What size is A4 paper?

210 × 297 mm, or 8.27 × 11.69 inches. Defined by ISO 216.

Why is A4 that particular size?

Because A0 is one square metre with sides in the ratio 1:√2, and each smaller size is the previous one halved. That ratio is the only one preserved by halving, which is what makes scaling between A sizes clean.

Does USB-C mean fast?

No. USB-C is a connector shape and says nothing about speed. The same port can be USB 2.0 at 480 Mbit/s or USB4 at 40 Gbit/s.

Is A4 the same as US Letter?

No. A4 is 210 × 297 mm and Letter is 215.9 × 279.4 — Letter is wider and shorter. A document laid out for one prints with wrong margins on the other.

Standard sizes

Paper, envelope, card, connector, container, photo and bed sizes, with dimensions in millimetres, centimetres and inches. Most of these are as permanent as reference data gets — ISO 216 has defined A4 since 1975.

Why A4 is 210 × 297 mm

A0 is defined as one square metre of paper with its sides in the ratio 1:√2. Every smaller A size is that sheet halved again. The √2 proportion is the only ratio preserved when a rectangle is cut in half, which is why scaling a document between A sizes never crops it or leaves odd margins, and why two A4 sheets side by side make exactly one A3. The idea dates to 1786 and was standardised in Germany in 1922 before becoming ISO 216.

Where the labels stop meaning anything

USB-C is a connector shape, not a speed. The same port can be USB 2.0 at 480 Mbit/s or USB4 at 40 Gbit/s — a factor of eighty between two identical-looking cables.
USB 3.0, 3.1 Gen 1 and 3.2 Gen 1 are the same 5 Gbit/s hardware renamed twice. The USB-IF now recommends marketing by speed for exactly this reason.
HDMI 2.0 was retired as a label in 2023 and folded into 2.1, so a device sold as HDMI 2.1 may support nothing beyond what 2.0 did. Check for the named features instead.

Paper sizes

A4 210 × 297 mm — 8.27 × 11.69 inches.
US Letter 8.5 × 11 inches — 215.9 × 279.4 mm.
A3 297 × 420 mm — 11.69 × 16.54 inches.
A5 148 × 210 mm — 5.83 × 8.27 inches.
A0 841 × 1189 mm — 33.11 × 46.81 inches.
A1 594 × 841 mm — 23.39 × 33.11 inches.
US Legal 8.5 × 14 inches — 215.9 × 355.6 mm.
A2 420 × 594 mm — 16.54 × 23.39 inches.
B5 176 × 250 mm — 6.93 × 9.84 inches.
Tabloid / Ledger 11 × 17 inches — 279.4 × 431.8 mm.
A6 105 × 148 mm — 4.13 × 5.83 inches.
A7 74 × 105 mm — 2.91 × 4.13 inches.
All 21

Envelope sizes

DL envelope 220 × 110 mm — fits A4 folded into three.
C4 envelope 229 × 324 mm — holds A4 unfolded.
C5 envelope 162 × 229 mm — holds A4 folded once, or A5 flat.

Card sizes

Credit card (ID-1) 85.60 × 53.98 × 0.76 mm.
Business card Varies by country: 85 × 55 mm in Europe, 89 × 51 mm in the US.

USB

USB-C connector A connector, not a speed — 8.34 × 2.56 mm, reversible.
USB 3.x 5 Gbit/s originally, renamed repeatedly — check the speed, not the name.
USB 2.0 480 Mbit/s. Still what most cables and keyboards use.
USB4 40 Gbit/s typical, built on the Thunderbolt 3 protocol.

Video connectors

HDMI 2.1 48 Gbit/s at full capability, but the label alone guarantees nothing.
HDMI 2.0 4K at 60 Hz. The practical minimum for a 4K desktop.
DisplayPort 1.4 4K at 120 Hz using display stream compression.
HDMI 1.4 4K but only at 30 Hz, which is why it feels wrong for desktop use.

Shipping containers

20-foot shipping container 6.06 × 2.44 × 2.59 m externally — about 33 m³.
40-foot shipping container 12.19 × 2.44 × 2.59 m externally — about 67 m³.

Photo sizes

6 × 4 inch photo 152 × 102 mm — the standard print size, and 3:2.

Bed sizes

Double bed UK 135 × 190 cm; US "full" 137 × 191 cm. Similar but not identical.
King bed UK king 150 × 200 cm; US king 193 × 203 cm — 43 cm wider.

Common questions

What size is A4 paper?: 210 × 297 mm, or 8.27 × 11.69 inches. Defined by ISO 216.
Why is A4 that particular size?: Because A0 is one square metre with sides in the ratio 1:√2, and each smaller size is the previous one halved. That ratio is the only one preserved by halving, which is what makes scaling between A sizes clean.
Does USB-C mean fast?: No. USB-C is a connector shape and says nothing about speed. The same port can be USB 2.0 at 480 Mbit/s or USB4 at 40 Gbit/s.
Is A4 the same as US Letter?: No. A4 is 210 × 297 mm and Letter is 215.9 × 279.4 — Letter is wider and shorter. A document laid out for one prints with wrong margins on the other.