This is a complex subject. Megapixels are a measure of quantity, not quality. An 8 megapixel camera phone isn’t the same as an 8 megapixel DSLR camera. In short, not all pixels are compared equally.
Simple formula to determine megapixels
Multiply the width (in pixels) by the height (in pixels) then divide the result by 1 million
For the sake of simplicity, we are going to assume you have a DSLR camera. I currently shoot with a Nikon D700 12.1MP camera for sports and a Nikon D810 36.3MP camera for portraits. To determine how many megapixels your camera shoots, use a program such as Lightroom or Window’s explorer to review the metadata. Look for dimensions under the Metadata panel.
The D700 shoots 4,256 by 2,832 pixels. Using our formula we multiply 4,256 by 2,832 which equals 12,052,992. Then we divide that by 1,000,000.
(4,256 x 2,832) / 1,000,000 = 12.052992 or 12.1MP
Calculate how many megapixels you need for a high quality print
( (300 x Width) x (300 x Height) ) / 1 million
300 ppi pixels per inch is recommended for high-quality prints. Determine the physical size of the print you want to make, such as 4 x 6 inches, 8 x 10 inches, etc. Now, multiply the width by 300, and the height by 300, which will give you the size in terms of pixels. Therefore, a 10 x 8 inch print would be 3,000 x 2,400 pixels equaling 7.2 million pixels. Dividing 7.2 million by 1 million will give you 7.2 megapixels, the minimum resolution you’ll want your camera to have.
( (300 x 10) x (300 x 8) ) / 1,000,000
(3,000 x 2,400) / 1,000.000
7,200,000 / 1,000,000
Calculate the largest size the image can print at 300ppi
(Pixel Width / 300) by (Pixel Height / 300)
Dividing an image’s pixel width by 300 will give you the largest length the image can print at full resolution. Dividing an image’s pixel height by 300 will give you the largest height the image can print at full resolution. Therefore, a 6399px by 4170px image can be printed at a maximum size of 21.33 by 13.9 inches.
6399px / 300 = 21.33 inches
4170px / 300 = 13.9 inches
21.33 in by 13.9 in maximum print size
Get the optimal detail
Jeff Schewe by far is the most authoritative expert on digital printing. In fact, he has written several must have books on the subject. The Digital Print and The Digital Negative are his most current ones. I reached out to Jeff for his expert advice. He confirmed 300 is the generally accepted resolution for inkjet prints (or silver chromogentic lab prints). But he added, it gets a bit more complicated if you want to get the optimal detail.
Epson pro printers “want” 360ppi (by want, he mean that’s what the printer reports to the printing pipeline). In fact, if you have enough native resolution you can print at 720ppi if you have “Finest Detail On” in the driver. For Canon & HP printers, the numbers are 300/600.
If you print out of Lightroom, Jeff says this is easy-just click on the Print Resolution and enter 360ppi (or 720 with Finest Detail selected). The numbers would be 300 or 600 for Canon & HP. Obviously, this is more complicated when printing out of Photoshop.
If going out to a lab, the labs generally do want only 300ppi which is fine.
A solution if your Print provider demands 300ppi
If the print service you use demands you send images that are 300ppi at full size, a simple solution is using onOne’s Perfect Resize. Perfect Resize use Genuine Fractals powered algorithms for optimizing clarity and detail for different image types. It gives you the best quality enlargements for your photos, whether taken with a DSLR or your mobile device.