Fixed and Floating Fastener Position Tolerance Calculator

How to use it

Enter the clearance hole at MMC, meaning its smallest allowed diameter, and the fastener at MMC, meaning the largest shank or major diameter. Choose how much tolerance to reserve for fastener tilt and thread error, and how to split the fixed case between the two parts. The results give the floating tolerance, the fixed total and equal split, the allocation after reserve, and the position allowed when the hole is at its largest size. Use the chart to see how stepping up the clearance hole size, for example from fine to medium series, multiplies the tolerance you can give.

• Floating: the fastener passes through clearance holes in all parts, such as a bolt and nut.
• Fixed: the fastener is held in one part, such as a tapped hole, stud or press-fit pin.
• Always use MMC sizes in both formulas.
• Apply the resulting tolerance at MMC in the feature control frame.

Worked example

An M8 bolt has a maximum major diameter of 7.972 mm for a 6g thread. The medium-series clearance hole of 9.000 mm gives a diametral clearance of 1.028 mm, so a floating pattern can use a position tolerance of 1.028 mm at MMC. If the bolt threads into a tapped part, the total of 1.028 mm is shared, so an equal split gives 0.514 mm to each. After a 10 percent reserve, the floating tolerance is about 0.925 mm. If the clearance hole is made to a tighter 8.400 mm fine-series size, the floating tolerance falls to 0.428 mm.