About this tool
The Tent & Tarp Sizing Calculator is a free, in-browser tool with two modes: sizing a footprint (groundsheet) tarp to fit under an existing tent, and estimating how much floor area a given number of campers actually needs. Everything is calculated on your device as you type, with nothing uploaded.
In footprint mode it follows the golden rule that a groundsheet should be slightly smaller than the tent floor so rain running off the fly lands on the ground, not on the tarp where it would pool under you. Enter the tent floor length and width and an inset per side (about 2-3 inches / 5-8 cm is typical); the footprint dimensions become floorLength − 2 × inset and floorWidth − 2 × inset, and the tool reports the resulting area in square feet and square metres. In sizing mode it multiplies the number of sleepers by a sleeping-pad width (about 20 inches each) laid side by side, keeps the pad length, and adds a margin for gear and shoulder room, returning a recommended minimum floor width, length and area.
Manufacturers rate tents tightly — a rated "2-person" tent usually means two pads touching with no spare room. For comfort, gear storage or wider pads, size up by one person or increase the margin. Use footprint mode to buy or cut a groundsheet, and sizing mode to sanity-check a tent's real capacity before you commit.