I was asked to change the images to 300 dpi tiff files at a width between 3.5" to 5". The image properties show the width in pixels only. How do I resize the width in pixels to fit the width between 3.5" to 5" at 300 dpi? Thank you.
300 PPI (pixels-per-inch) means 1 inch = 300 pixels. From that you should be able to work out your sizes relatively easy.
If you know the pixel dimensions and need to work out the physical dimensions based on 300 PPI, just divide your pixel value by 300. For example:
You have an image that is 1,200x1,200 pixels.
1,200 / 300 = 4
So if you are printing at 300PPI, your image will be 4x4".
If you know the physical dimensions and need to know what pixel dimensions you need, just multiply your inches by 300. For example:
You need a 2x2" image at 300PPI.
2 x 300 = 600
So your image needs to be 600x600 pixels.
Note DPI and PPI are used interchangeably a lot of the time but it should really be PPI.
Assuming you're using Photoshop, here's a step by step guide to change your default units preferences. Though you may also be able to just toggle the ruler on (CTRL + R) and right click on it to change the unit preference.
Also here's a link to another question that might address some issues you could run into.
Hope this helps!