An estate agent’s floor plan, a photo of the plan on the wall of a flat, the drawing that came with a lease — all of them have the room you want to measure in them, at some scale nobody wrote down in a form you can use. This page turns one into square feet: mark something you know the length of, trace the outline, and read the area.
The image never leaves your browser. It is read straight off your device, drawn here, and never uploaded, stored, or sent anywhere. There is no server in this at all — close the tab and the image is gone.
The scale is the whole job, and it is one line: the two ends of a wall with a dimension printed on it, a scale bar, or a door you know is 2 ft 6 in. Everything else is measured against that, so the better the line you pick, the better every number on the page. Change it later and every length follows — nothing you traced moves, because the corners are kept where you put them on the image and converted only to be shown.
Walls that come out nearly straight are straightened as you trace, because a plan photographed by hand is never quite square. If the room really is at an angle, turn that off.
When the outline is closed, the room opens in the calculator or in the floor plan editor like any other: it can be costed, sent to an estimator, shared as a link or printed.