A feature of size is any geometric shape on a part that can be described by a size dimension, specifically a cylindrical surface, a spherical surface, or two opposed parallel surfaces. Think of a hole drilled through a plate, the thickness of that plate, or the diameter of a pin. Each of these has a measurable size (diameter or width) and qualifies as a feature of size. This concept is foundational in geometric dimensioning and tolerancing (GD&T) because it determines which tolerancing rules and modifiers you can apply to a given part feature.
What Qualifies as a Feature of Size
To count as a feature of size, a geometry needs to meet a straightforward physical requirement: it must be a surface or pair of surfaces that can be measured with a single size dimension. The most common examples are:
- A hole diameter: the cylindrical interior surface of any round hole
- A pin or shaft diameter: the cylindrical exterior surface
- A plate thickness: two flat, opposed parallel surfaces with a width dimension between them
- A ball bearing diameter: a spherical surface
The key idea is “opposed points.” For a flat feature like a slot or a tab, you need two parallel surfaces facing each other so that a single dimension captures the distance between them. A single flat surface on its own, like the top face of a block, is just a “feature” but not a feature of size because there’s nothing opposing it to define a measurable width or thickness.
Why It Matters in GD&T
Features of size unlock specific tolerancing tools that plain surfaces cannot use. The most important is Rule #1 in the ASME Y14.5 standard, which states that the size limits of an individual feature of size automatically control its form. A shaft dimensioned at 20 mm plus or minus 0.1 mm, for example, cannot bow or taper beyond what that size range allows. At its maximum size of 20.1 mm, the shaft must fit inside a perfect cylinder of 20.1 mm diameter. This built-in form control only applies to features of size.
Features of size also establish derived geometry that GD&T relies on for further tolerancing. A cylindrical hole defines a center axis. Two opposed parallel surfaces define a center plane. These derived elements, the axis or center plane, become the targets for tolerances like true position. When a drawing calls out the position of a hole, it’s actually controlling where that hole’s center axis falls relative to the part’s datums.
Material Condition Modifiers
Only features of size can use material condition modifiers, which are among the most powerful tools in GD&T for balancing fit and function. These modifiers adjust how much positional or geometric tolerance a feature gets based on its actual produced size.
Maximum Material Condition (MMC) is used when easy assembly is the priority. For a hole, maximum material means the smallest allowable diameter (least material removed). As the hole is produced larger than that minimum, extra positional tolerance becomes available, called “bonus tolerance.” The logic is simple: a bigger hole is easier to fit a pin into, so you can afford to let the hole’s location be slightly less precise.
Least Material Condition (LMC) works the opposite way and is used when tight location accuracy matters more than easy assembly. It encourages tighter fits by applying bonus tolerance as a hole gets smaller or a pin gets larger. LMC is less common and carries some manufacturing risk, since pushing toward a tighter fit can accidentally create an interference fit if tolerances aren’t carefully managed.
Neither of these modifiers can be applied to a plain surface. They only make sense for geometries that have a measurable size that can shift between material conditions.
Regular vs. Irregular Features of Size
The ASME standard splits features of size into two categories: regular and irregular. The distinction matters for how tolerances are interpreted, though the vast majority of features you’ll encounter in practice are regular.
A regular feature of size (RFOS) is one cylindrical surface, one spherical surface, or two opposed parallel surfaces associated with a directly toleranced dimension. Holes, shafts, slots, tabs, and plate thicknesses all fall here. These are the classic, intuitive examples.
An irregular feature of size (IFOS) is a directly toleranced feature, or collection of features, that can be contained by or can contain a mating envelope but doesn’t fit neatly into the cylinder, sphere, or parallel plane categories. An oblong shape (sometimes called an “obround”) is a textbook example. It has a definable mating envelope, meaning you could imagine a perfectly shaped boundary that just contains or fits inside it, but that boundary isn’t a simple cylinder or pair of planes. Irregular features of size can also be collections of features treated together, like a pattern of slots that collectively define a mating condition.
The practical difference between regular and irregular types comes down to one thing: regular features involve a single cylindrical, spherical, or parallel-plane geometry, while irregular features involve one or a collection of directly toleranced geometries that require a more complex mating envelope.
Continuous Features of Size
Sometimes a feature of size is physically interrupted but still needs to function as one feature. Imagine a long bearing surface on a shaft that’s broken by a groove in the middle. The two remaining cylindrical segments are separate surfaces, but functionally they need to act as a single diameter for assembly purposes.
In the ASME system, the “CF” (continuous feature) symbol tells inspectors and manufacturers to treat those interrupted surfaces as one feature of size. The ISO GPS system handles the same concept with a “CT” (common tolerance) designation. The practical effect is the same: a single mating envelope, like one perfect cylinder, must be able to fit across all the interrupted segments simultaneously.
How ISO and ASME Handle Size Differently
If you work with international drawings, it’s worth knowing that the ISO GPS system and ASME Y14.5 don’t define size measurement identically. ASME’s Rule #1 bundles form control into size limits by default. A shaft’s size tolerance automatically prevents it from being too bent or tapered. In ISO, you have to specify form control separately. To replicate ASME’s default behavior in ISO, engineers must explicitly call out both a global size (like a minimum circumscribed diameter) and a local size (measured at individual cross sections).
About 20% of companies surveyed in one industry study reported that they expect to use both ASME and ISO systems going forward. Some symbols share the same meaning across systems, some share the same meaning only in certain contexts, and some exist in only one system. If you’re reading a drawing, checking which standard it references is essential before interpreting any feature of size tolerance.