Gage tolerance selection tool
Works out the gage tolerance your ratio requirement allows, and what a given gage tolerance actually costs you in part tolerance. Companion to the three-wire, pre-plate, TAR/TUR, process capability, and thermal expansion calculators.
This tool contains no gage tolerance data and cannot name a class.
ASME B89.1.5 gagemaker tolerance values are tabulated in a copyrighted standard and none of them appear in
this page. Every figure below is derived arithmetically from values you enter. The output is a target
— the largest gage tolerance that satisfies the requirement you stated — which you then match
against a class in your own copy of the standard or against a supplier’s quotation. Mapping a target to
a class designation at a given nominal size is the copyrighted table itself, so this tool does not attempt
it. Gage tolerance conventions, including GO plus and NO-GO minus, vary and must be confirmed against your
print. This page makes no determination of whether a selection satisfies a customer requirement, a quality
system, or an accreditation scope. No warranty of any kind is made as to accuracy, completeness, or fitness
for a particular purpose, and no liability is accepted for any loss or damage arising from its use.
Version 1.0 · All values derived from user input · Last reviewed August 2026
How the classes are structured
Gagemaker tolerance classes are conventionally designated XXX, XX, X, Y, Z and ZZ, running from the
tightest at XXX to the loosest at ZZ. Within any one class the tolerance increases with nominal size, so a
class designation on its own is not a tolerance — it is a tolerance per size, and you need both
to get a number.
The numbers live in your copy of ASME B89.1.5, and nowhere in this page. No tolerance value, size
range, or relationship between classes appears here, because that information is the copyrighted content of
the standard. Take the target this tool computes, go to your own copy or to your gage supplier, and find the
class whose tolerance at your nominal size does not exceed it.
Tighter classes cost more and take longer to procure, and below a point the gage’s form tolerance
rather than its size tolerance becomes the binding constraint on what you can actually buy. Specifying the
tightest available class is not automatically the right answer; specifying the loosest one that meets your
ratio, with margin for wear, usually is.
Formulas
| Part tolerance span | T = UL − LL |
| Requirement basis | T or T/2, your selection |
| Percentage and ratio | fraction = 1 / ratio they are reciprocal |
| Maximum gage tolerance | gmax = basis × fraction = basis / ratio |
| Achieved per-member ratio | basis / g |
| Pair ratio, GO and NO-GO together | T / (2g) |
| Pair ratio when new, with wear | T / (2g + w) |
| Guaranteed acceptance zone, hole | NOGOlo − GOhi |
| Possible acceptance zone, hole | NOGOhi − GOlo |
| Guaranteed acceptance zone, shaft | GOlo − NOGOhi |
| Possible acceptance zone, shaft | GOhi − NOGOlo |
| Tolerance consumed | T − guaranteed good parts rejected |
| Over-acceptance risk | possible − T bad parts accepted |
Bilateral is interpreted as ±g/2, so the total band on each member equals the
stated gage tolerance and the three placements stay comparable on the same g. Read as
±g the band doubles.
Not covered. Class designation from a tolerance target. Gage tolerance values of any
kind. Geometric form tolerance on the gage, which is separate from size tolerance and is often the binding
constraint on a precision gage. Thread gages, which have their own tolerance system. Taper, spline, and
functional gages. Measurement uncertainty of the gaging process. Calibration interval.