This is one of the most common questions on photography forums. If you are thinking of buying, for example, an 8 megapixel digital camera, you want to know how large a print you can expect to be able to produce from its image files. Unfortunately, we can't simply say 8x10" or 16x20" or any such single answer. Assuming an excellent lens and flawless technique, maximum print size still varies enormously depending on your subject matter and how close you are to your subject. It also depends on how acute the viewer's close vision is and how close she gets to the print. Billboards are routinely printed at 15 pixels per inch – which would make a 5 mp image weigh in at 10x14 feet (3x4 meters)! – but assumes the viewer is many yards away. For the following discussion, however, we'll assume excellent vision and close inspection (12" or 30cm from eye to print).

Pixels per inch is an essential concept for this discussion. For example, the Nikon D40 is a 6 megapixel dSLR, which means it produces image files that are 2000 pixels by 3000 pixels in size. If I tell the printer I want a print that is 10 inches wide, the printer simply divides 2000 by 10, which tells it to use 200 pixels per inch. The length of the print will then be 3000 divided by 200 or 15 inches.

Consider the following close-up portrait:

A 6 mp image like this can be enlarged almost indefinitely. Certainly, a 16x20" (40x50cm) print should be possible.

Now consider the opposite extreme:

In this 6 mp picture the fine detail of the tree branches and houses needs to have absolutely crisp edges. A 9x13" (22x33cm) print is crisp; a 10x15" (25x40cm) print begins to show marginal enlargement blur; anything larger will not pass inspection by anyone able to read a typical paperback book.

So, if you want a simple answer to how large you can print while still maintaining reasonable quality at close inspection: the best that I can give you is:

For photo-realistic prints you will need a minimum of 220 pixels per inch for your most highly detail-dependent images to a minimum of 120 pixels per inch for your least detail-dependent images. (These ppi numbers are before any upsampling.)

This translates to:

In inches:

Sensor size	Pixel array	Ideal print size	Max for fine detail	Max for low detail
2 mp	1200 x 1600	4 x 6	5 x 7	10 x 13
3 mp	1536 x 2048	5 x 7	7 x 9.5	13 x 17
3 mp (dSLR)	1440 x 2160	5 x 7	6.5 x 10	12 x 18
4 mp	1704 x 2272	5.5 x 7.5	8 x 10	14 x 19
5 mp	1920 x 2560	6.5 x 8.5	9 x 11.5	16 x 21
6 mp	2136 x 2848	7 x 9.5	10 x 13	18 x 24
6 mp (dSLR)	2000 x 3000	6.5 x 10	9 x 13.5	16 x 24
7 mp	2304 x 3072	7.5 x 10	10.5 x 14	19 x 25
8 mp	2448 x 3264	8 x 11	11 x 15	20 x 27
8 mp (dSLR)	2336 x 3504	8 x 11.5	10.5 x 16	19.5 x 29
9 mp	2616 x 3488	9 x 11.5	12 x 16	22 x 29
10 mp	2736 x 3648	9 x 12	12.5 x 16.5	23 x 30.5
10 mp (dSLR)	2592 x 3872	8.5 x 13	12 x 17.5	21.5 x 32
12 mp	3000 x 4000	10 x 13	13.5 x 18	25 x 33
12 mp (dSLR)	2832 x 4256	9.5 x 14	13 x 19	23.5 x 35.5

In centimeters:

Sensor size	Pixel array	Ideal print size	Max for fine detail	Max for low detail
2 mp	1200 x 1600	10 x 15	13 x 18	25 x 33
3 mp	1536 x 2048	13 x 18	18 x 24	33 x 43
3 mp (dSLR)	1440 x 2160	13 x 18	17 x 25	30 x 46
4 mp	1704 x 2272	14 x 19	20 x 25	36 x 48
5 mp	1920 x 2560	17 x 22	23 x 29	41 x 53
6 mp	2136 x 2848	18 x 24	25 x 33	46 x 61
6 mp (dSLR)	2000 x 3000	17 x 25	23 x 34	41 x 61
7 mp	2304 x 3072	19 x 25	27 x 36	48 x 64
8 mp	2448 x 3264	20 x 28	28 x 38	51 x 69
8 mp (dSLR)	2336 x 3504	20 x 29	19 x 41	48 x 74
9 mp	2616 x 3488	23 x 29	31 x 41	56 x 74
10 mp	2736 x 3648	23 x 30	32 x 42	58 x 77
10 mp (dSLR)	2592 x 3872	22 x 33	31 x 45	55 x 81
12 mp	3000 x 4000	25 x 33	34 x 46	64 x 84
12 mp (dSLR)	2832 x 4256	24 x 36	33 x 48	60 x 90

Please remember that these numbers are meant as guidelines, not decrees.

Also keep in mind that detail rendering will improve by printing smaller than maximum size. 300 pixels per inch (uninterpolated) is usually considered to be the finest print resolution that the human eye can fully resolve, so I used this figure to calculate the ideal print size column. Old timers might think of this as contact print quality. And of course printing at even higher pixels per inch for even smaller print sizes will retain the impression of extremely fine detail and may give the impression of even greater fineness of detail.