How do I calculate how many plants I need?

Divide the bed area by the area one plant owns. On a square grid that is spacing squared: 96 sq ft at 18 in (1.5 ft) is 96 ÷ 2.25 = 42.7, so 43 plants. That is a good estimate. To get the number you can actually plant, lay out the rows: 5 rows across the 8 ft side and 8 plants along each, 40 plants, because the bed is not a whole number of 18 in spacings wide.

Why does the laid-out count differ from area ÷ spacing²?

Area ÷ spacing² counts every plant as owning a full cell, including the ones at the edge, as if the bed had no edge. A real bed ends partway through a cell, and that cell either gets a plant or it does not. With a half-spacing setback and a bed that is a whole number of spacings on each side the two agree exactly: a 12 × 9 ft bed at 18 in is 48 plants either way. For the 12 × 8 ft bed, 8 ft is 5.33 spacings, so there are 5 rows (not 5.33) and the count is 40 against 43.

How many more plants does a triangular layout take?

About 15.5% more. Each plant in a triangular grid owns s² × √3/2, 1.1547 times less area than the s² of a square grid, so the same bed holds 2 ÷ √3 = 1.1547 times as many plants. At 12 in that is 100 plants per 100 sq ft on a square grid and 115.5 on a triangular one, with the rows 10.4 in apart. The bed in the example holds 45 plants triangular against 40 square once edges are counted: the difference in a small bed is not exactly 15.5%. Triangular spacing leaves less bare soil between plants, so ground cover fills in evenly; it is a design choice, not a requirement.

How far from the edge should the first plants be?

A common rule of thumb is half the spacing, so 9 in for an 18 in spacing: an edge plant then has the same room on its outer side as it has halfway to its neighbors. It is a rule of thumb, not a requirement, and the calculator lets you choose. With no setback the plants sit on the edge line, which is what you want against a wall or hard edging, and it takes more plants: 54 for the example bed against 40. A full spacing from the edge leaves a wider margin and takes fewer.

What does on center mean?

On center is the distance from the middle of one plant to the middle of the next, not the gap between their edges. A plant tag that says "space 18 in" normally means 18 in on center. Use the spacing for the size the plant will grow to if you want the bed to look spaced, or a smaller spacing if you want it to fill in fast; the plant’s grower or nursery is the right source for what to choose.

How many flats of ground cover do I need?

Enter the number of plants in the flat you will buy (flats come in different counts; read the label) and the calculator divides your plant count, spares included, by it and rounds up. The example bed with a flat of 32 plants needs 2 flats for 40 plants. The table of flats for 100 sq ft shows the arithmetic for a few common counts.

How do I allow for a path, boulder or tree in the bed?

Add each as an opening and the calculator removes its share of the bed from the count. A 3 × 2 ft opening in the example bed brings the plants from 40 to 38. This scales the count by area, so it is an estimate: for a large opening, lay the rows out on the ground and count the plants that fall outside it.

Should I buy extra plants?

That is your call and depends on the plant, the season and how you plant. A few spares replace the ones that fail or are damaged; this calculator offers 0, 5 and 10% and does not claim any of them is the right figure. With 10% extra the example bed takes 44 plants.

Which side do the rows run along?

Along the bed length. If a bed is long and thin, the count can differ when the rows run the other way, so enter the dimensions swapped and compare. On a square grid the count is the same either way; on a triangular grid it can change by a few plants.

Plant Spacing Calculator: How Many Plants Do I Need?

A 12 × 8 ft bed planted on an 18 in square grid, with the first plants 9 in from the edge, needs 40 plants: 5 rows of 8. The quick area estimate, 96 sq ft ÷ (1.5 × 1.5 ft), gives 43; laying the rows out gives fewer because the bed is not a whole number of spacings wide. On a triangular (offset) grid the same bed holds 45 plants, with rows 15.6 in apart instead of 18.

Figures based on the sources listed belowUpdated 2026-10-04How we calculate

Bed length

Bed width

Spare plants

None (0%)A few spares for losses (5%)Extra spares (10%)Custom

Plant spacing, on center (in)

Center to center, as on the plant tag: the distance between the middles of two neighbors.

LayoutIn a triangular grid every plant is the same distance from its six neighbors, so rows sit 0.866 × the spacing apart and the bed holds about 15% more plants. Rows run along the bed length.

Distance from the edgeHalf the spacing from the edge gives each edge plant the room its neighbors have. It is a rule of thumb, not a requirement; choose none to plant right up to a wall or edging.

Plants per flat (optional)

For ground cover sold in flats: enter the number of plants in the flat you will buy (check the label). Leave 0 to skip flats.

