Understanding Commercial Pattern Sizing

The first sewing pattern most people buy comes with an unpleasant surprise: the size on the envelope is two or three numbers higher than the size they buy in shops. Nothing is wrong. The two systems are measuring different things, and only one of them is anchored to the body at all.

Two Different Systems

A pattern size chart lists body measurements. You match your bust, waist and hip to the chart and the pattern provides the garment dimensions. A ready to wear size label lists nothing in particular. It is a marketing designation set by each brand, adjusted over time, and not tied to any enforced standard.

The historical reason is documented. A National Institute of Standards and Technology review of body dimensions for apparel noted that the sizing standards in use relied on body measurement data gathered by the United States Department of Agriculture in the late 1930s and that those standards no longer reflected the diversity of the population. Voluntary standards drifted away from the bodies they described, brands rescaled their own charts independently, and the result is a system where a single nominal size means different measurements in every shop. The underlying population data has meanwhile moved: the National Center for Health Statistics publishes current nationally representative body measurements from the National Health and Nutrition Examination Survey, covering height, weight and circumferences by sex and age, and those figures differ substantially from mid twentieth century assumptions.

The practical consequence: ignore the number entirely and work from measurements.

Wearing Ease and Design Ease

Ease is the difference between the body measurement and the finished garment measurement, and it comes in two kinds.

Wearing ease is the minimum extra room needed to move, sit and breathe. Typical values in woven fabric are around 5 to 10 cm at the bust, 2 to 3 cm at the waist and 5 cm at the hip. Below that the garment restricts movement. Knit garments can have zero or negative ease because the fabric itself supplies the movement.

Design ease is everything the designer added on top for the intended silhouette. A fitted sheath dress might have only wearing ease. A relaxed shirt might have 20 cm of design ease at the bust, and an oversized coat 40 cm or more.

This is why comparing your bust measurement to the finished garment measurement is meaningless without knowing which silhouette was intended. Look instead at the finished garment measurements printed on the pattern tissue or in the instructions, subtract your body measurement, and ask whether the resulting ease matches the amount of room you actually want. A useful calibration is to measure a garment you already own and love, and compare.

Choosing by Upper Bust

Commercial patterns are drafted for a specific bust cup proportion, historically a B cup, meaning a fixed difference between the full bust and the upper bust. If your own difference is larger, selecting by full bust gives you a pattern that is too wide across the shoulders, chest and back, and you will spend the fitting process taking in everything except the part that actually needed room.

The standard approach is to select the size by your upper bust measurement, which gives a correct shoulder and upper chest fit, then perform a full bust adjustment to add the volume needed over the bust itself. If your upper bust and full bust are close together, or the full bust is smaller in proportion, the reverse applies and a small bust adjustment removes surplus.

Cup Sized Patterns

Some pattern companies now publish separate pattern pieces for different cup proportions, typically A through DD or beyond. These have the bust adjustment already drafted in, so you select your size by upper bust and then select the cup piece matching the difference between your upper bust and full bust. Where they are available they save considerable work, though the cup letters correspond to that measurement difference and not to bra cup size, which is calculated differently.

A Working Method

Measure yourself and write the numbers down. Choose the size by upper bust for tops and dresses, by waist or hip for skirts and trousers depending on which is proportionally larger. Read the finished garment measurements and calculate the ease you will actually get. Compare that against a garment you own. Then, if the pattern spans several sizes at once, grade between them rather than compromising on one. The size number itself carries no information worth acting on.