A QR is "scannable" when a phone camera can find it, focus, read each module, and decode the data — from across the room, often in bad light, often held by someone who's also walking. A few things need to be true for that to work.
The QR needs to be big enough for the distance it'll be scanned from (the Size Calculator handles this). It needs real contrast between the dark and light modules — gray on white is the classic failure. It needs the white border around it (the "quiet zone") to be at least 4 modules wide. It needs enough error correction for whatever wear it'll see. And the module edges need to be sharp, which mostly means: don't save it as a heavily compressed JPG.
The tests this tool runs
First, the baseline: does the QR decode at the size you uploaded? Then we scale it down to 50%, 25%, and 12.5% — that simulates seeing the code from further away. A code that fails at 25% usually has too many modules for its physical size. We try a reduced-contrast version, which catches faded print and dim-lighting scans. And we rotate it 90°, 180°, and 270°; failures here usually mean a damaged or asymmetric finder pattern.