ISO Fits Calculator

An ISO fit names a hole and a shaft by a letter and a number, as in 25 H7/g6. The letter places the tolerance zone against the basic size, capitals for holes and lower case for shafts: H starts at the basic size and goes up, h ends at it and goes down, and the letters away from them move further off. The number is the tolerance grade, how wide the zone is for that size: the larger the number, the wider the zone.

largest clearance = largest hole − smallest shaft  ·  smallest clearance = smallest hole − largest shaft

A negative clearance is interference. If the smallest clearance is zero or more the fit is a clearance fit, if the largest is zero or less an interference fit, and if one is positive and the other negative a transition fit. Hole basis fits keep the hole at H and change the shaft; shaft basis fits keep the shaft at h and change the hole. Letters C to G give the same fit in either basis. K to U do only where the Δ rule below applies, over 3 mm with the hole one grade coarser than the shaft: 25 H7/k6 and 25 K7/h6 match, but at 1 mm H7/k6 runs +0.010 to −0.006 and K7/h6 +0.006 to −0.010.

Where the numbers come from. ISO 286 gives formulas for its tolerances, but its tables are the values that count, and the formulas do not always reproduce them. So the calculator holds the published values for the tolerance grades and the shaft letters, and works out every hole letter from them by the standard's rules, including the Δ correction on K, M and N to grade 8 and on P to U to grade 7. They were checked against every row of ANSI B4.2's preferred-fit tables as printed in Machinery's Handbook.

Worked example. 25 H7/g6, a sliding fit. The size is in the 18 to 30 mm range, where IT7 is 21 µm and IT6 is 13 µm. The hole is 25.000 to 25.021. The g shaft's upper deviation is −7 µm, so it runs 24.980 to 24.993. The clearance is 0.007 to 0.041 mm. Load it with the button under the inputs.

Not covered: sizes over 500 mm, grades outside 5 to 11, the letters a, b, cd, ef, fg, j, t and v to zc, K, M and N holes above grade 8, and the ANSI B4.1 inch fit classes (RC, LC, FN). The fit is between sizes only: form, surface finish, temperature and the parts' materials change how it behaves.

Reference: ISO 286-1:2010 and ISO 286-2:2010; ANSI B4.2-1978 (R1999), Preferred Metric Limits and Fits. Values as printed in Machinery's Handbook, 27th ed., pp. 670 to 688. js and JS are exactly ±IT/2, as the 2010 edition gives them; the 1969 BS 4500 rounded an odd IT down to the even value below for grades 7 to 11, so older charts print those half a micrometer tighter.

Background on the Drafter blog: Why your dowel pin doesn't fit.