Formula
Plants = area ÷ (spacing in feet)². Seeds = plants ÷ (germination rate ÷ 100), rounded up. Example: 32 sq ft at 12 in spacing needs 32 plants; at 85% germination, sow about 38 seeds.
Germination falls with old seed and cold soil — if your seed is old, sow more, or test a few seeds on damp paper first.