Build a correct graphic scale bar and a north arrow for your sheet. Pick a scale — metric ratio (1:50, 1:100…) or an imperial architect's scale (¼ in = 1 ft, 1 in = 20 ft…) — set the length, divisions and a finely-divided end, choose a north-arrow style and rotation, then export. The SVG is sized in real millimetres, so placed at 100% on a layout it reads true; PNG too.
Runs entirely in your browser — nothing is uploaded. The bar is drawn at the real paper size for the chosen scale; drop the SVG into your layout at 1:1.