ISO 286 Limits & Fits Calculator

Recommended ISO Fits

The table below gives common hole-basis fits engineers use as starting points. Exact selection depends on load, speed, lubrication, temperature, material, plating, surface finish, and process capability.

Common ISO Fit Examples

25 mm H7/g6 fit example

For a 25 mm nominal size, H7/g6 is commonly used as a sliding or close clearance fit. The H7 hole starts at the basic size, while the g6 shaft tolerance zone sits below the basic size, so assembly should have positive clearance. Use this when parts must locate accurately but still slide or rotate without force.

25 mm H7/h6 fit example

H7/h6 is a location clearance fit. The H7 hole has zero lower deviation and the h6 shaft has zero upper deviation, so the tightest case approaches line-to-line contact. It is useful for accurate location with easy assembly when you do not want a deliberate press fit.

25 mm H8/f7 fit example

H8/f7 is often used for close running clearance. Compared with H7/g6, it is generally looser and better suited to rotating parts, lubrication allowance, and less demanding location requirements.

25 mm H7/p6 fit example

H7/p6 is a press or interference fit. The shaft tolerance zone sits above the basic size, so the shaft can be larger than the hole even at the limiting conditions. Check assembly force, hub stress, material strength, and whether heating or cooling is needed before using it on production parts.

Engineering Tolerances, Fits & Limits — A Friendly Primer

In precision manufacturing, two parts rarely measure exactly their nominal size. Instead, drawings specify a basic size plus permitted tolerances, producing a range of acceptable dimensions called limits. For mating parts (a hole and a shaft), these limits determine the fit: whether assembly results in guaranteed clearance, guaranteed interference, or a transition where either outcome is possible. This calculator lets you explore limits and fits either by entering deviations directly (µm) or by choosing ISO-style hole and shaft tolerance classes such as H7/g6, H7/h6, H8/f7, and H7/p6.

Key Terms (Plain English)

• Basic size (D): The nominal size from which limits are derived (e.g., 25 mm).
• Deviation: Offset from the basic size. For holes we use \(E_i, E_s\); for shafts \(e_i, e_s\) (in µm).
• Limits: The smallest and largest permissible sizes (e.g., 25.000–25.025 mm).
• Clearance: Hole diameter minus shaft diameter. Positive = “slack”, negative = overlap.
• Fit types: Clearance (always +), Interference (always −), Transition (can be + or −).

How Limits & Fits Are Computed

Deviations are given in micrometres (µm). Converting to millimetres is simply division by 1000. If the basic size is \(D\) (mm), then:

• Hole limits: \(D_\text{hole,min} = D + E_i/1000\),   \(D_\text{hole,max} = D + E_s/1000\).
• Shaft limits: \(D_\text{shaft,min} = D + e_i/1000\),   \(D_\text{shaft,max} = D + e_s/1000\).
• Minimum clearance: \(C_\text{min} = D_\text{hole,min} - D_\text{shaft,max}\).
• Maximum clearance: \(C_\text{max} = D_\text{hole,max} - D_\text{shaft,min}\).

Classification is then straightforward: both clearances > 0 → clearance fit; both < 0 → interference fit; mixed signs → transition fit.

What Are IT Grades?

ISO 286 defines tolerance grades such as IT5, IT6, IT7, IT8, IT9, IT10, IT11, IT12, and IT13. A smaller number is tighter. The fit symbol combines a letter and a grade: the letter positions the tolerance zone relative to the basic size, and the number controls the tolerance width. The calculator supports common ISO letters for holes and shafts:

• H hole: lower deviation \(E_i = 0\); hole lies entirely above the basic size.
• h shaft: upper deviation \(e_s = 0\); shaft lies entirely below the basic size.
• js shaft: deviations are symmetric about 0 (±IT/2).
• g, f, d, c shafts: zones below the basic size, commonly used for clearance fits.
• k, n, p, u shafts: zones near or above the basic size, commonly used for transition and interference fits.

Exact ISO tables include grade and diameter details for every zone. Use official tables for certification work; this tool is a practical browser-side calculator for quick planning and sanity checks.

Selecting a Fit (Rule-of-Thumb)

• Free sliding: H8/h7 or H9/h9 for general assemblies that must move without binding.
• Location/transition: H7/js6 where some parts may be tight, others loose; alignment prioritized.
• Light press: H7/p6 or H7/k6 for locating with light interference (bearing inner rings on shafts often use k/m).
• Heavy press/shrink: H7/u6 or similar for permanent joints. Thermal methods are common for assembly.

Common Pitfalls (and Easy Fixes)

• Forgetting units: Deviations are in µm, basic size and limits in mm. Convert consistently.
• Mismatched grade severity: A very tight hole with a very loose shaft can yield unpredictable fits. Pair grades sensibly (e.g., H7/h6).
• Ignoring process capability: Choose grades your machining process and metrology can actually hit.
• Skipping function: Rotating parts may need clearance for thermal growth and lubrication; static pins may require interference.

Practical Tips

• Prototype with slightly looser combinations first; tighten once function is proven.
• For bearings, consult the manufacturer’s fit recommendations—load direction and rotation matter.
• Consider environment: temperature swings, corrosion allowance, coatings/plating thickness, and surface finish all influence real fits.

Disclaimer: This primer and calculator provide engineering approximations for education and early planning. For production drawings, QA, or regulated applications, verify with ISO 286 tables and your organization’s standards.

FAQ

What is an engineering tolerance?

An engineering tolerance is the allowed variation from a nominal size. It defines the largest and smallest acceptable dimensions for a part.

What is ISO 286?

ISO 286 is the metric limits and fits system for linear sizes. It defines tolerance grades, fundamental deviations, and how hole and shaft fit classes are written.

What is the difference between H7/g6 and H7/h6?

H7/g6 gives more guaranteed clearance because the g6 shaft zone is below the basic size. H7/h6 is a closer location clearance fit because the h6 shaft upper deviation is zero.

What is a clearance fit?

A clearance fit always leaves the hole larger than the shaft at the limiting conditions.

What is an interference fit?

An interference fit always makes the shaft larger than the hole at the limiting conditions, so assembly usually needs force, heating, or cooling.

What is a transition fit?

A transition fit can produce either small clearance or small interference depending on the manufactured sizes.

How do I calculate maximum and minimum clearance?

Minimum clearance is the smallest hole minus the largest shaft. Maximum clearance is the largest hole minus the smallest shaft.

What does H7 mean?

H7 is a hole class. H means the lower hole deviation is zero, and 7 is the tolerance grade.

What does g6 mean?

g6 is a shaft class. g positions the shaft tolerance zone below the basic size, and 6 is the tolerance grade.

Why is hole basis more common?

Hole basis is common because standard hole-making tools are easier to standardize, while shafts can often be turned or ground to match the required fit.