Price per plant (optional)

Buy 40 plants

96 sq ft + 0% waste = 96 sq ft

Rows along the length: 5 rows (every 18 in), 8 plants per row
Plants by area (area ÷ one plant’s share, no spares): 43 plants, the laid-out count is 40: edge effects

How this number is calculated

96 sq ft (no waste added) ÷ 2.4 sq ft per plant = 40 plants

Shopping list

Garden measuring tape (100 ft)
Marking paint for laying out a bed
Hori hori garden knife
Bulb and plug planter
Garden kneeler

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Plants per 100 sq ft by spacing, square and triangular grid

By area: 100 ÷ s² for a square grid, 100 ÷ (s² × √3/2) for a triangular grid, with s in feet. The triangular grid holds 1.1547 times as many, 15.5% more, at every spacing. Worked by this calculator’s formula; a real bed is a little more or less at the edges.
Spacing on center	Square grid	Triangular grid	Extra plants (triangular)
6 in (15 cm)	400	461.9	61.9
8 in (20 cm)	225	259.8	34.8
9 in (23 cm)	177.8	205.3	27.5
10 in (25 cm)	144	166.3	22.3
12 in (30 cm)	100	115.5	15.5
15 in (38 cm)	64	73.9	9.9
18 in (46 cm)	44.4	51.3	6.9
24 in (61 cm)	25	28.9	3.9
30 in (76 cm)	16	18.5	2.5
36 in (91 cm)	11.1	12.8	1.7

By area against laid out in a 10 × 10 ft bed

Half-spacing setback, rows along the length, no spares, worked by this calculator. Where the two columns differ, the laid-out count is the one to plant: a bed that is not a whole number of spacings wide ends in a partial cell.
Spacing on center	Square, by area	Square, laid out	Triangular, by area	Triangular, laid out
6 in (15 cm)	400	400	462	429
12 in (30 cm)	100	100	116	105
18 in (46 cm)	45	36	52	42
24 in (61 cm)	25	25	29	23
36 in (91 cm)	12	9	13	8

Flats for 100 sq ft by plants per flat

Plants for 100 sq ft on a square grid, by area, then divided by the number of plants in a flat and rounded up. The flat sizes (18, 32, 36 and 72 plants) are examples only; use the count on the label of the flat you buy.
Spacing on center	Plants	18 per flat	32 per flat	36 per flat	72 per flat
6 in (15 cm)	400	23	13	12	6
8 in (20 cm)	225	13	8	7	4
12 in (30 cm)	100	6	4	3	2
18 in (46 cm)	45	3	2	2	1

Methodology & assumptions

By area: each plant owns one cell of the grid. On a square grid at spacing s that cell is s × s; on a triangular grid the rows are s × √3/2 (0.866 × s) apart and each plant owns s² × √3/2. Plants = bed area ÷ that cell, so a triangular grid holds 2 ÷ √3 = 1.1547 times as many plants, 15.5% more, at the same spacing. This is exact geometry for an unbounded bed. Laid out: a real bed has edges. The calculator takes the setback off every side (half the spacing by default, or a full spacing, or none), then fits rows along the bed length: rows = floor(usable width ÷ row pitch) + 1 and plants per row = floor(usable length ÷ spacing) + 1. In a triangular grid every second row is offset by half the spacing, so it holds one plant fewer when the row ends fall short. The two counts agree only in special cases: with a half-spacing setback and a bed that is a whole number of spacings on each side, a square grid has exactly area ÷ s² plants. Otherwise the laid-out count is the one you can actually plant, and the difference is the edge effect: a partial cell at the edge either holds a plant or it does not. The headline is the laid-out count plus any spares you choose, rounded up; the by-area count is shown beside it when they differ. Openings (a path, a boulder, a tree) reduce the laid-out count in proportion to the area they take, which is an estimate; lay out the rows on the ground around them. Rows run along the bed length; a long thin bed can hold a different number when laid out the other way, so try swapping length and width. Flats are the plants (with spares) divided by the plants per flat you enter, rounded up. Spacing is on center, the distance between plant centers, as on a plant tag; the spacing a plant needs is the plant maker’s or nursery’s figure, not something this calculator knows. The metric toggle shows lengths in m and the spacing in cm.

Hexagonal (triangular) and square lattices: the cell area of a triangular grid at spacing s is s² × √3/2, from the area of an equilateral triangle (plane geometry; no outside source needed)
Horticulture magazine, "Plant Spacing" (hortmag.com/weekly-tips/plant-spacing): square grid versus triangular layout, about 15% more plants and rows 0.866 of the spacing apart for a triangular layout (found in a web search; the page itself could not be opened from this build environment, so check it)
Garden.org, "Planting Fundamentals" (garden.org/learn/articles/view/2449): the first plant about half the spacing from the bed edge is a rule of thumb, not a standard (found in a web search; the page itself could not be opened from this build environment, so check it)
The plant tag, nursery or grower for the plant you are buying: the spacing to use and the number of plants in a flat. This calculator takes both as entries and makes no claims about them

How to calculate plant spacing

Measure the bed. Measure the length and width in feet. Subtract any path, boulder or tree as an opening.
Pick the spacing. Use the on-center spacing from the plant tag or your nursery: the distance from the middle of one plant to the middle of the next.
Estimate by area. Divide the area by the spacing squared (in feet) for a square grid, or by the spacing squared × 0.866 for a triangular grid.
Lay out the rows. Take the edge setback (half the spacing is common) off each side, then rows = usable width ÷ row spacing + 1, rounded down, and plants per row = usable length ÷ spacing + 1, rounded down. In a triangular grid offset every second row by half the spacing.
Add spares and find flats. Add spares if you want them, round up, and divide by the plants in a flat to find the flats.

