Pupillary distance

Measured, not guessed.

PD is the distance between your pupils, in millimetres — glasses need it to centre your lenses. Any standard bank card is an exact 85.60 mm ruler.

1. Card photoHold the card flat on your forehead through a 10-second countdown.
2. Eyes photoPut the card down and keep your head still.
3. Mark the cornersPlace four rings on the card's corners and read your PD.

Sit at arm's length. A plain background helps.

Everything runs in your browser — photos never leave this device. Hold the card face-down if you prefer.