Size Standards Reference: Who Sets the Sizes of Everyday Things

Size standards at a glance

The table below lists the standards that fix the sizes covered most often on this site. Where a standard defines a family of sizes, the key figure shows one representative size.

Who writes size standards

A standard is only as binding as the law or contract that adopts it. Most are voluntary: companies follow them because parts must fit together and buyers expect them to.

• ISO (International Organization for Standardization) and IEC (electrotechnical) publish international standards, adopted nationally as EN, BS, DIN or JIS versions.
• ANSI does not write most standards itself. It accredits developers such as ASME and ASTM International and approves their documents as American National Standards.
• CEN publishes European standards (EN), such as EN 13402 for clothing labels. National bodies like DIN (Germany) and BSI (UK) adopt and publish them.
• NIST maintains US measurement units and some Voluntary Product Standards, including PS 20 for lumber.
• Regulators such as the US Consumer Product Safety Commission (CPSC) and NHTSA set mandatory rules, usually for child safety and vehicles.

Underneath everything sit the units themselves. Since the 1959 International Yard and Pound Agreement, 1 inch is exactly 25.4 mm and 1 pound exactly 0.45359237 kg. The US survey foot, which differed by two parts per million, was retired for new work at the end of 2022. The unit converter uses these exact definitions.

Paper, envelope and card standards

ISO 216 is the best-known size standard in the world. It starts from A0, a sheet of 1 m² with sides in the ratio 1:√2 (841 × 1189 mm), and halves it to get each smaller size, so every A size has the same shape. The B series sits between A sizes, and the C envelope series (originally ISO 269) is sized so an A sheet slides into the matching C envelope.

US Legal (8.5 × 14 in) and the No. 10 business envelope (4 1/8 × 9 1/2 in) are long-standing trade sizes rather than ANSI drawing sizes. The full charts are in the paper size guide, the A4 page and the envelope size chart.

Clothing, shoe and jewelry standards

Here the standards define how to measure the body and how to label a size, not what a size must be. That gap is why two "medium" shirts can differ by a full size.

The common shoe systems are older than any of these standards. UK and US sizes step in barleycorns of 1/3 in (8.47 mm); continental European sizes step in Paris points of 2/3 cm (6.67 mm), so EU sizes go up in smaller increments. US hat sizes are the head circumference divided by π, so a 7 1/8 hat fits about 22.4 in (56.9 cm). The shoe size chart, clothing size chart and ring size chart map each label to its measurement.

Screen and electronics standards

• Screen size is the diagonal of the viewable area in inches. A label such as "65-inch class" may cover a diagonal of 64.5 in. Width and height depend on the aspect ratio; for 16:9 the width is 0.8716 × the diagonal. See aspect ratios.
• VESA mounting (FDMI) fixes the bolt-hole patterns on the back of monitors and TVs: 75 × 75 and 100 × 100 mm for most monitors, then 200 × 200 mm up to 600 × 400 mm and larger for TVs.
• Batteries: IEC 60086 (and ANSI C18 in the US) sets maximum cell sizes: AA 14.5 × 50.5 mm, AAA 10.5 × 44.5 mm. See the battery size chart.
• Phone and laptop sizes have no standard beyond the screen diagonal. Body dimensions come only from manufacturer spec sheets.

Building and hardware standards

Construction mixes nominal names with actual sizes, and the standards say which is which.

Building codes apply only where a jurisdiction adopts them, often with local amendments, so check with your building department. The lumber size chart, brick sizes and concrete block sizes have the full tables.

Vehicle, shipping and logistics standards

The meaning of every part of a tire sidewall is covered in the tire size guide, and container interiors and payloads in the shipping container guide.

Safety standards with mandatory dimensions

These are the size rules most likely to be enforced by law in the US. They are minimums for safety, not just conventions.

• Cribs (16 CFR 1219 and 1220): slats no more than 2 3/8 in (6 cm) apart. A full-size crib mattress must be at least 27 1/4 × 51 5/8 in and no more than 6 in thick, so no dangerous gap forms at the sides. See the crib size guide.
• Small parts (16 CFR 1501): toys for children under 3 must not have parts that fit in a test cylinder 1.25 in (31.7 mm) wide and 1 to 2.25 in deep.
• Child car seats (FMVSS 213): sets the crash performance and labeling rules for every seat sold in the US, including the weight and height limits printed on the label. See car seat sizes.

Sizes with no formal standard

Plenty of familiar sizes are conventions only. They stay stable because related products must fit, but nothing stops a manufacturer from changing them, so measure before you buy.

• Mattresses and bedding: queen 60 × 80 in and king 76 × 80 in in the US are industry norms; European and UK bed sizes differ.
• Carry-on bags: each airline sets its own limit. 22 × 14 × 9 in is common in the US and about 55 × 40 × 23 cm in Europe; see carry-on rules.
• Furniture heights: 30 in dining tables and 18 in chair seats are ergonomic norms, not standards.
• Pizza, cake pans and food containers: sizes are named by diameter or capacity with no agreed tolerances.

Frequently asked questions