How can you buy an engagement ring without knowing their ring size?

Buying an engagement ring without confirmed size information is possible, but a close measurement significantly reduces the chance of a costly resize. The most reliable covert method is borrowing a ring the recipient already wears on their ring finger, placing it flat on paper, and tracing the interior circle with a pencil. Measure the diameter across the widest point using a ruler; note that slight oval distortion in older rings can introduce a millimetre of error, which corresponds to roughly half a size. If the proposal is not a surprise, use a printed ring-sizer guide or a reusable plastic sizer tool, both of which produce an IGI-compatible circumference reading in millimetres. Timing also matters: finger circumference is measurably larger in the evening due to warmth and fluid retention, so an evening measurement gives the safer, slightly larger reference. When in doubt, size up by half a step rather than down, as resizing a lab-grown diamond ring wider is the more straightforward adjustment.