What File Size Do You Need?

Often when a buyer is looking for a photo, they have one piece of information in their head: they want 300dpi. But this is misleading. Here I will show you how to work out which image size you need to buy.

Most people will know that 300dpi is the resolution they need for printing a photo, but this may not always be the case. 300dpi is a measurement which tells you how many Dots Per Inch make up the photo. All photos are made up of dots so the more dots you have the better quality the photo is. But this figure alone doesn't tell you if the image is big enough to use in a given situation.

When you are searching for royalty free stock images on Fotolia, having clicked a thumbnail, under the stock image you will see the filesize (megapixels), the dpi and the size of image produced at that dpi. So you don't have to calculate a thing!

But if you want to understand how to calculate it for yourself, read on ...

You need to know four things to buy the right sized image:
1) What do you want the image for?
2) How big do you want the image?
3) Which orientation do you need it (horizontal or vertical)?
4) How much quality loss are you prepared to accept to get the size you want?

The Maths

Let's suppose you want to print an image onto a portrait A5 flyer.

You know three things:
(1) the image is for print, so you need to start by looking for 300dpi images (not 72dpi images which are used for websites)
(2) A5 is 148mm x 210 mm.
(3) You will know whether you need a wide picture (landscape/horizontal) or a tall picture (portrait/vertical)

Calculate this Example
- First convert the mm to inches.
- 148mm=5.83"x8.27"
- Next calculate how many dots you will need. 300 dots per inch is:
5.83x300 = 1749 dots; 8.27x300 = 2481 dots
- Multiply these two together to give you the number of dots needed to cover the space you have (A5)

You Will Have:
- 300 dpi
- Portrait (upright) orientation
- 5.83" wide, 8.27" tall
- 1749x2481 pixels in size - so the file size needs to contain 4,339,269 pixels.

There are 1million pixels in a megapixel. So the image you would need should be 4.34MP or larger.

Larger is never wrong. That is one thing you can rely on. If the picture you can buy is larger than you need, this is never a bad thing. You can simply resize the photo using any standard graphics package.

Smaller can be an issue. If the megapixels or the physical dimensions of the image are smaller than you need, then you will have to accept some quality loss as you have to stretch it to fit the space. The more you stretch an image, the less sharp it looks.

For a lot of instances this might not be a problem. This decision is something that you need to make bearing in mind what you are using it for, your target audience, what your designer suggests and your budget. If you've found the ideal image tht fits your budget for a flyer being handed out at a trade fair, you might decide the quality is good enough. But if you are a branding and marketing expert launching your company with a glossy A5 brochure, you would probably not accept any loss of quality and prefer to look for a larger filesize.

As a tip, if you are trying to make an image stretch, never go below 240dpi for print items or the quality will probably deteriorate too much.