The Wrong Size: A $300 Ring Turned Into “Electronic Trash”

It was my first time buying a smart ring. Following the instructions on the official website, I wrapped a piece of paper around my finger to measure the circumference and chose size 12. When I received it and put it on, it felt a bit tight, but I thought, “If it’s a little tight, it won’t fall off easily.” As it turned out, after wearing it for one night, my finger was swollen the next morning, and I almost had to use dish soap to get it off.

To make matters worse, the ring’s data was completely off: the sleep tracker showed I’d had “3 hours of deep sleep” all night (in reality, I woke up frequently because it was so uncomfortable), and my heart rate readings fluctuated wildly. I consulted customer service and learned that if the ring is too tight, it compresses blood vessels and affects the PPG signal; if it’s too loose, the sensor lifts away from the skin, allowing light to escape and causing data distortion.

According to the official Oura Ring technical documentation, the proper fit is when the ring can be easily rotated 90 degrees while on your finger but doesn’t slide past the knuckle on its own; removing it requires a bit of force but doesn’t leave red marks. However, without trying it on in person and relying solely on paper measurements, the error rate can be as high as 30%.

I found the perfect size using a trial kit, and the data immediately returned to normal

Venus Smart Shop offers a “Size Trial Kit”—you’ll receive a set of eight resin trial rings in different sizes (US sizes 6–13). Each trial ring has nearly the same weight and thickness as a real ring.

I spent an afternoon trying on different sizes on various fingers (left ring finger, right middle finger, and index finger) and conducted several tests:

Rotation test: Gently rotate the ring—it should move smoothly without spinning on its own.

Fist test: When making a fist, the ring shouldn’t leave deep indentations.

Swing test: Shake your hand vigorously—the ring shouldn’t slip off.

Overnight simulation: I slept with the trial ring on all night and felt no swelling in the morning.

Ultimately, I found that size 10 fit my left ring finger and size 12 fit my right middle finger (I decided to wear it on my left hand). Only then did I place an order for the official version. After switching to the correct size, the ring’s data quality improved significantly: sleep tracking aligned with my subjective experience, and the HRV curve became smoother.

I consulted Venus’s technical team, who explained that the smart ring’s optical sensors are extremely sensitive to skin contact pressure. Excessive pressure can cause capillaries to flatten, reducing signal amplitude; insufficient pressure allows ambient light to enter, introducing noise. The optimal pressure range is 1–2 Newtons (equivalent to the force of gently pressing your finger against your skin). The trial ring helps you find this balance.

A single size difference in a ring can make a world of difference.

Thanks to the professional service at Venus Smart Shop, the ring I chose has been comfortable to wear for a long time.