Fixed Fastener Formula

The Fixed Fastener Formula applies when a fastener is fixed in one mating part, for example, a bolt threaded into a tapped hole or a dowel pin pressed into a bore, and the other part has a clearance hole through which the fastener must pass.

The formula T = H − F gives the total positional tolerance diameter available across both mating parts. It represents the maximum permissible deviation of a hole center from its true position, ensuring assembly regardless of where each feature sits within its tolerance zone.

The total T is split between Part 1 (the clearance hole part) and Part 2 (the fixed fastener part). The standard default is a 50/50 equal split, expressed as T = (H − F) / 2 per part. The slider lets you shift that budget toward whichever part has lower manufacturing precision.

Why divide by 2? In a 50/50 split, we assume both parts could be manufactured off-center in opposite directions. Dividing the total clearance by 2 ensures that even if Part 1 is at its maximum error in one direction and Part 2 is at its maximum error in the other, the bolt will still clear the hole.

Note: While the standard formula defaults to T = (H − F) / 2, this calculator allows for unequal distribution. As long as T₁ + T₂ ≤ (H − F), the assembly remains valid. Shift the slider to accommodate parts that are more expensive or difficult to manufacture to tight tolerances.

Reference: ASME Y14.5-2018 §8.3: Fixed and Floating Fastener Formulas