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How to Find a Woman’s Ring Size (Even Without Her Knowing!)

by David Weliver

So, you’ve finally found “the one” and are ready to propose? Congrats! But do you know her ring size? Here, we’ll show you how to find a woman’s ring size. We’ll even include a few tricks to use to find her size on the DL if you want to surprise her.

How ring size is measured

Ring size is based upon the circumference (distance around) the thickest part of your finger in millimeters. This measurement is the converted to standard ring sizes.

For example, if you measure your ring finger and determine it is 54 mm in circumference, this measurement translates to an American ring size of 7 on a scale between 3 and 13.

If you’re in the United Kingdom, your ring size would be an O on a scale of F to Z. If you happen to live in France, your conversion is easier, as the French simply express ring sizes in the nearest quarter of a millimeter.

Ring size conversion chart (millimeters to ring size)

The U.S. ring size scale goes from size 3 (44.1 mm) to size 13 (69.6 mm).

Use the following ring size conversion chart to convert a measurement in millimeters to either U.S. or U.K. ring sizes.

Ring size conversion chart - millimeters to US and UK ring sizes.
Ring size conversion chart: Millimeters (mm) to U.S. and U.K. ring sizes.

How to measure for ring size at home

You can easily measure for ring size at home with either measuring tape or a piece of string (or dental floss) and a ruler. You can also buy a ring sizing tool like the one’s jewelers use for less than $10.

The best way to find a woman’s ring size is to either ask or to visit an experienced jeweler who will measure her finger for you. Obviously, if you want to surprise her (or save money by buying from a top online jeweler), you’re going to need to find her ring size on your own!

Find ring size with measuring tape

The easiest way to do this is to measure the thickest part of her ring finger with a sewing tape measure like the one pictured below. You can pick up a measuring take for less than $5 on Amazon, at Walmart, or any fabric store.

Sewing tape measures can be used to find her ring size at home.
Inexpensive sewing measuring tape is the best way to find her ring size at home.

Most measuring tape has inches on one side and metric measurements (centimeters and millimeters) on the back. Just be sure your tape has millimeters because inches (or even centimeters) will not be precise enough.

Find ring size with string or dental floss

If you do not have measuring tape, you can find her ring size at home using a piece or string or dental floss and either a ruler or a carpenter’s tape measure.

Simply wrap the string or floss around the thickest part of her ring finger and either cut or mark the string. Then measure it with the ruler or tape measure.

Find ring size with a ring sizing tool

Finally, you can also buy a ring sizing tool like the ones jeweler use. They are quite inexpensive, with several options available on Amazon for under $10.

James Allen will also send you a free one as we explain below.

Here’s what they look like:

Use this virtual ring sizer

Top-rated online jeweler James Allen offers this free virtual ring sizer tool to help you find your girlfriend’s engagement ring size. If you want to be precise as possible, they’ll even mail you a free ring sizer tool.

James Allen virtual ring size tool.
James Allen’s virtual ring sizer tool will help you find her ring size using an existing ring, by measuring her finger. They’ll even mail you a free ring sizer.

When is the best time to measure for ring size?

If you’ve ever worn a ring before, you know that your fingers can swell throughout the day or even based upon the weather or your diet. Therefore, it’s important to take multiple measurements to get the most accurate ring size.

At the very least, try to measure her ring size both in the morning and in the evening. To be as accurate as possible, aim to get a few measurements over the course of a few days.

You’ll want to use the largest ring size you measure. If your fingers swell up and you’re wearing too small of a ring, it will be difficult and even painful to take off. In extreme cases, you may need to use a ring cutter to cut it off.

How to find a woman’s ring size without her knowing

If you need to find your girlfriend’s ring size secretly, you can try to use the string-and-ruler method while she’s sleeping or measure one of her existing rings.

If you’re feeling sneaky, you can try to wrap a string around her ring finger while she’s sleeping and then measure the string with a ruler.

What if you can’t do this, or she’s a light sleeper? The next best thing to do would be to find an existing ring she owns. (Just make sure it’s something she wears on her ring finger, like a promise ring.) Every finger is a different size!

With an existing ring, it will be easier to measure the diameter (the distance across) rather than the circumference. Measure the diameter inside the ring, not outside it. You can then calculate the circumference with this formula:

Circumference = π x Diameter (π = 3.14159)

How to buy an engagement ring if you don’t know her ring size

If you want to guarantee your proposal stays a surprise, you may have to buy an engagement ring without knowing your soon-to-be fiancée’s ring size. It’s not a big deal! Jewelers resize rings all the time.

Better yet, good ones should offer to do it for free as part of your purchase.

Of course, you’ll want to make an educated guess about her ring size before you buy. Most women have U.S. ring sizes between 5 and 7.

Try to observe the size of her hands relative to other women. Are they smaller, average, or larger? Buy a size 5 if they’re on the smaller side, a size 6 if they’re average, and a size 7 if they’re larger.

You might just get lucky. She’ll likely be able to wear it until you can get it resized if you’re only off a half size or so. The ring may just feel a little loose or tight.

In the worst-case scenario, your fiancée may not be able to wear her ring immediately after you propose.

How much does it cost to resize a ring?

Most jewelers will offer at least one complimentary ring resizing when you buy a ring from them. The fee to resize a ring again — or a ring you purchased elsewhere — varies by jeweler but is usually less than $100.

Even online jewelers will happily resize a ring that you purchased from them for free. Of course, you’ll need to ship it back and forth.

For example, here is Blue Nile’s official resizing policy.

We provide complimentary resizing for Build Your Own Rings™, and Diamond Bands when within the manufacturer’s recommended sizing range, during the first year. Simply contact a Diamond & Jewelry Consultant at 1-800-242-2728 to arrange for your free resize.

Brilliant Earth will send you a free ring sizer but also offers a very useful printable ring size guide that you can use to quickly find a ring size if you don’t want to wait for a sizer to arrive in the mail. Brilliant Earth also offers a free 60-day resize policy, too. (Request a free ring sizer or get the size guide here.)

James Allen also offers free ring resizing within one year of purchase with free shipping within the U.S. and Canada for the first resize. Additional resizes within the first year cost $25 for gold rings and $50 for platinum.

How do jewelers resize rings?

Jewelers resize rings by making a delicate cut at the bottom of the ring and either adding or removing a small amount of metal to the ring.

As a result, resize fees may differ depending on whether you are sizing up (adding metal) or sizing down (removing metal) as well as the material the ring is made of.

Watch how a jeweler resizes a ring.

Some rings cannot be resized. These include rings made of alternative metals like Tungsten, rings with milgrain or ornate bands, and some silver rings.


Finding the perfect ring is easier once you know how to find a woman’s ring size. You can do this by measuring her finger with a measuring tape, piece of string and a ruler, or a specialized ring sizing tool that can be purchased for less than $10.

If you need to secretly find your girlfriend’s ring size for an engagement ring, you can try to measure her finger at night. But it might be easier to measure an existing ring if she has one!

Finally, if you simply can’t find out her ring size ahead of time, it’s not difficult to resize a ring. Most jewelers will do this for free after you buy a ring from them. Otherwise, resizing a ring typically costs between $25 and $100.

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James Allen

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James Allen is the most competitive diamond vendor with more than 150,000 loose diamonds in inventory. They have been a leader in selling diamonds online and pioneered 3D imaging so you can see the exact diamond you're buying in exquisite detail. You'll find it hard to beat James Allen in terms of price or selection. Read our James Allen review.

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