Bulb Spacing Calculator
Bulbs are planted staggered rather than in rows, which is why the count beats the square grid.
The formula
square grid: area / spacing^2 ; triangular: area / (spacing^2 * sqrt(3)/2)
Triangular planting fits 15 % more
On a square grid each plant owns a square of ground. Stagger alternate rows and the rows nest, so the row spacing drops to √3/2 of the plant spacing and each plant owns 13 % less ground. That is 15 % more plants in the same bed, at the same distance from every neighbour — which is why orchards and bulb plantings are laid out this way and vegetable rows usually are not. Square is easier to hoe.
Spacing is centre to centre
The figure quoted on a label is the distance between plant centres, not the gap between the edges of mature plants. A plant said to need 18 inches occupies an 18-inch square whatever its eventual spread, and the spread is what tells you whether that is sensible.
The edge is where the count goes wrong
Dividing area by spacing squared ignores what happens at the boundary, and on a small bed that is most of the plants. A 2 m bed at 0.5 m spacing holds five plants across, not four — so on small areas count the rows out rather than trusting the division.
Rules of thumb, and the field
Every figure here describes an average season on an average site. Soil type, aspect, drainage, variety and weather each move the answer by more than the arithmetic does, and a grower who has kept records for a few years on one piece of ground knows it better than any formula. Use these to plan an order or size an implement, then trust what the ground has actually done before.