How to find her ring size without her knowing

By The Raul Diamonds team · October 15, 2026 · 8 min read

1 carat emerald cut lab-grown diamond solitaire engagement ring in 14K gold

By the Raul Diamonds team · Updated October 2026

The short answer: The most accurate secret method is to borrow a ring she already wears on her left ring finger and have a jeweler size it on a mandrel, or measure its inside diameter in millimeters and match it to a US size chart. If you cannot borrow a ring, ask a close friend or family member. When you are between sizes, choose the larger one, because a ring is easier to size down than up.

A surprise proposal leaves one practical question: what size is her finger? This guide ranks eight discreet ways to find out, from most to least accurate, gives you a US size chart in millimeters, and explains how to choose when you are unsure. It ends with the safety net that makes a small miss easy to fix.

Key takeaways

A ring she wears on the left ring finger is the best clue; a jeweler can size it in under a minute on a ring mandrel.
US ring size 6 has an inside diameter of about 16.5 mm; size 7 is about 17.3 mm. Each whole size adds roughly 0.8 mm of diameter.
The string-around-the-finger trick while she sleeps is the least reliable method, because string stretches and fingers change size overnight.
Raul Diamonds includes one complimentary resize within 60 days of delivery, with shipping covered both ways.

Which finger and hand should you size?

In the US, an engagement ring is worn on the ring finger of the left hand, the fourth finger counting from the thumb. Size that finger, not any other.

The detail matters more than it seems. The same finger on the right hand is often slightly larger, especially if she is right-handed, and a ring from her middle or index finger will be a different size entirely. If the only ring you can borrow comes from her right hand, treat its size as a close estimate and lean toward the smaller end of your range for the left hand.

Knuckles matter too. A ring has to slide over the knuckle and then sit at the base of the finger without spinning. A ring she wears daily has already passed that test, which is why it is the best evidence you can find.

How can you find her ring size without her knowing?

Borrowing a ring she wears on the right finger and having it measured is the most accurate method. The other methods below are ranked by how close they usually get you.

Have a jeweler size a borrowed ring. Take a ring she wears on her left ring finger to any jeweler. They slide it onto a tapered steel rod called a ring mandrel and read the size to the quarter. Have it back in her jewelry box the same day.
Measure the inside diameter yourself. Lay the borrowed ring on a ruler or, better, a digital caliper, and measure straight across the inside edge in millimeters. Match the number to the chart below. Measure twice, at right angles, and use the average.
Trace or match it on a printable chart. Trace the inside of the ring on paper with a sharp pencil, or place it on the circles of our printable ring size guide and find the circle that lines up with the inside edge. Print at 100% scale.
Press it into soap or clay. If the ring cannot leave the house, press it halfway into a bar of soap or a little modeling clay to leave a clean impression. Measure the impression, or take it to a jeweler.
Ask a friend or family member. Her sister, mother or closest friend may already know her size, or can find it on a shopping trip by trying on rings together. Choose someone who can keep a secret.
Compare against your own finger. Slide her ring onto your own fingers and mark where it stops, for example halfway down your pinky. A jeweler can then size that spot on your finger. Useful, but rough.
Use the typical range as a fallback. Many jewelers treat US size 6 to 7 as the common range for women. It is a starting point, not a measurement, and it can be off by two sizes or more.
The string trick while she sleeps. Wrapping string or paper around her finger at night is the method most people try first and the least reliable. String stretches, it is hard to pull to the right tension, and fingers change size with temperature and time of day. It also skips the knuckle entirely.

What is the US ring size chart in millimeters?

A US ring size maps to an inside diameter and circumference: size 6 is about 16.5 mm across and 51.9 mm around, and each half size adds about 0.4 mm of diameter.

The values below follow the standard US scale, where whole sizes are 0.032 inch (about 0.81 mm) apart in inside diameter, as summarized here. They match the full chart in our ring size guide, which runs from size 3 to 12.

US size	Inside diameter (mm)	Inside circumference (mm)
4	14.9	46.8
4.5	15.3	48.0
5	15.7	49.3
5.5	16.1	50.6
6	16.5	51.9
6.5	16.9	53.1
7	17.3	54.4
7.5	17.7	55.7
8	18.1	57.0
8.5	18.5	58.2
9	18.9	59.5

Use the diameter column for a borrowed ring and the circumference column for a strip of paper. If your measurement falls between two rows, note both sizes and read the next section.

