ISO Fit & Tolerance Calculator
Calculate hole and shaft tolerances per ISO 286. Supports H-basis fits with clearance, transition, and interference classifications.
Introduction
The ISO Fit & Tolerance Calculator implements the deviation tables in ISO 286-1 for the hole-basis (H) system. Enter a nominal diameter from 1 to 500 mm and a fit designation like H7/g6, H7/k6, or H7/p6, and the tool returns upper and lower deviations in micrometres for both the hole and shaft, plus the maximum and minimum clearance (or interference) for the assembly. Coverage spans the shaft letters d, e, f, g, h, j, k, m, n, p with grades IT5–IT11 for d–h and IT5–IT7 for k–p, matching the ranges actually tabulated in the standard.
How it Works
The fundamental deviation for each shaft letter and IT grade comes from ISO 286-1 Table 2 (fundamental deviations) and Table 4 (IT values by size range). For an H7 hole at 30–50 mm, the IT7 band is 25 μm with the lower deviation pinned at 0 (the H-basis convention). For a g6 shaft at the same size range, the upper deviation is −9 μm and the lower is computed by subtracting the IT6 band (16 μm) from that upper deviation, giving −25 μm. The calculator subtracts the maximum shaft size from the minimum hole size to get the minimum clearance, and the maximum hole from the minimum shaft to get the maximum clearance. Negative values indicate interference. The classification is clearance when minimum clearance is positive, interference when maximum clearance is negative, and transition when the two have opposite signs.
Usage Scenarios
- Specifying a sliding-fit bearing seat: a 30 mm shaft in a plain bronze bushing typically uses H7/g6 — the calculator confirms a clearance range of 7–34 μm, enough for an oil film but tight enough to control radial play.
- Designing a press-fit pulley hub: H7/p6 on a 20 mm shaft yields 15–35 μm of interference, requiring a hydraulic arbor press during assembly and producing a torque-transmitting joint that resists slip up to design loads without keys.
- Selecting a dowel pin fit between mating jig plates: a 6 mm pin pressed to P7 in the slave plate (interference 6–18 μm) and reamed to H7 in the master plate (clearance 0–12 μm) gives repeatable location with removable assembly.
FAQ
Why does the calculator block k/m/n shaft letters at IT8 and higher?
ISO 286-1 only tabulates fundamental deviations for the interference-region letters (k, m, n, p, r, s) at the precision IT grades IT5 through IT7. At IT8 and coarser, the tolerance band is wide enough that the fit becomes unpredictable — straddling clearance and interference depending on actual measured dimensions. The standard intentionally omits these combinations to discourage their use. If you need an interference fit, work in IT6 or IT7 and accept the manufacturing cost.
How do I convert an ISO 286 fit to absolute hole and shaft dimensions on the drawing?
Add the deviations to the basic size. For a 50 mm H7/g6 fit: hole = 50.000 + 0/+0.025 mm (write as 50.000/50.025 or as 50 H7); shaft = 50.000 + (−0.009)/(−0.025) mm (write as 49.991/49.975 or as 50 g6). On the drawing, prefer the letter-grade callout (50 H7) over the explicit limits when working with experienced suppliers — it is more compact and links directly to the standard.
Can ISO 286 fits be used for non-round features like keyways and splines?
No. ISO 286 covers cylindrical (round-hole, round-shaft) features only. Keyway widths use ISO 2768 general tolerances or part-specific limits. Involute splines use ISO 4156 (metric) or ANSI B92.1 (inch) with their own fit classes. Press-fits between non-round mating features (rectangular bars in slots, square shafts in square holes) are governed by ANSI/ASME B4.1 or shop-defined tolerance schemes — never extrapolate H7/g6 deviation values to non-round features.
What surface roughness is needed to actually verify an H7 tolerance?
Surface roughness eats into the dimensional tolerance during measurement. For an H7 hole with a 25 μm band, target Ra 0.8–1.6 μm (Rz around 6 μm) — achievable by reaming, fine boring, or honing. Drilled-only holes typically run Ra 3.2 μm or rougher and cannot be reliably verified to H7 because measurement results scatter across peaks and valleys. As a rule of thumb, the average peak-to-valley roughness should not exceed 25% of the dimensional tolerance band.
Do thermal effects change the fit between assembly and operating temperature?
Yes, especially for dissimilar materials. Steel-on-steel grows together (~11.5 × 10⁻⁶ /°C) so a fit barely shifts with temperature. But a steel shaft in an aluminum housing (23 × 10⁻⁶ /°C) sees the bore open 2× faster than the shaft when heated. For a 50 mm joint heated by 50 °C, the steel shaft grows 30 μm while the aluminum bore grows 58 μm — that 28 μm shift can turn an H7/p6 interference fit (max 42 μm interference) into a joint that slips. Always check operating-temperature dimensions for high-temperature assemblies.