Print sizes and shapes: why a photo gets trimmed to fit the paper

Every print size has a shape as well as a size. When a photo has a different shape from the paper, part of the photo has to be trimmed to fill the sheet, or the print gets blank borders. This page explains how to find the shape of a photo and of a print, which edges get trimmed, and how much each common photo shape loses at each of the four Purrealism print sizes. For how big each size is in centimetres and where it suits a room, read how big is an 8 × 10 print. This page belongs to the guide to art prints and paper.

The shape of a print

The shape of a rectangle is its aspect ratio: its width compared with its height, reduced to the smallest whole numbers. An 8 × 10 print reduces to 4:5, because both numbers divide by 2. A 12 × 16 print and an 18 × 24 print both reduce to 3:4. A 24 × 36 print reduces to 2:3, because both numbers divide by 12.

A second way to compare shapes is to divide the short side by the long side. The result is the same for any size of the same shape.

The higher the number, the closer the shape is to a square. The 8 × 10 print is the squarest of the four, and the 24 × 36 print is the longest and narrowest.

The shape of a photo

A photo's shape comes from its pixel width and height. Divide both by the same number until they cannot be reduced further. A photo of 3,024 × 4,032 pixels divides by 1,008 to give 3:4. A photo of 4,000 × 6,000 pixels divides by 2,000 to give 2:3. A photo of 1,080 × 1,920 pixels divides by 120 to give 9:16.

The University of Chicago Press, in its guide for authors, says the pixel dimensions are listed in the file's properties and shown by most photo editing programs. So you can find the shape of any photo before you print it.

Why a photo gets trimmed

To fill a sheet from edge to edge, the photo is enlarged until it covers the whole paper. It stops growing when it covers the paper in the direction that runs out first. The rest of the photo then hangs over the paper in the other direction, and that part is trimmed.

Which edges lose the strip depends on which shape is wider:

• The print is wider in shape than the photo. The photo fills the width first, so strips come off the top and bottom.
• The print is narrower in shape than the photo. The photo fills the height first, so strips come off the two sides.

Here is one example of each. An upright 3:4 photo (0.75) on an 8 × 10 print (0.8) meets a print that is wider in shape, so its top and bottom are trimmed. The same photo on a 24 × 36 print (0.667) meets a narrower shape, so its sides are trimmed.

The trimmed share of the photo is one shape number divided by the other, taken away from 1. For the 3:4 photo on 8 × 10 paper, 0.75 ÷ 0.8 = 0.9375, so 6.25 percent of the photo is trimmed. On 24 × 36 paper, 0.667 ÷ 0.75 = 0.889, so 11.1 percent is trimmed.

How much each common photo shape loses

The table below uses that method for five photo shapes. Each figure is the percent of the photo's area trimmed to fill the paper, rounded to a whole number, with the photo and the print both upright.

Two patterns stand out. Every photo shape matches one print shape exactly, except the two shapes at the bottom, which lose a share at every size. And a square photo loses the most on the tallest print, because the gap between the two shapes is widest there.

Borders instead of a trim

If you want to keep the whole photo, you can print it smaller than the paper and leave blank borders. The size of the borders follows from the same shape numbers.

Take a 3:4 photo on 8 × 10 inch paper. Printed to the full 10 inch height, it is 7.5 inches wide (10 × 0.75). That leaves 0.5 inches of blank paper across the width, which is 0.25 inches at each side when the photo is centred. A 2:3 photo on the same paper is 6.67 inches wide at full height, which leaves 0.67 inches at each side.

If the print will be framed, plan the borders together with the frame and any mat. The guide to framing and hanging covers frames and mats.

Upright or wide

A print can hang upright or on its side. Turning the paper changes its shape from 3:4 to 4:3, for example. The trim is smallest when the print is turned the same way as the photo. An upright 3:4 photo on an upright 12 × 16 print loses nothing. On the same paper turned wide, 16 × 12, it would lose 44 percent of its area, because the photo's shape number (0.75) would be compared with the paper's (1.333).

The print resolution calculator further down this page works this way. You type a photo's pixel width and height, or let it read them from a photo you choose. For each of the four sizes it turns the print to match the photo, trims the photo at two opposite edges to match the print's shape, and shows the pixels per inch after the trim, the band that figure falls in and the percent of the photo trimmed. The page on print resolution explains the pixels per inch.

Inch sizes and the ISO A sizes

The four print sizes are measured in inches. The United States defines the inch as exactly 25.4 millimetres, according to the National Institute of Standards and Technology, which dates that definition to July 1, 1959.

Most countries use a different family of paper sizes. Markus Kuhn's guide to international paper sizes at the University of Cambridge explains the ISO 216 system. In it, the long side of every sheet is the square root of two times the short side, 1.4142. This shape has one useful property: cutting a sheet in half, parallel to its short side, gives two sheets of the same shape. A0 has an area of one square metre, A1 is half of A0, and so on down to A4, which measures 210 × 297 millimetres.

The guide says Germany adopted these sizes as the standard DIN 476 in 1922. They became the international standard ISO 216 in 1975, and they are now used in almost every country except those in North America.

For a print, the A shape (short side ÷ long side = 0.707) falls between the 2:3 shape of a 24 × 36 print and the 3:4 shape of a 12 × 16 print. A photo printed on A4 paper and later framed in an inch-sized frame will not match its shape. Choose the print size and the frame size in the same system.

Planning a photo for the trim

Before you choose a size, look at the edges of the image that will be trimmed. For an upright 3:4 image on 8 × 10 paper, those are the top and bottom, which in a pet portrait may hold the ears, a crown or the paws. For an upright 3:4 image on the tall 24 × 36 print, they are the two sides, which may hold part of the background or a second animal.

Rollins College's library, in its advice for poster printing, suggests previewing a file at "actual size" before printing to check it. Do the same with the trim in mind: if the part you care about sits close to an edge that will be trimmed, pick the print size whose shape matches the image instead.

What this means for a royal pet portrait

Purrealism art prints come in 8 × 10, 12 × 16, 18 × 24 and 24 × 36 inches, on matte or glossy paper. You choose the size in the portrait studio. If you pick the digital download and plan to print it yourself, find the image's shape first, then choose a paper size that matches it or accept the trim shown in the table above. The page on printing a digital download covers the printer settings that control trimming and borders.

A portrait with detail near its edges gives you less room to trim. In the Rajput Warrior portrait in the gallery, a dog in armour with a fort at sunset behind, the fort's tower stands at the right edge of the image and the dog's ears reach close to the top. A print shape that trims the sides would cut into the tower, and one that trims the top and bottom would come close to the ears.