Ring Size Calculator

There are three reliable ways, and you don't need a jeweller for any of them. First, the on-screen sizer: take a ring that already fits the right finger, lay it flat on the circle above, and drag the slider until the red circle exactly fills the inside of the band — for best accuracy, calibrate the screen to a bank card first. Second, the diameter method: with a ruler, measure straight across the inside of a ring you own (in millimetres) and type that number into the “ring diameter” tab. Third, the finger method: wrap a thin strip of paper or a piece of string snugly around the base of your finger, mark where the two ends overlap, lay it flat, measure the length to the mark in millimetres, and enter that as your circumference. Whichever method you use, measure two or three times and take the average — and remember the calculator instantly converts your result into US, UK, EU and Japanese sizes.

Both describe the round hole inside the ring, just measured differently. The diameter is the straight-line distance across the inside, from one edge to the opposite edge through the centre. The circumference is the full distance around the inside edge — the number you'd get if you could unroll the inner circle into a straight line. They're linked by a fixed formula: circumference = diameter × π (pi ≈ 3.14159), so if you know one you can always work out the other. For example, a US 7 ring has an inner diameter of about 17.3 mm and an inner circumference of about 54.4 mm (17.3 × 3.14 ≈ 54.4). This matters because different methods give you different numbers — a ruler across an existing ring gives diameter, while a string around your finger gives circumference — but our calculator accepts either and converts internally, so you never have to do the maths yourself.

Each region built its own scale on top of the same physical measurement. The US/Canada system uses numbers from about 3 to 13.5 in half and quarter steps. The UK, Ireland and Australia use letters from F up to Z (with “½” sizes in between, like N½). The EU/ISO system is the most logical because the size number simply is the inner circumference in millimetres — so an EU 54 ring is exactly 54 mm around. Japan and much of Asia use their own numeric scale from roughly 1 to 27. As a reference point, a single size translates across all of them: US 7 = UK N½ = EU 54 = Japan 13 = 17.3 mm diameter. Because there's no perfect overlap between the scales (a US half-size doesn't always land on a clean UK letter), conversions round to the nearest standard size — so when in doubt between two, choose the larger. The full chart below lists every size in all five systems.

Measure at the end of the day, indoors at normal room temperature — that's when your fingers are at their largest and most stable. Fingers are surprisingly variable: they swell in heat, after exercise, after salty food, and during travel or pregnancy, and they shrink noticeably in cold weather and first thing in the morning. The difference between a cold-morning measurement and a warm-evening one can easily be half a ring size or more, which is enough to make a ring that fit in the shop feel tight or loose later. To get a size that stays comfortable all day, take your measurement when your hands are warm and relaxed, avoid measuring right after a workout or a cold walk, and if you can, check it on two or three different days and average the results.

Yes — band width changes how a ring fits, even at the same nominal size. A wide band (anything 6 mm or more, like many men's rings and chunky statement pieces) covers more of your finger and sits tighter than a thin band, so the standard rule is to go up by about half a size for comfort. A very narrow band (2 mm or less) can feel slightly loose at your measured size. If you measured using a thin ring or a string but plan to buy a wide band, add the half size; if you measured an existing wide band, you're already accounting for it. The tool handles this for you: tick the “wide band (6 mm+)” box above and your result moves up half a size automatically. When you're between options, a fractionally larger wide ring is always more comfortable than one that's too tight to slide over the knuckle.

It's completely free, needs no signup, and runs entirely in your browser — no measurements are uploaded or stored. On accuracy: the conversions themselves are based on the international ISO 8653:2016 standard, so once you have a correct measurement the size mapping is precise. The thing that affects accuracy is the measurement, not the maths. The on-screen sizer is only exact if you calibrate it to a bank card first, because screens differ in pixel density; the ruler and string methods are accurate to within about half a size if you measure carefully and snugly. For everyday rings and gifts, that's plenty. For a high-value purchase or an engagement ring — where a resize is costly or impossible — use the tool to get your size, then confirm it with a jeweller's physical sizer set before you buy, especially if you're between two sizes or buying a wide band.

If you're buying a ring as a surprise and have no measurement to go on, the averages are a sensible starting point. For women, the most common ring size is US 6, and roughly two-thirds of women fall in the US 5.5 to 7 range; petite hands often land around US 4 to 5, and taller or larger frames around US 7 to 8. For men, the average is US 9, with most men between US 9 and 11; slim hands sit nearer US 8 and broad hands around US 12. Hand size correlates loosely with overall build and height, so you can refine the guess from what you know about the person. That said, these are population averages, not a substitute for measuring — there's natural variation, and ordering one size off is the single most common reason rings need resizing. If at all possible, get a real measurement using one of the secret methods below.

For a surprise proposal, you have several discreet options. The most reliable is to borrow a ring they already wear on the relevant finger (make sure it's the right finger — don't size a pinky ring for a ring finger): set it on paper and trace the inside circle, or measure its inner diameter with a ruler, then enter that here. If they don't wear rings, gently wrap a strip of paper or string around their finger while they sleep and mark the overlap — only attempt this with a deep sleeper. You can also enlist a recently-engaged friend to invite them to try on an engagement ring, which makes asking the size feel completely natural in the moment. A subtler trick: slip their ring onto one of your own fingers, note exactly where it stops, and a jeweller can estimate the size from that. Whichever route you take, double-check the measurement before ordering, and lean toward the larger size — a slightly loose ring can be resized down easily, whereas too-small means it won't even go on at the proposal.

An inner circumference of 54 mm corresponds to about a US 7 — which is also UK N½, EU 54 (the EU number is just the circumference), Japan 13, and an inner diameter of 17.3 mm. To put that in context, every extra ~2.5 mm of circumference is roughly one full US size: 49 mm ≈ US 5, 52 mm ≈ US 6, 54 mm ≈ US 7, 57 mm ≈ US 8, and 59.5 mm ≈ US 9. So if your string measurement came out a millimetre or two either side of 54, you're still in the US 6.5–7.5 band. For your exact size, type your measured circumference into the “finger size” tab above and the calculator snaps it to the nearest standard size and shows every regional equivalent at once.

This is common, and it's the main reason a ring can feel both “too tight” and “too loose” at once. A ring has to pass over your knuckle to go on, but it sits and is worn at the base of your finger — so if your knuckle is noticeably wider than the base, a ring sized to the base won't fit over the knuckle, while one sized to the knuckle will spin at the base. The fix is to measure both points: take the knuckle measurement and the base measurement, and choose a size in between (closer to the knuckle if the gap is large, so it goes on comfortably). Designs with a slightly heavier or rounded inner band, or sizing beads added by a jeweller, also help a knuckle-heavy finger keep the ring centred. When you can't decide, err toward the size that clears the knuckle — a ring you can't get on is no use, and a base that's a touch loose is easily snugged.

A well-fitting ring is snug but not painful: it should slide on with a little resistance over the knuckle, sit firmly at the base without leaving a deep red groove, and require a gentle wiggle to pull back off. A couple of quick tests — if the ring spins loosely around your finger or slips off when you shake your hand, it's too big; if it goes on but you have to twist hard or use soap to remove it, or your finger throbs after a while, it's too small. Aim for the size that comes off with a small amount of effort over the knuckle. Keep in mind your fingers change through the day and the seasons, so a perfect fit in a warm room may feel slightly loose on a cold morning — that's normal. When you're genuinely between two sizes, choose the larger one: a fraction loose is comfortable and safe, whereas too tight can cut off circulation and is harder to fix.