Parallelism Calculator
Enter the flange diameter and the measured face-to-face gap difference around the joint.
Use the flange outside diameter (or the bolt-circle diameter the gap is measured across).
Largest gap minus smallest gap, measured with feeler gauge at 4 points around the face.
Awaiting input
Allowable
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Measured
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Margin
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Enter a diameter and a measured gap difference. The check uses the API 686-aligned criterion
of 0.001 in per inch of flange diameter (equivalently 1 mil/in or 0.001 mm/mm).
Criterion (API 686-aligned)
Maximum allowable parallelism error ≈ 0.001 × D, where D is the flange diameter.
This is the same as 1 mil per inch in US units or 0.001 mm per mm in SI — a
common field-acceptance check for piping flanges connecting to rotating equipment.
Always verify the latest project specification and the current edition of the applicable
API standard before final acceptance — project specs may be tighter or differ for specific service.
Visual Reference
What the field measurement is looking for.
Parallel — Acceptable
Gap is uniform around the face. Measured difference ≤ 0.001 × D.
Angular — Not Acceptable
Gap varies across the face. If (max − min) > 0.001 × D, the joint fails the check.
Measurement Pattern
Take feeler-gauge readings at 4 points (12/3/6/9). Record max and min — the difference is the parallelism error.