There are four spacing guidelines:
Small – 16 per square for 3 inch spacing.
Medium – 9 per square for 4 inch spacing
Large – 4 per square for 6 inch spacing
Extra Large – one per square for 12 inch spacing
Refer to the seed packet to determine which spacing your plants will need.
For example: a tomato or green pepper plant needs one per square. While radishes and carrots need 16 per square.
The secret to Square Foot Gardening is the grid. The grid is one of the most important things to include while following a Square Foot method.