Glossary & Reference
MMC is the size limit that guarantees two mating parts will always assemble — and the trigger that unlocks bonus tolerance everywhere else on the print.
Maximum Material Condition (MMC) is one of the feature-of-size concepts defined in ASME Y14.5: the condition of a feature — a hole, a pin, a slot, a tab — where the maximum amount of material is present within its size tolerance. It's called out on a print with a circled M directly after a tolerance value in a feature control frame. Applying it also invokes bonus tolerance, the same concept covered on our GD&T page: as a feature's actual produced size departs from its MMC limit, the geometric tolerance allowed on that feature grows by exactly the amount of that departure.
Because MMC means "the most material," it lands on opposite ends of the size range depending on whether the feature is internal or external:
The opposite limit — Least Material Condition (LMC) — is simply the other end of the same size tolerance.
Not every geometric symbol accepts an MMC modifier. ASME Y14.5 permits it only on straightness (axis), parallelism, perpendicularity, angularity, and — most commonly — true position. Form controls tied strictly to surface shape, like circularity and flatness, don't take a material-condition modifier at all: MMC only applies where a feature's size and its geometric location or orientation interact.
This is the same bonus tolerance concept explained on our GD&T page. With no modifier shown, a tolerance applies Regardless of Feature Size (RFS) and never changes. With the M modifier, the stated tolerance is the minimum guaranteed value — it applies exactly as written only when the feature sits at its MMC limit. As the feature's real size drifts toward LMC, it earns additional bonus tolerance, matched to how far it departed from MMC. A hole drilled larger than its MMC size, for example, can be positioned less precisely and still pass.
Take a shaft that has to pass through a hole with clearance, no interference, every time. The shaft's MMC is its largest allowed diameter; the hole's MMC is its smallest allowed diameter. As long as the shaft's MMC diameter is kept smaller than the hole's MMC diameter, clearance is guaranteed at the worst-case condition — the largest the shaft could ever be, paired against the smallest the hole could ever be. Every other combination of actual sizes only opens the gap further, which is exactly why MMC is the calculation point tolerance stacks are built around.
An MMC callout tells you two things on the shop floor: which size limit represents the worst case you have to guarantee clearance against, and that a part measuring away from that limit may still be fully acceptable even if a naive positional check looks tight — because bonus tolerance is real, usable tolerance once you calculate it correctly.