If you have a reference ring, measuring its inner diameter is one of the easiest and most accurate ways to find your ring size. Use a ruler or measuring tape to check the diameter in millimeters or inches, then convert it to a standard size using our calculator.
How to size a ring by diameter at home
Time:
2 minutes
Tools:
Ruler or tape measure
Step 1: Place your ring on a flat surface
Place the ring you want to size on a flat and stable surface to prepare for measuring its inner diameter accurately.
Step 2: Measure the inner diameter
Measure the inner diameter in inches or millimeters from one inner edge to the opposite, making sure to align the center and look straight down from above for an accurate result.
Step 3: Determine your ring size
Input your measured diameter in millimeters or inches into the calculator below to get your US ring size, based on the official conversion between inner diameter and US ring sizes.
This calculator uses the official internal diameter values defined by the ISO 8653:2016 standard to provide the closest US ring size. If your measurement falls between two sizes, both will be displayed.
Small differences matter
Ring size systems are based on small, precise increments, whether they use inner diameter or inner circumference as a reference. For example, in the US system, each full size increases by 0.032 inches (about 0.81 mm) in inner diameter, so a variation of 0.016 inches (around 0.4 mm) equals half a size. This is why it’s important to measure your ring’s inner diameter as accurately as possible, ideally with at least 0.02 inches (0.5 mm) of precision. A careful measurement will give you a more reliable result.
Convert your ring size
After measuring your ring, you can use our conversion chart to find your size in any international system, including UK, EU, French, and more.
View conversion chart
Frequently asked questions about measuring a ring
If you’re between two sizes, consider two main factors: the width of the ring and its interior shape. Wider bands usually feel tighter because they cover more of the finger, while narrow rings feel looser. Flat interiors create more friction against the skin, whereas comfort fit designs slide on more easily. If your final ring is wide or has a flat inner wall, choosing the larger size is often more comfortable. For narrow or comfort fit rings, the smaller size may be a better fit.
You can use a measuring tape or a caliper (digital or manual). Both offer precise results if handled correctly. You can also use a printed ruler, as long as you’ve confirmed it’s correctly scaled, since even a small distortion may affect accuracy.
Yes. Rings with non-circular shapes or irregular inner edges can’t be measured accurately using a ruler, as they lack a consistent diameter. For designs with thick decorative settings, using a caliper may help, but it’s not always reliable. In these cases, it’s better to size your finger directly using a ring sizer or printable tool.
There’s no difference in accuracy as long as you use a precise ruler and measure carefully. However, millimeters make it easier to be precise, since the units are smaller and less prone to rounding errors. If you can choose, measuring in millimeters is usually more convenient.
Alternative methods to determine the size of a ring
Printable ring size chart
Print a ring size chart and place your ring on the circles to measure it accurately at home.
On-screen ring sizer
Measure your ring directly on your screen using visual guides to find your size instantly.
Metal ring sizer
Use a metal stick to measure your ring size with precision—ideal for regular or professional use.
Ratings from users following our ring measurement guide