Plant Calculator

Background.

A 240 square foot bed planted at 12 inches on centre takes 240 plants in a straight grid and 278 in staggered rows. Same bed, same spacing, every plant exactly 12 inches from its nearest neighbours, and 38 plants of difference, which at $3.75 each is $142.50. The difference comes entirely from how the rows sit against each other, and that is the first thing to settle before buying groundcover, bedding plants or perennials by the flat.

In a square grid each plant owns a square of ground as wide as the spacing. Shift every other row sideways by half a spacing and the rows can slide closer together, nesting into the gaps of the row before, until each plant sits at the corner of an equilateral triangle. The rows are then only 0.866 of a spacing apart, each plant owns 0.866 of the ground it had in the grid, and the bed holds 15.5% more plants. Groundcover is often planted this way because it closes over evenly and leaves no straight lines of bare soil.

The spacing itself is a decision about the plant, not about arithmetic. It belongs on the tag or in a species reference, and it should reflect how wide the plant grows and how quickly you need the ground covered. University of Maryland Extension's groundcover guidance describes the trade: close spacing covers bare soil sooner, so there is less erosion and less room for weeds, but it raises the risk of disease and eventually leaves individual plants struggling against their neighbours. Because the count rises with the square of the spacing, small changes cost a lot: 12-inch spacing needs 2.25 times as many plants as 18-inch.

The numbers have been checked against two extension publications: Table 31 of University of Georgia Extension Bulletin 931, which lists plants per 100 square feet for both patterns at spacings from 4 to 14 inches, and Iowa State's worked daylily example.

What is plant calculator?

On-centre spacing, written o.c., is the distance from the centre of one plant to the centre of the next. It is measured centre to centre because the edges of a plant move as it grows, while its centre stays where you planted it.

A plant count for a bed is the bed's area divided by the ground each plant is given. In a square pattern that ground is a square with sides equal to the spacing, s², so 12-inch spacing gives each plant 144 square inches, exactly one square foot. In a triangular pattern, also called offset, staggered or equilateral, each plant gets a parallelogram s wide and s × 0.866 deep, about 124.71 square inches at 12 inches.

Density is quoted two ways. UGA's bulletin uses plants per 100 square feet, which suits landscape beds; Iowa State uses plants per square foot, which suits small plantings. They differ only by a factor of 100. Plants of this kind are usually sold in flats or trays of 18, 32, 38 or 50 cells, and it is the number of whole flats that decides how many plants actually come home.

How to use this calculator.

1. 01Measure the bed and enter its area in square feet. For an irregular bed, split it into rectangles, triangles and half-circles, work out each, and add them.
2. 02Enter anything inside the outline that will not be planted, such as stepping stones, an existing shrub or a downspout splash block, as the area to deduct; leave it at 0 if there is nothing.
3. 03Enter the on-centre spacing in inches from the plant tag; for a spacing in centimetres divide by 2.54, so 30 cm is 11.81 inches. If the tag gives a range, run both ends and compare the plant counts and costs.
4. 04Choose triangular for staggered rows or square for a straight grid, which is easier to mark out with a string line.
5. 05Enter the cells per flat, or 1 if you are buying single pots, and the price per plant, which is the tray price divided by its cell count. Enter 0 for the price to skip the cost.

The formula.

The ground each plant occupies is A_p = s² square inches in a square grid and A_p = s² × √3⁄2 in a triangular one, where s is the on-centre spacing. Dividing by 144 converts square inches to square feet. The exact count is the net area, the bed less any deduction, divided by A_p in square feet; density per 100 square feet is 100 divided by the same figure; and the distance between rows is s in a grid and s × √3⁄2 when the rows are staggered.

The √3⁄2 is pure geometry. Three plants that are each s apart form an equilateral triangle, and the height of that triangle, the perpendicular distance from one row to the next, is s × √3⁄2 = 0.8660254 s. Each plant's share of the ground shrinks by that factor, so the count rises by its reciprocal, 1.1547, which is where the 15.5% comes from. The factor is computed from the square root of three, not from a rounded 0.866, so the results match published table values to their last printed digit.

Spacing enters squared, which makes it the input with the biggest effect on the answer: a 10% closer spacing needs about 23% more plants. Bed area enters only in proportion, which is why a printed density table works for any bed: multiply the area by the plants per 100 square feet and divide by 100, so 240 × 115.47 ÷ 100 gives the same 277.13.

Two roundings follow, both because shops sell whole things. The count is rounded up to whole plants, and flats are rounded up from that rounded count. The cost is the whole-plant count times the price. The unrounded geometric count is shown separately, so you can see exactly how much rounding added.

The count assumes the pattern fills the bed exactly. In a square grid, that is what you get when the outermost plants sit half a spacing in from each edge; other edge rules give different totals, and on a small bed the difference is large.

A worked example.

Frequently asked questions.

References& sources.

1. [1]University of Georgia Cooperative Extension, Bulletin 931, "Conversion Tables, Formulas and Suggested Guidelines for Horticultural Use", Pennisi, B.V., published 7 November 2024. Table 31, "Estimated number of plants to fill 100 ft² of bed area for square (row) and triangular (equilateral) planting patterns using 4- to 14-in. spacing distances", adapted from Bailey, D.A. and Powell, M.A., 1999. Primary source for both patterns and for the row-offset column. Note that the table's prose states the offset as 0.886 X while its own printed Y values (3.46, 5.20, 6.93, 8.66, 10.39, 12.12) and its plant counts are 0.866 X; this page follows the numbers. Retrieved 2026-07-31; open access.
2. [2]Iowa State University Extension and Outreach, Yard and Garden, "How to Determine Plant Quantity for Planting Beds", Steil, A., last reviewed February 2025. Source of the independent check: the plants-per-square-foot table (4 in → 9, 15 in → 0.64, 48 in → 0.06) and the worked daylily example, 28 square feet × 0.64 = 17.92 rounded to 18 plants, all three of which this calculator reproduces exactly. Retrieved 2026-07-31; open access.
3. [3]University of Maryland Extension, "Groundcovers", updated 10 April 2026. Source of the spacing guidance quoted beside the result: spacing depends on the species chosen; close spacing gives faster coverage and less opportunity for erosion and weed colonisation, but greater risk of plant disease and the eventual decline of individuals facing too much competition. Retrieved 2026-07-31; open access.
4. [4]National Institute of Standards and Technology, Handbook 44 – 2026, Appendix C, "General Tables of Units of Measurement". Units of Area: 144 square inches = 1 square foot, and 43,560 square feet = 1 acre. Source of the square-inch to square-foot conversion used throughout this page. Retrieved 2026-07-31; open access PDF, downloaded and text-extracted.
5. [5]Iowa State University Extension and Outreach, Yard and Garden, "How do I determine the size of my garden bed or lawn in square feet?". Source of the method for measuring an irregular bed by breaking it into rectangles, triangles and circles and adding the results. Retrieved 2026-07-31; open access.