What if you are between two sizes?

Choose the larger size, or the middle of your range. A ring that is slightly loose can still go on and be worn safely until it is resized; a ring that is too small cannot get past the knuckle at all.

Making a ring smaller is also usually the simpler job for a jeweler: a small section of the band is removed and the shank is rejoined. Making it larger means stretching the metal or adding to it, which has limits. Most plain-band rings can move up or down one to two sizes, which gives a sensible guess plenty of room.

Two more things tilt the choice. Wider bands, above about 6 mm, fit more snugly, so they often call for half a size up. And fingers swell in heat and after salty meals, then shrink in the cold, so a size taken on a winter morning reads small.

Which rings are hardest to resize?

Rings with diamonds set all the way around the band, such as full eternity bands, are the hardest to resize, because there is no plain metal to cut and rejoin without disturbing the stones.

The same caution applies to some channel-set and intricate pavé designs, and to tension or unusual shanks. If you are guessing her size, a solitaire or a ring with a plain lower band is the forgiving choice: a jeweler can adjust it cleanly and the change is invisible. A half eternity band, with stones across the top only, keeps a plain section underneath for the same reason.

What happens if the ring does not fit?

If the ring does not fit, Raul Diamonds resizes it for you. Every Raul ring includes one complimentary resize within 60 days of delivery, with insured shipping covered both ways.

There is one detail to know before you ask her. A resized ring cannot be returned, so if there is any chance she would prefer a different design, decide on the design first and the size second. Our standard returns policy covers unworn pieces within 30 days of delivery, and the resizing policy explains how to request a new size.

If your proposal is close to the holidays, plan for timing as well. Every piece is made to order in 2–3 weeks, then delivered in 2–5 business days. Our Christmas proposal ring timeline works backward from the date.

The Raul tip If you borrow a ring, photograph it on her dresser before you touch it, so it goes back in exactly the same spot and direction. Measure the same evening at room temperature, and return it before morning. Then give us both numbers you found, the diameter and the size, and we will tell you which one we would order.

Our picks

Solitaires with a plain band are the most forgiving rings to buy when her size is an educated guess, because they resize cleanly.

1 Carat Round Brilliant Lab Grown Diamond Solitaire Ring in 14K Yellow Gold: a slim, classic band that adjusts easily up or down.
1 Carat Oval Cut Lab Grown Diamond Solitaire Ring in 14K Yellow Gold: an elongated oval that flatters the finger, on an uncomplicated band.
Teardrop Pear Lab-Grown Diamond Solitaire Engagement Rings in 14K Gold: a graceful teardrop on a plain shank, simple to resize.
Oval Lab Grown Diamond Bypass Engagement Ring in 14K Gold: a softer, modern bypass silhouette with a plain lower band.

See every design in our solitaire engagement rings collection.

Questions

What is the average ring size for women in the US?

Many jewelers treat US size 6 to 7 as the common range for women. It is only a starting point, since individual sizes vary widely, so use a borrowed ring or a trusted friend whenever you can.

Can I use a ring she wears on her right hand?

Yes, as an estimate. The right ring finger is often slightly larger than the left, especially for right-handed people, so its size usually sits at or just above her left-hand size.

How accurate is the string method?

It is the least reliable method. String stretches, it is hard to wrap at the right tension, and it ignores the knuckle, so results are often off by half a size or more.

Should I size up or down if I am unsure?

Size up slightly or choose the middle of your range. A slightly loose ring can still be worn until it is resized, and making a ring smaller is usually simpler than making it larger.

Is resizing free at Raul Diamonds?

Yes, once. Every Raul ring includes one complimentary resize within 60 days of delivery, with insured shipping covered both ways. A resized ring cannot be returned.

Can every engagement ring be resized?

Most rings with a plain band can move up or down one to two sizes. Full eternity bands and designs with stones all the way around are difficult or impossible to resize, so they are best bought with a confirmed size.

Still unsure? Send us what you have, a borrowed ring's measurement or a size from her best friend, and we will help you choose a size and a design that resizes cleanly. Book a free consultation by video or phone.

Written by the Raul Diamonds team

Every guide is checked against our published policies and the grades on our IGI reports. Read the Raul Standard.

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