Giving Slic3r the correct filament diameter is a critical step for getting quality prints. Each spool of filament (even if from the same manufacturer) will have slightly different diameters, and the diameter will vary as you go through the spool.
Digital Calipers - Available at Harbor Freight for less than $15
Calculator - Phone, tablet, or computer will work great
Measuring Filament Diameter
1. Zero the calipers, and pinch the filament between them. Write the first measurement down.
2. Move down the filament (1-3 feet), and take another measurement. Write the second measurement down.
3. Move down the filament (1-3 feet), and take another measurement. Write the final measurement down.
4. Average all three measurements by adding them, and then dividing the final number by 3. For example: 1.53 + 1.50 + 1.53 / 3 = 1.52
5. Input the final number into Slic3r, or whatever software you use to slice models for printing. Creating a filament profile for each spool of filament is a good idea.