US ring sizes convert to inside circumference and diameter by a fixed formula, so the chart below is exact. What is not exact is measuring your own finger — and four specific, avoidable things are what make home measurements come out wrong.
One clarification first, because two different "size charts" get confused constantly. This page is about the ring — how big the band has to be to fit your finger. If you are trying to work out how big the stone should be, that is a different chart, and we have it in our moissanite mm-to-carat size chart.
Ring Size Chart (US)
Ring sizing is a fixed scale rather than a convention: each whole US size adds 2.55mm of inside circumference and 0.81mm of inside diameter. The figures below are Angara's published conversion chart.
| US size | Inside diameter | Inside circumference |
|---|---|---|
| 3 | 14.1 mm | 44.3 mm |
| 3.5 | 14.5 mm | 45.6 mm |
| 4 | 14.9 mm | 46.8 mm |
| 4.5 | 15.3 mm | 48.1 mm |
| 5 | 15.7 mm | 49.3 mm |
| 5.5 | 16.1 mm | 50.6 mm |
| 6 | 16.5 mm | 51.9 mm |
| 6.5 | 16.9 mm | 53.1 mm |
| 7 | 17.3 mm | 54.4 mm |
| 7.5 | 17.7 mm | 55.6 mm |
| 8 | 18.1 mm | 56.9 mm |
| 8.5 | 18.5 mm | 58.1 mm |
| 9 | 19.0 mm | 59.7 mm |
| 9.5 | 19.4 mm | 61.0 mm |
| 10 | 19.8 mm | 62.2 mm |
| 10.5 | 20.2 mm | 63.5 mm |
| 11 | 20.6 mm | 64.7 mm |
| 11.5 | 21.0 mm | 66.0 mm |
| 12 | 21.4 mm | 67.2 mm |
| 12.5 | 21.8 mm | 68.5 mm |
| 13 | 22.3 mm | 70.1 mm |
Two anchors worth memorising, because they let you sanity-check any chart you find elsewhere: a US 6 is 16.5mm across, and a US 7 is 17.3mm. If a chart disagrees with those by more than a tenth of a millimetre, it is the chart that is wrong.
The practical consequence of the step size: half a ring size is about 1.3mm of circumference — less than the thickness of two stacked credit cards. It is also the difference between a ring that stays on and one that doesn't, which is why the measuring matters more than the chart does.
Three Ways to Measure at Home
In descending order of how much you can trust the answer.
1. Measure a ring that already fits. The most reliable, because it removes your finger from the equation. Lay the ring flat, measure the inside diameter across the widest point in millimetres, and read it off the chart above. Make sure it is a ring worn on the same finger of the same hand.
2. A plastic or paper ring sizer. Most sellers will send one free, and printable versions work if — and only if — you print at 100% scale with no page scaling. Blue Nile's fit test is the right one: the sizer should be "snug enough that the plastic ring doesn't slip off, but loose enough to slide over the knuckle."
3. String or paper strip and a ruler. The least accurate and the most popular, because it needs nothing you don't already have. Wrap a strip around the base of the finger, mark where it overlaps, measure the length in millimetres — that is your circumference. Read it off the chart, or divide by 3.14 for the diameter.
The string method's weakness is that string stretches and paper strips get pulled tight. If it is all you have, Blue Nile's advice applies: "Repeat the process three to four times to confirm the result."
| Method | What you need | How far it can be out |
|---|---|---|
| Measure a ring that fits | A ring and a ruler | Least — you are measuring metal, not skin |
| Plastic or printed sizer | A sizer, or a printer set to 100% | Small, if the print scale is right |
| String or paper strip | Nothing special | Most — stretch and tension both read small or large |
The Four Things That Make Home Measurements Wrong
Almost every mis-sized ring comes from one of these.
1. Measuring at the wrong time of day. Fingers change size through the day and with the weather. Blue Nile: "Finger size can vary with temperature, time of day, and even which hand you use most. It's better to take your measurement at the end of the day when your fingers are warm and slightly larger." Measuring cold hands first thing in the morning is how you end up with a ring you can't get past your knuckle in August.
2. Ignoring the knuckle. If your knuckle is wider than the base of your finger, a ring sized to the base will not go on, and one sized to the knuckle will spin. The guidance is to "measure both and choose a size in between for the best fit."
3. Assuming a wide band fits like a thin one. A wide band sits against more of your finger and feels tighter at the same measurement. For bands from roughly 6mm upward, "consider going up half a size for a better fit."
4. Measuring the wrong hand. Your dominant hand is usually slightly larger. Measure the exact finger, on the exact hand, that will wear the ring.
When Not to Measure at Home
Three situations where a jeweller's steel sizer is worth the trip.
- It is a surprise proposal. A ring you can borrow and measure beats any guess. If there is nothing to borrow, a jeweller can size a ring she already owns from a tracing or a photograph against a scale — which is still better than string.
- Your fingers change a lot. Swelling with heat, exercise or over the course of a day is normal, but if the variation is large, a fitting at a couple of different times gives you a range rather than a point.
- It is an eternity band or a very wide band. Neither can be resized much — an eternity band is set all the way round, so there is no plain section to cut. Get those measured properly the first time.
Once You Know Your Size
Your ring size is also the input for the other question people ask straight afterwards: how big should the stone be? Finger width scales with ring size, so the two decisions are connected — a stone that looks balanced at size 5 looks undersized at size 9.
Our moissanite size chart gives the mm-to-carat conversions and matches stone diameters to ring sizes, so it picks up exactly where this page stops. And if you are buying online, our guide to buying moissanite safely covers what a seller should be telling you before you order.
Every Oglowa ring listing states the band width alongside the size, because that half-size adjustment for wide bands only works if you know which one you are looking at.
FAQ
What is the most common ring size?
For women, US 6 to 7 covers the majority; for men, US 9 to 10. But "most common" is a poor substitute for measuring — the range either side is wide enough that guessing is a coin flip.
How do I measure my ring size without a ring sizer?
Wrap a strip of paper around the base of your finger, mark the overlap, and measure the length in millimetres. That is your inside circumference — read it off the chart above. Repeat three or four times, because paper strips are easy to pull tight.
How many mm is a size 7 ring?
17.3mm inside diameter, 54.4mm inside circumference. Each whole size up adds 0.81mm of diameter and 2.55mm of circumference.
Should I size up for a wide band?
Usually yes — about half a size. Wider bands contact more of your finger and feel tighter at the same measurement.
When is the best time of day to measure?
The end of the day, when your hands are warm. Fingers are at their smallest when cold, so a morning measurement in winter will read low.
What if I'm between two sizes?
Go up, particularly if your knuckle is prominent. A slightly loose ring can be padded or resized down; one that will not pass the knuckle is unwearable until it is resized up.
Can every ring be resized?
No. Plain bands resize easily; rings set with stones all the way around, and some wide or patterned bands, cannot be resized much or at all. Check before you buy rather than after.
Keep Reading
- Moissanite Size Chart: mm to Carat (With Real Size Comparison)
- Moissanite Ring Settings Guide: Prong vs Bezel vs Pavé
- How to Buy Moissanite Safely: What to Look For in a Seller
Know your size? Browse the Oglowa moissanite collection — GRA certified, with dimensions listed on every product page.
