On this page
One nominal size, a range of actual sizes
A shaft marked 20 mm does not have to measure exactly 20.000 mm at every accepted size. Nominal size provides a common reference. A tolerance describes how far a size may lie below or above that reference. Two features sharing the same nominal size do not necessarily assemble in the same way. Whether their combination leaves clearance or produces interference depends on the actual size of each feature.
The ISO system expresses these limits in a compact code. A class such as h9 or H7 becomes meaningful when read together with nominal size. Although 10 mm h9 and 100 mm h9 use identical letters and numbers, the permitted difference in millimetres is not the same. Reading the calculated minimum and maximum sizes alongside the class gives a more tangible picture of what the designation allows.
The letter locates the interval; the number defines its grade
Lowercase letters identify shafts and external sizes; uppercase letters identify holes and internal sizes. For this reason, h7 and H7 are different designations. With h, the largest permitted size equals nominal size and the interval extends downwards. With H, the smallest permitted size equals nominal size and the interval extends upwards. Letter case is therefore part of the calculation rather than a choice of writing style.
The number identifies the tolerance grade. At the same nominal size, h9 has a narrower interval than h11. This means tighter dimensional limits, rather than a ranking of alloy quality. Material names such as 303 and 316L answer a different question: they identify the material, while a tolerance class describes the permitted variation in its size.
Which dimension describes each section?
A round section uses diameter, while a square section uses the distance between the opposite faces defining its side dimension. A hexagon uses across flats, the distance between opposite flat faces. Confusing this with the largest distance across corners applies the calculation to the wrong dimension even when the class itself is correct. Section shape establishes which faces the entered size refers to.
Flat bar has separate width and thickness dimensions. Assigning h9 to both dimensions of a 50 × 10 mm section does not mean that both can be undersize by the same amount. The two dimensions can fall into different table ranges. Treating them independently gives actual limits for width and thickness instead of a single ambiguous flat-bar tolerance value.
When two size intervals meet
A hole-and-shaft fit considers two intervals together. The smallest hole and largest shaft give the tightest combination; the largest hole and smallest shaft give the loosest. A clearance fit allows the hole to be at least as large as the shaft throughout the permitted combinations. An interference fit keeps the shaft larger than the hole. When the intervals overlap, actual dimensions can produce either result: this is a transition fit.
A designation such as H7/g6 describes both features in one line. The clearance or interference reported by the calculator is a geometric difference between dimensions. An identical interval does not mean that different assemblies require the same mounting force or support the same load. The value of a tolerance calculation is that it makes the size limits, and the resulting range between them, visible.