Whether it is an engagement ring or an anniversary gift, ring size is the detail that most often goes wrong when the recipient does not know it is coming. The good news is that there are several reliable ways to find a size without giving the game away, and a few things to do to protect yourself if you guess wrong.

Method 1: Borrow a ring they already wear

This is the most accurate approach. Choose a ring that fits the same finger you plan to put the new ring on, since sizes differ between hands and fingers. A ring worn on the right hand is often a bit larger than one worn on the left. Then you have two ways to use it:

  • Measure the inside diameter. Place the ring on a ruler and measure straight across the inside, from inner edge to inner edge, in millimeters. Enter that number in our ring size checker using the known-diameter mode.
  • Trace it. Press the ring's inside edge on paper and mark the circle, or push it into a bar of soap to leave an impression. Measure the opening afterward.

Take the ring only briefly and put it back where you found it. A ring that goes missing for an hour is the quickest way to spoil a surprise.

Method 2: Ask someone who knows

A close friend, sibling or parent may already know the size, particularly if the person has had a ring resized or sized professionally. Ask a question that does not give the game away, such as, "Do you remember what size her mom's ring was?" Be aware that people often remember a size from many years ago, or from a different finger.

Method 3: Use the tool during a normal conversation

If the person is comfortable with technology, it is easy to run our calculator together casually, for example while browsing jewelry for a gift for someone else, or while talking about whether a device like a smart ring would suit them. Our Oura ring guide is a good example of a topic that comes up naturally.

Method 4: Take a size from a photo (with caution)

Some people estimate size from a photo of the ring on the hand next to a known object like a coin. This is the least reliable method, because camera angle and distance distort proportions. Use it only to decide between very different sizes, such as "closer to a 5 or a 9," never to pin down a precise number.

Method 5: Buy with a plan for resizing

If you cannot get a reliable measurement, plan for it. Buy from a jeweler that offers free or low-cost resizing, check the return policy before you buy, and tell the seller the purchase is a surprise. When you propose, say that the ring can be exchanged or adjusted. For rings that are difficult to resize, see ourresizing guide before choosing the design.

Which way to err

If you have to guess between two sizes, going slightly larger is usually safer: a loose ring can be held in place with a guard, while one that is too small may not go over the knuckle, which can be awkward in a proposal. Wide bands usually need a bit more room, so read our guide if you are choosing one.

Common mistakes to avoid

  • Using a ring from the wrong finger or hand.
  • Measuring the outside of the ring instead of the inside.
  • Using a ring that was loose or too tight to begin with, since it will pass on the same error.
  • Assuming that a fashion ring size matches the size of the finger it was designed for.

For a full list of errors to avoid, see 10 common ring sizing mistakes.