Don't Assume Your Size
You want to be accurate from the outset when you're getting your boot size. Boots and gym shoes are often sized differentlty, and what you need from a boot may differ from what you need from a gym shoe, especially if you're wearing thick or multiple pairs of socks.
Measure Your Foot
The most accurate way to know your foot size is to measure your foot. You can do this by using a Brannock device (the device that shoe stores use to size feet) or by standing on a piece of paper and tracing your foot to measure it. Make sure you measure both feet, as one might be larger than the other!
Check the Size Chart
Typically, you want to use a size chart from the shoe company you plan to purchase from. However, there are standard size charts for Men's and Women's US sizes that you can also reference if your company has a conversion chart instead of absolute measurements.