Formula
On a rectangle the count along a side is one more than the number of gaps that fit in the length after both margins: floor of (length minus two margins) divided by the spacing, plus one. Triangular rows use a row spacing of spacing times √3/2 unless you override it. Odd rows use the full count. Even rows start half a spacing in from the margin and keep a center only while it still clears the far margin. A known-area estimate divides the area by spacing squared, or by spacing squared times √3/2, and floors the result. Edges are unknown in that mode, so it is an estimate.
n = floor((L − 2 × margin) / spacing) + 1
row spacing = spacing × √3 / 2
Spacing near the mature width is a starting idea for canopies that touch. The tag knows the cultivar. NC State Extension’s square-foot guide, adapted from Junior Master Gardener, lists 3 inch crops at 16 per square foot, 12 inch crops such as tomatoes, peppers, and eggplant at one per square foot, and 18 to 24 inch squash at one plant per two square feet. MSU Extension lists a typical tomato at 24 inches in 36 inch rows. Those are examples from those pages.
Worked examples
10 × 4 ft, 12 in, no margin, square. 11 by 5, 55 plants. With a 6 inch margin, 10 by 4, 40 plants. 8 × 8 ft at 18 in. 6 by 6, 36 plants.
The same 10 × 4 ft bed, triangular. Row spacing 10.39 in, five rows, 53 plants. 100 ft² at 18 in. About 44.44 square, floored to 44; about 51.32 triangular, floored to 51. Ten percent extras on 55 is 61. At an example $4.50, 55 plants are $247.50.
3 m × 1.2 m at 30 cm. The same 11 by 5, 55 plants.
Reference
Example spacings from the sources below. Check the cultivar.
| Example | What the source says |
|---|---|
| Carrots, radishes | NC State square-foot guide: 3 in, 16 per ft² |
| Tomato, pepper, eggplant | Same guide: 12 in, 1 per ft² |
| Tomato, typical | MSU Extension: 24 in plants in 36 in rows |
| Staked tomato | UNH Extension: 18 to 24 in, rows at least 5 ft |
Questions
How do I calculate how many plants I need?
For a rectangle, count how many centers fit along the length and across the width. At 12 inch spacing with no margin, a 10 by 4 foot bed is 11 by 5, 55 plants. The plant tag still sets the spacing.
What is triangular plant spacing?
Rows are staggered. The row spacing is the on-center distance times √3/2, about 0.866, unless you type a different row spacing. Even rows start half a space in, so they often hold one fewer plant.
How many plants per square foot?
Divide the layout count by the bed area. NC State Extension’s square-foot guide puts one tomato, pepper, or eggplant in a square foot, and 16 carrots at 3 inch spacing. Those are that guide’s examples, not a rule for every cultivar.
Should I use square or hexagonal spacing?
Triangular spacing fits more plants at the same on-center distance because the rows sit closer. Square is easier to mark. Neither one chooses the distance.
How far apart should I plant?
Follow the plant tag and your extension service. This page turns a distance you type into a count. MSU Extension lists a typical tomato at 24 inches in 36 inch rows. UNH lists staked tomatoes at 18 to 24 inches. The cultivar wins.
How do I account for paths or obstacles?
Shrink the bed, or raise the edge margin, so the centers stay off the path. This page lays out one rectangle.
Does this work for trees and shrubs?
Yes for the count, once you have a spacing. The nursery tag or the extension note is where that spacing comes from.
Sources
- NC State Extension, square-foot gardening guide by Liz Driscoll, adapted from Junior Master Gardener. 3 inch spacing is 16 plants per square foot. 12 inch spacing, including tomatoes, eggplant, and peppers, is one per square foot. 18–24 inch squash is one per two square feet.
- Michigan State University Extension, “How to Grow Tomatoes,” typical spacing 24 inches in 36 inch rows, 0.5 plants per square foot.
- University of New Hampshire Extension, growing tomatoes: staked or caged plants 18 to 24 inches apart, in rows at least 5 feet apart. Cultivar and habit change it.