How the count is worked out
The calculator places plants in a simple grid. Each plant gets half a spacing of room at the edges, so the number of plants along the length is the length divided by the spacing, rounded down, and the same for the width. The total is the two multiplied together.
Take the spacing from the seed packet or plant label for the variety you grow. Packets often give two numbers, one between plants in a row and one between rows; enter both if yours does.
Note: A grid count is a ceiling, not a recommendation. Large crops in containers also depend on the volume of soil each plant gets, so check the crop's own guidance too, as in our guide to how many tomato plants per container.