A3 vs A4 size: dimensions, pixels and which to choose

A3 size is 297×420 mm, which is 29.7×42 cm or 11.7×16.5 in. A4 is exactly half of an A3 sheet, and A5 is half of an A4. This guide compares the three in millimetres, centimetres, inches and pixels, then covers what each is used for, how to frame it, and how A3 relates to A2 and A1.

A3 size compared with A4 and A5

To convert millimetres to inches, divide by 25.4. For A3, 297 ÷ 25.4 is 11.69 and 420 ÷ 25.4 is 16.54, which round to 11.7×16.5 in. Every A size is the one above it cut in half across the long side, so A3 holds exactly two A4 sheets. Place two A4 sheets side by side along their short edges and you have A3 (2 × 21 cm = 42 cm). The long side of A4, 29.7 cm, is the short side of A3.

A3 is 1.41 times the width and 1.41 times the height of A4, which is 41% more in each direction and twice the area.

What A5, A4 and A3 are used for

• A5 (14.8×21 cm): notebooks, cards and small desk prints. Too small to carry a wall on its own.
• A4 (21×29.7 cm): the everyday document and letter size in most countries outside North America, where US Letter is used instead. It is also the largest size many home printers take.
• A3 (29.7×42 cm): small posters, drawings and wall prints. It is often the smallest size that reads as a poster rather than a page. Many home printers do not take A3, so it is usually printed at a print shop.

A3 size in pixels: resolution for printing

Pixels equal inches multiplied by pixels per inch (ppi). 300 ppi is the print-shop ideal, and around 150 ppi still looks sharp on a poster seen from a metre or more. At 300 ppi, A4 needs 2480×3508 px and A3 needs 3508×4961 px. At 150 ppi, A3 needs 1754×2480 px.

Enlarging a file costs resolution by a factor of 1.414 for each A step. An A4 file of 2480×3508 px stretched to A3 comes to about 212 ppi (300 divided by 1.414), which is still above the 150 ppi that looks sharp from a metre. One more step, to A2, brings it to about 150 ppi.

To check any file, divide its pixel width by the print width in inches. A file 3508 px wide printed 11.7 in wide is 300 ppi.

Framing A3, A4 and A5

A frame takes the size of the print it holds. With a mat, the frame is one standard step up: an A4 print in an A3 frame, an A3 print in an A2 frame. The mat opening is cut slightly smaller than the print. Our guide to framing a poster with a mat, or without one covers when a mat is worth it.

Match the shape as well as the size. A3 (29.7×42 cm) does not fit a 30×40 cm frame or a 12×16 in frame (30.5×40.6 cm), because A3 is 1:1.414 and those are 3:4. A3 is 2 cm taller than 30×40 cm and 1.4 cm taller than 12×16 in. In the same way, A4 (8.3×11.7 in) does not fit a US Letter frame (8.5×11 in), because it is 0.7 in too tall. Buy an A-size frame for an A-size print.

The US range has no exact A3. The nearest sizes are 12×16 in (30.5×40.6 cm) and 12×18 in (30.5×45.7 cm), and A3 is 42 cm tall, between the two.

How A3 relates to A2 and A1

A2 is double A3 and A1 is double A2, so A1 has four times the area of A3. Each step up multiplies both sides by about 1.414, the square root of 2, which is why the shape never changes.

The rule behind all of this, starting from A0 at one square metre, is in our guide to A1, A2 and A3.

Choosing A3 or A4 for a print

A4 suits a desk, a shelf, or a group of several prints on one wall. A3 is large enough to stand alone on a narrow wall or in a hallway. A pair of A3 prints side by side with a 5 cm gap is 64.4 cm wide. With the rule of thumb that art should be about two-thirds of the furniture width, that suits a desk or console about 95 to 100 cm wide. Our art print size guide sets each size against real walls.

Montclair prints are made to order on 200gsm archival matte paper with pigment inks, framed or unframed. Frames come in black, white, natural wood or dark wood and arrive ready to hang. Unframed prints ship rolled in a protective tube, with free tracked shipping worldwide. Sizes run from A4 (21×29.7 cm) through A3 and A2 to A1 and A0. Browse the motorsports prints, or open one such as Rally Car Mid-Jump, and pick a size on the product page.

Most prints also come as an $8 Print Ready Package: seven print-ready files covering 4×6 in to 24×36 in and A4 to A1, a guide to printing them, and a personal-use licence. A4 and A3 both sit inside that range, so a local print shop can print either from the same package.