The Fixed Fastener Formula calculates your Total Tolerance Budget, the maximum wiggle room available before a bolt interferes with the side of a hole. Since the fastener is fixed (threaded or pressed) into one part, the available clearance must be shared between both mating components.
Video by Dean Odell: a concise visual explanation of the fixed fastener formula and its application to bolt hole patterns.
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
Maximum fastener shank diameter
Minimum clearance hole diameter — the smallest the hole can be at Maximum Material Condition
Allocate the total tolerance budget T between the two mating parts.
Equal split: both parts share the tolerance budget equally.
Drafter recommends a 60/40 split — 60% of the tolerance to the tapped hole and 40% to the clearance hole.
MMC = Maximum Material Condition. For a clearance hole, MMC is its smallest diameter.