Worked examples

Project	Result
12 × 8 ft bed, 18 in square grid, first plants 9 in from the edge	96 sq ft → 40 plants (5 rows of 8); by area 43
The same bed on a triangular grid (alternate rows offset)	96 sq ft → 45 plants (6 rows, 8 and 7 alternately); by area 50
The same bed with plants on the edge line (no setback)	96 sq ft → 54 plants (6 rows of 9)
12 × 9 ft bed, 18 in square grid, 9 in from the edge (a whole number of spacings: the two methods agree)	108 sq ft → 48 plants (6 rows of 8); by area 48
The 12 × 8 ft bed with a 3 × 2 ft opening and 10% spares	90 sq ft + 10% → 42 plants (estimate: openings scale the count by area)

Frequently asked questions

How do I calculate how many plants I need?: Divide the bed area by the area one plant owns. On a square grid that is spacing squared: 96 sq ft at 18 in (1.5 ft) is 96 ÷ 2.25 = 42.7, so 43 plants. That is a good estimate. To get the number you can actually plant, lay out the rows: 5 rows across the 8 ft side and 8 plants along each, 40 plants, because the bed is not a whole number of 18 in spacings wide.
Why does the laid-out count differ from area ÷ spacing²?: Area ÷ spacing² counts every plant as owning a full cell, including the ones at the edge, as if the bed had no edge. A real bed ends partway through a cell, and that cell either gets a plant or it does not. With a half-spacing setback and a bed that is a whole number of spacings on each side the two agree exactly: a 12 × 9 ft bed at 18 in is 48 plants either way. For the 12 × 8 ft bed, 8 ft is 5.33 spacings, so there are 5 rows (not 5.33) and the count is 40 against 43.
How many more plants does a triangular layout take?: About 15.5% more. Each plant in a triangular grid owns s² × √3/2, 1.1547 times less area than the s² of a square grid, so the same bed holds 2 ÷ √3 = 1.1547 times as many plants. At 12 in that is 100 plants per 100 sq ft on a square grid and 115.5 on a triangular one, with the rows 10.4 in apart. The bed in the example holds 45 plants triangular against 40 square once edges are counted: the difference in a small bed is not exactly 15.5%. Triangular spacing leaves less bare soil between plants, so ground cover fills in evenly; it is a design choice, not a requirement.
How far from the edge should the first plants be?: A common rule of thumb is half the spacing, so 9 in for an 18 in spacing: an edge plant then has the same room on its outer side as it has halfway to its neighbors. It is a rule of thumb, not a requirement, and the calculator lets you choose. With no setback the plants sit on the edge line, which is what you want against a wall or hard edging, and it takes more plants: 54 for the example bed against 40. A full spacing from the edge leaves a wider margin and takes fewer.
What does on center mean?: On center is the distance from the middle of one plant to the middle of the next, not the gap between their edges. A plant tag that says "space 18 in" normally means 18 in on center. Use the spacing for the size the plant will grow to if you want the bed to look spaced, or a smaller spacing if you want it to fill in fast; the plant’s grower or nursery is the right source for what to choose.
How many flats of ground cover do I need?: Enter the number of plants in the flat you will buy (flats come in different counts; read the label) and the calculator divides your plant count, spares included, by it and rounds up. The example bed with a flat of 32 plants needs 2 flats for 40 plants. The table of flats for 100 sq ft shows the arithmetic for a few common counts.
How do I allow for a path, boulder or tree in the bed?: Add each as an opening and the calculator removes its share of the bed from the count. A 3 × 2 ft opening in the example bed brings the plants from 40 to 38. This scales the count by area, so it is an estimate: for a large opening, lay the rows out on the ground and count the plants that fall outside it.
Should I buy extra plants?: That is your call and depends on the plant, the season and how you plant. A few spares replace the ones that fail or are damaged; this calculator offers 0, 5 and 10% and does not claim any of them is the right figure. With 10% extra the example bed takes 44 plants.
Which side do the rows run along?: Along the bed length. If a bed is long and thin, the count can differ when the rows run the other way, so enter the dimensions swapped and compare. On a square grid the count is the same either way; on a triangular grid it can change by a few plants.