Don't measure cold fingers. This is when fingers are their smallest.

For the most accurate reading, measure the finger at the end of the day when the finger is the largest.

The best size is usually the ring that fits snugly and gives a little resistance when you take it off.

Bear in mind that a wide band ring will cling more to the finger and catch more on the knuckle, and so you may need a larger size than normal.
Men very often lose their wedding rings as they choose a size that is too loose; if you are not used to wearing a ring, the proper size may feel tight at first.

You will probably find that your dominant hand is usually one UK size larger than your other hand, so if you are right handed and have a size M ring that fits the left, you will probably wear a size N ring on the right hand.

Ask your partner's mother or one of her friends if they know her finger size.

Borrow one of your partner's rings (that they wear on the finger you want the ring for) and slide it down a tapered candle. Then measure using the chart above.