How to measure without a ring sizer:

Wrap a piece of paper around your finger, mark with a pen and measure the length. This will give you the circumference of your finger.

If choosing a ring with a wide band width, you may need to go one size up.

Alternatively you can order a ring sizer for a more precise measurement.

When to measure:

The best time of day to measure is in the evening, when your fingers are largest.
Avoid measuring when your fingers are cold, as they can be at least half a size smaller.