Printable Ring Sizer and Ring Size Chart: Check the Print First
To measure ring size at home, find the inside diameter or inside circumference of the finger (or of a ring that already fits) in millimetres and look it up in the chart below. A printable ring sizer does that lookup for you on paper - but only if it printed at exactly 100%. If your print dialog shrank the page by 3%, a true size 7 reads as about 7.66, and you end up ordering a size 8. So the first step with any printed sizer is to check its scale with a ruler or a bank card, before you measure anything.
Ring size chart (US / Canada, mm and ISO / EU)
US and Canadian sizes from 3 to 13 in half sizes, with inside diameter, inside circumference, and the ISO size. The ISO size used in Europe is simply the inside circumference in millimetres, rounded.
| US / Canada | Inside diameter | Inside circumference | ISO / EU |
|---|---|---|---|
| 3 | 14.07 mm | 44.2 mm | 44 |
| 3.5 | 14.47 mm | 45.5 mm | 45 |
| 4 | 14.88 mm | 46.8 mm | 47 |
| 4.5 | 15.29 mm | 48.0 mm | 48 |
| 5 | 15.69 mm | 49.3 mm | 49 |
| 5.5 | 16.10 mm | 50.6 mm | 51 |
| 6 | 16.51 mm | 51.9 mm | 52 |
| 6.5 | 16.91 mm | 53.1 mm | 53 |
| 7 | 17.32 mm | 54.4 mm | 54 |
| 7.5 | 17.73 mm | 55.7 mm | 56 |
| 8 | 18.13 mm | 57.0 mm | 57 |
| 8.5 | 18.54 mm | 58.2 mm | 58 |
| 9 | 18.95 mm | 59.5 mm | 60 |
| 9.5 | 19.35 mm | 60.8 mm | 61 |
| 10 | 19.76 mm | 62.1 mm | 62 |
| 10.5 | 20.16 mm | 63.3 mm | 63 |
| 11 | 20.57 mm | 64.6 mm | 65 |
| 11.5 | 20.98 mm | 65.9 mm | 66 |
| 12 | 21.38 mm | 67.2 mm | 67 |
| 12.5 | 21.79 mm | 68.5 mm | 68 |
| 13 | 22.20 mm | 69.7 mm | 70 |
The formula behind the chart
US ring sizes are a straight line in inside diameter:
- Inside diameter (mm) = 11.63 + 0.8128 x US size
- Inside circumference (mm) = pi x inside diameter
- ISO / EU size = inside circumference in mm
The same scale in inches is 0.458 in plus 0.032 in per size, which is where the odd-looking 0.8128 comes from (0.032 x 25.4). Each whole size adds 0.8128 mm of diameter, or 2.55 mm of circumference. A half size is only 0.41 mm across, or 1.28 mm around - keep that number in mind, because it is the tolerance a printed sizer has to hit.
To go backwards from a measurement: US size = (diameter - 11.63) / 0.8128, and diameter = circumference / pi.
How to measure ring size at home: three methods
1. From a ring that already fits. Lay the ring on a set of printed circles. The right size is the circle whose line just shows all the way round inside the ring. It has an advantage: a ring that is comfortable already accounts for your knuckle - the other two methods leave that to your judgment.
2. With a paper strip. Wrap a printed strip round the base of the finger, snug but loose enough to slide over the knuckle, and read the size where the end meets the scale.
3. With string and a ruler. Wrap string or a thin strip of paper round the finger, mark where it overlaps, lay it flat and measure in millimetres. That is the circumference; find it in the chart, or use the formula.
Worked example (illustrative): the string measures 56 mm.
- Diameter = 56 / pi = 17.83 mm
- US size = (17.83 - 11.63) / 0.8128 = 7.62
That falls between two half sizes, and the rule printed on the sizer is to go to the larger one: size 8. Measure at the end of the day when fingers are largest, not when your hands are cold, and allow a little extra for a wide band.
Why printer scaling ruins a printed ring sizer
A printed sizer is a ruler. If the printer shrinks the page, every circle and every strip mark shrinks with it, by the same percentage. “Fit to page” or “shrink to fit” does exactly that, quietly, to make room for the printer’s margins.
Here is what a 3% shrink does to someone whose true size is 7:
- Their ring has an inside diameter of 17.32 mm.
- The circle labelled “7” printed at 97% is only 16.80 mm across, so the ring sits loose around it.
- The circle the ring actually matches is the one labelled 7.66. The same thing happens with the paper strip: the marks are closer together, so the end of the strip lands further along the scale.
- That is between 7.5 and 8, and the “go larger” rule says size 8 - a ring 0.81 mm wider inside than the finger it was meant for.
A shrunk print always reads too large, never too small, and the error grows with the size, because it is a percentage of the diameter:
| Print shrink | True size 5 | True size 7 | True size 9 | True size 11 |
|---|---|---|---|---|
| 1% | reads 5.20 (+0.20) | reads 7.22 (+0.22) | reads 9.24 (+0.24) | reads 11.26 (+0.26) |
| 2% | reads 5.39 (+0.39) | reads 7.43 (+0.43) | reads 9.48 (+0.48) | reads 11.52 (+0.52) |
| 3% | reads 5.60 (+0.60) | reads 7.66 (+0.66) | reads 9.72 (+0.72) | reads 11.78 (+0.78) |
| 5% | reads 6.02 (+1.02) | reads 8.12 (+1.12) | reads 10.23 (+1.23) | reads 12.33 (+1.33) |
At 3%, a true size 7 moves by 0.66 of a size - more than a half-size step. At 5%, a size 11 moves by 1.33 sizes. Enlarging works the other way: printed at 103%, a true size 7 reads 6.38.
The check that catches it
A shrink of a few percent is invisible on a page of circles, but easy to see against something whose size you know:
- A 100 mm ruler. Printed at 97%, it measures 97 mm - 3 mm short against a real ruler.
- A bank card outline. Bank and debit cards are made to one standard size, ISO/IEC 7810 ID-1: 85.60 x 53.98 mm. Printed at 97%, the outline is 83.03 mm wide, so a real card overhangs it by 2.57 mm.
A useful rule is to reprint if the 100 mm ruler is off by more than 1 mm. That is a 1% scale error, and across the whole chart from size 3 to 13 it moves a reading by 0.17 to 0.28 of a size - under a half-size step. (On the card, 1 mm is about 1.2% of its width.)
To print at true size, choose 100%, Actual size or Scale: 100 in the print dialog, and turn off “fit to page”, “shrink oversized pages” and “scale to fit”. Then check the ruler before you trust a single circle.
How these were calculated
The chart uses the US formula above: inside diameter = 11.63 + 0.8128 x size, circumference = pi x diameter, ISO size = circumference rounded to the millimetre. Every row was computed twice - with the same code that draws the sizer below, and independently from the inch definition (0.458 in + 0.032 in per size); the two agree to within 0.0032 mm, and the page only builds if they do. The shrink figures were computed with a closed form (reading = size whose diameter is the true diameter divided by the print scale) and checked by a numeric search over the printed circles. The 56 mm string and the shrink percentages are illustrative, not measured from any particular printer. Shops and brands vary slightly in how they label sizes, so when a seller publishes their own chart, use theirs. This is general measuring guidance, not professional advice.
If you would rather print a sizer than measure string, this one carries its own scale check on page one - a 100 mm ruler, a 4 inch ruler and an ID-1 card outline - plus circles and strips for sizes 3 to 13, and jewelry sellers may send it to their own customers.