It is possible to measure ring size at home without specialized tool.
There are two easy ways to measure ring size. The first way is if you have a ring that fits already. Download this Ring Size Chart and print out the chart. Select “Actual Size” or “No Scaling” on your printing option. Then measure your ring’s inside edge against the images on the chart.
The second way is to measure the circumference of your finger with a piece of paper and a ruler. Follow this guide and measure in millimeter to get the ring size that matches our numbering system. Remember that measuring with paper is better than string as strings can stretch and reduce the measurement accuracy.