choose a fit by function.
• Fits – Metric (ISO System): IT grades, fundamental deviations, preferred hole-basis and shaft-basis fits, with a computed table.
• Fits – Inch (ANSI B4.1): RC, LC, LT, LN and FN fit classes with limits, plus modified fits and worked examples.
• Press, Shrink and Expansion Fits: Interference stress, assembly force, shrink temperatures, cold expansion fits and holding torque.
• Fits in Practice: Bearing seats, shafts, keys and keyseats, dowel pins, threads and other everyday fits.
• Temperature, Coatings and Fit Problems: Thermal expansion, plating and anodize build-up, surface finish, and why good parts still fail to fit.

Tolerances: how much variation is allowed

• Dimensional Tolerancing Basics: Limits of size, plus-minus and limit tolerances, unilateral and bilateral, and how to dimension so tolerances do not pile up.
• General Tolerances and Drawing Notes: ISO 2768 classes, title-block tolerances, how to write the note, and what it does and does not cover.
• Tolerances by Process and Surface Finish: What each manufacturing process can hold, how roughness uses up tolerance, and what a tight tolerance costs.
• Tolerance Stack-up Analysis: Chains and loops, worst case, RSS, modified RSS and Monte Carlo, with worked assembly examples.
• Statistical Tolerancing and Process Capability: Cp and Cpk, defect rates, setting tolerances from a capability, and spending tolerance wisely.

Start here: what do you need?

I need to…	Go to
understand limits of size, plus-minus and limit tolerances	Dimensional Tolerancing Basics
know what tolerance applies when none is written	General Tolerances and Drawing Notes
pick a fit for a shaft and hole, or understand what H7/g6 means	How Fits Work
look up or calculate a metric (ISO) fit	Fits – Metric (ISO System)
look up an inch fit (RC, LC, LT, LN, FN)	Fits – Inch (ANSI B4.1)
design a press, shrink or cold-expansion fit	Press, Shrink and Expansion Fits
fit a bearing, key, dowel pin or thread	Fits in Practice
find out why parts that are in tolerance still do not fit	Temperature, Coatings and Fit Problems
know what my shop or supplier can hold, and what it costs	Tolerances by Process and Surface Finish
check that an assembly will always go together	Tolerance Stack-up Analysis
loosen tolerances safely using process capability	Statistical Tolerancing and Process Capability
control form, orientation, location or runout	The GD&T guide

Calculators

• Fit Calculator (ISO 286): limits and clearance or interference for any ISO hole and shaft, 3 to 500 mm.
• Tolerance Stack-up Calculator: worst case, RSS, modified RSS and Monte Carlo for a stack-up loop.
• Process Capability Calculator: Cp, Cpk and defect rates from measurements or a mean and standard deviation.

Key terms

Term	Meaning
Basic size	The size from which the limits of size are derived
Limits of size	The largest and smallest sizes a feature may have
Tolerance	The total permitted variation: maximum limit minus minimum limit
Allowance	The prescribed difference between the maximum-material limits of mating parts: the tightest fit that can occur
Fit	The relationship between two mating parts: clearance, transition or interference
Clearance	The shaft is always smaller than the hole
Interference	The shaft is always larger than the hole
Transition fit	Either clearance or interference may result
Hole basis / shaft basis	A system of fits in which the hole (or the shaft) has a fixed basic size and the other part is varied
IT grade	An International Tolerance grade: the size of the tolerance zone. A higher number is a looser tolerance
Fundamental deviation	The letter that places the tolerance zone relative to the basic size (H for holes at the basic size, h for shafts, and so on)
Stack-up	The accumulated variation of several dimensions in an assembly
Cp and Cpk	Measures of how well a process fits inside a tolerance: the width of its spread against the tolerance, and how centered it is

Quick reference: preferred fits

The Handbook’s Table 1 (from ANSI/ASME B4.2) lists the ten preferred metric fits. Each is shown here as hole basis, with the shaft-basis equivalent. The full limits, by size, are on the Fits – Metric (ISO System) page.

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

Quick reference: general tolerances

ISO 2768-1 class m (medium), the usual default for machined parts, in millimeters. All four classes are on the General Tolerances and Drawing Notes page.

Nominal size	0.5–3	3–6	6–30	30–120	120–400	400–1000	1000–2000	2000–4000
Class m (±)	0.1	0.1	0.2	0.3	0.5	0.8	1.2	2.0

FAQ

What is the difference between a tolerance and a fit?

A tolerance applies to one part: how much its size may vary. A fit applies to two parts together: how tight or loose they are when assembled. A fit is the combination of two tolerances and an allowance.

Where do I start if I only need a fit for a shaft in a hole?

Read How Fits Work for the idea, then use the ISO or inch fits pages to find the limits for your size.

How is this different from the GD&T guide?

This guide is about size: how big a feature may be, and how parts of different sizes fit together. GD&T is about shape, orientation and location: how flat, square or well positioned a feature must be. A real drawing needs both.

Which standards does this guide follow?

ANSI/ASME B4.1 and B4.2, ISO 286 and ISO 2768, with ASME Y14.5 for drawing practice, using the Machinery’s Handbook as its main reference. Each page names its sources, and notes where the metric and inch systems or ASME and ISO differ.

Are the values on these pages safe to use directly on a drawing?

They are intended as a working reference. For a contract drawing or a safety-critical part, check the limits against the current edition of the standard that the drawing names.

References

• Machinery’s Handbook, Industrial Press: “Allowances and Tolerances for Fits” (limits and fits, ANSI/ASME B4.1 and B4.2, ISO and British Standard metric limits and fits, tolerance analysis and assignment).
• ANSI/ASME B4.1, Preferred Limits and Fits for Cylindrical Parts; ANSI/ASME B4.2, Preferred Metric Limits and Fits.
• ISO 286-1 and 286-2, ISO code system for tolerances on linear sizes; ISO 2768-1, general tolerances.
• ASME Y14.5-2018, Dimensioning and Tolerancing.

Last reviewed: October 2026.