How to measure your ring size at home

What a ring size actually measures

Every ring size system describes the same thing: the opening of the ring, measured on the inside. In the US and Canada that number is a scale from about 3 to 13 in quarter steps, in the UK it is a letter, in continental Europe (ISO 8653) it is the inside circumference in millimetres, and in Japan it is a number that grows by a third of a millimetre of diameter per step. All of them can be converted to two plain numbers: the inside diameter and the inside circumference in millimetres. So measuring your ring size at home comes down to getting one of those two numbers as accurately as you can, then reading it off a ring size chart.

Half a millimetre of diameter is roughly half a US size, the difference between a ring that spins and one that sits well. A ruler marked in whole millimetres is a blunt instrument for this job.

Method 1: measure a ring that already fits

If you own a ring that sits comfortably on the finger you are buying for, this is the most reliable home method, because the ring already accounts for your knuckle and the way you like a ring to feel. What you measure is the inside diameter, edge to edge across the widest point of the opening, not the outside of the band.

1. Lay the ring flat on a ruler or, better, use digital calipers if you have them. Calipers read to a tenth of a millimetre; a ruler reads to a whole millimetre at best.
2. Measure across the inside in two directions at right angles. Older rings are often slightly oval. Take the average.
3. Look the diameter up on the chart, or multiply by 3.14 to get the circumference if the shop lists EU sizes.

Two things trip people up here. Reading the outside diameter by mistake: a band 1.5 to 2 mm thick adds 3 to 4 mm, which is four or five US sizes. And using a ring from a different finger: index, middle and ring fingers differ, and the dominant hand is often a quarter to half a size larger.

Method 2: string, paper strip or a tape measure

This is the method you will see on most jewellers' websites, and it is the only option when there is no ring to copy. You are measuring the circumference of the finger at the base, where the ring will sit.

1. Cut a strip of plain paper about 5 to 7 mm wide, or use a thin tape measure. Thread and string stretch and cut into the skin, so paper is usually more accurate.
2. Wrap it around the base of the finger. It should be snug but not tight: you want the strip to behave like a ring, not like a tourniquet.
3. Mark where the end overlaps the start with a pen.
4. Unwrap and measure the length to the mark in millimetres. That is your circumference, which is also your EU size.
5. Divide by 3.14 to get the inside diameter if you need it, or go straight to the chart.

Blue Nile and Brilliant Earth both recommend repeating this three or four times rather than trusting one wrap. GIA points out that string stretches and paper curls with humidity, so treat this as a cross-check rather than the final word for an expensive ring.

If your knuckle is noticeably wider than the base of your finger, measure both and pick a size between the two readings: the ring has to pass the knuckle but should not spin at the base.

Method 3: a printable ring sizer

Many jewellers publish a PDF with a row of circles labelled by size. You place a well-fitting ring on the circles until the inside edge lines up with one of them. The idea is sound, and the failure mode is always the same: the printer scaled the page. Print at 100 percent with "fit to page" switched off, and check the control ruler on the sheet against a real ruler before you trust a single circle. A 2 percent scaling error puts every circle off by a third of a millimetre at 17 mm.

Method 4: an on-screen ring sizer

The on-screen version of the printable chart skips the printer. The app draws a circle, you place your ring on the display and adjust the circle until it matches the inside edge. For this to be accurate the app must know the physical size of the screen, because a 17 mm circle has to be 17 mm on an iPhone SE and on an iPad Pro alike. That is how Orbita works: it converts millimetres to screen points using the actual diagonal of the device model, the slider moves in 0.1 mm steps from 14 to 23 mm, and the result is shown in US, UK, EU, JP and RU sizes at once, offline. It measures a ring, not a finger, so you still need a ring that fits, and you should look straight down at the screen rather than from an angle.

Method 5: a sizer set

A set of graduated metal or plastic rings is what jewellers use, and cheap sets are easy to buy online. Try them on until one slides over the knuckle with slight resistance and sits at the base without spinning. This is the most accurate way to measure a finger at home; belt-style plastic sizers are easy to overtighten.

When to measure

• Time of day. Fingers are smallest first thing in the morning and larger by the afternoon and evening. Blue Nile suggests measuring at the end of the day; Brilliant Earth says the middle or end of the day. Either way, not at 7 a.m.
• Temperature. Cold hands shrink, hot hands swell. Do not measure straight after a run, a hot shower or a walk in the cold; wait until the hand is at a normal room temperature.
• Which hand and finger. Measure the exact finger the ring will live on, on the correct hand. In the US, UK and most of Europe an engagement ring goes on the left ring finger; in Russia and several other countries it is the right.
• More than once. Two or three readings on different days are worth far more than one careful reading.

Band width changes the answer

A size on a chart assumes a fairly narrow band. Wide bands cover more skin and feel tighter at the same nominal size. Blue Nile suggests going up half a size for wider styles; Brilliant Earth gives the same advice for bands over about 4 mm. Comfort-fit rings, with a rounded inside profile, slide on more easily than flat ones and are sometimes bought a touch smaller.

The mistakes that cost people a resize

• Measuring the outside of the ring. Always the inside edge.
• Pulling the string tight. A taut string gives a circumference that is too small and a ring that will not pass the knuckle.
• Printing the sizer "fit to page". Check the control ruler.
• Trusting one reading on a cold morning. Repeat later in the day.
• Guessing from a photo. Without a reference object next to the hand there is no scale, and errors of a full size or two are normal.
• Ignoring whether the ring can be resized. Eternity bands with stones all the way round, some tension settings and some alloys cannot be sized up or down. Ask before buying if you are between sizes.

If you are between sizes

For a slim band, most jewellers recommend rounding up: a slightly loose ring is easier to live with, and easier to fix with a sizing bead, than one that will not come off. For a wide band, go up. For an everyday ring, measure again on another day before deciding.

The quick version

Have a ring that fits? Measure its inside diameter, with calipers or on screen, and skip the string. No ring? Wrap a paper strip three times and sanity-check the result with a second method. Buying something expensive that cannot be resized? Get a sizer set or visit a jeweller. If you do have a ring, Orbita turns the measurement into a thirty-second job: put the ring on the screen of your iPhone or iPad, drag the circle to the inside edge and read off the size in five systems. It is free and works offline.