Limits and fits define the dimensional tolerances required for two mating parts (such as a shaft and a hole) to assemble and function correctly. “Limits” establish the maximum and minimum acceptable physical dimensions for a manufactured part. The “fit” refers to the resulting tightness or looseness—the clearance or interference—between parts when they are joined.

ISO 286 is the internationally recognized engineering standard for calculating metric tolerances and fits. It uses an alphanumeric coding system (for example, H7/g6) to define the fundamental deviation (the letter) and the International Tolerance or IT grade (the number). This ensures components designed in one country can be seamlessly manufactured and assembled anywhere in the world.

Depending on the mechanical application, mating parts are designed using one of three standard fits:

• Clearance Fit: A gap always exists between the parts. The maximum shaft diameter is always smaller than the minimum hole diameter, allowing for sliding or rotation.
• Interference Fit (Press Fit): The parts physically overlap. The minimum shaft diameter is always larger than the maximum hole diameter, requiring mechanical force or thermal expansion to assemble.
• Transition Fit: A hybrid fit that can result in either a slight clearance or a slight interference. It is primarily used for precise alignment where parts must be assembled and disassembled without high force.

• Hole Basis System: The hole’s minimum size is kept constant at the nominal dimension (denoted by an uppercase H). The size of the shaft is varied to achieve the desired fit.
• Shaft Basis System: The shaft’s maximum size is kept constant at the nominal dimension (denoted by a lowercase h). The size of the hole is varied to change the fit.

The Hole Basis system is the industry standard because it is significantly more cost-effective and easier to manufacture. Machining a hole requires fixed-size tooling like drill bits and reamers. It is much easier to buy standard-sized tooling for the hole and adjust the exterior diameter of the shaft on a CNC lathe to achieve the specific fit.

IT grades determine the level of precision and manufacturing cost.

• Lower IT grades (IT1 to IT6): Extremely tight tolerances. These require high-precision manufacturing methods like cylindrical grinding and lapping, making them more expensive.
• Medium IT grades (IT7 to IT11): Standard engineering tolerances. These are easily achievable with standard CNC turning and milling operations.
• Higher IT grades (IT12 to IT18): Loose tolerances suitable for casting, forging, or sheet metal cutting.

This callout gives a manufacturer all the information needed to produce the mating parts:

• 25: The nominal (basic) target size in millimeters.
• H7: The tolerance for the hole (Uppercase letter means hole; ‘7’ is the IT grade).
• g6: The tolerance for the shaft (Lowercase letter means shaft; ‘6’ is the IT grade).

Calculating ISO 286 tolerances manually requires cross-referencing multiple complex engineering charts to find fundamental deviations and IT grade ranges. A limits and fits calculator automates this process. By simply entering your nominal dimension and desired fit, the tool instantly outputs the exact maximum sizes, minimum sizes, and micrometer deviations, completely eliminating the risk of human error in your CAD drawings.