Ring Size Conversion Chart
If you already own a ring that's the right fit then you can find its US size here.
A quick way to estimate the size of your finger is to:
take a strip of paper and wrap it around your finger, cutting it where ends overlap.
Lay the paper flat next to a ruler and measure its length to find the circumference of your finger, then check circumference against this chart.
Alternatively you can head to your local mall and ask a jewellery store assistant to measure your size, or get in touch to have a sizing belt posted out to you or an appointment set up if you live locally.