How Many Plants Do You Need? A Simple Formula for Garden Bed Planning

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How many flowers or plants do you actually need for your garden bed or space? Avoid the frustration of buying too many—or too few—at the nursery. We consulted the New York Botanical Garden in the Bronx for their tried-and-true formula for estimating plant quantities. While exact numbers depend on the plant type, this method gives you a solid starting point for garden design.

Steps for Calculating Plant Quantities

1. Determine the square footage of your planting area:

• Rectangle: Multiply length by width
• Circle: Multiply the radius by itself, then by pi (3.14)
• Oval: Multiply the average radius by itself, then by pi (3.14)
• Triangle: Multiply ½ the height by the base

2. Convert to square inches: Multiply square feet by 144 (the number of square inches in one square foot).

3. Determine space per mature plant: Multiply the suggested spacing (in inches) between plants by itself.

4. Divide total square inches by square inches per plant: The result is the total number of plants needed.

Example A: Wax Begonias in a 6×8-Foot Bed

Step 1: 6 ft × 8 ft = 48 sq ft.

Step 2: 48 × 144 = 6,912 sq in.

Step 3: Suggested spacing for wax begonia (Begonia semperflorens-cultorum) is 8–10 inches.

• At 8-inch centers: 8 × 8 = 64 sq in per plant
• At 9-inch centers: 9 × 9 = 81 sq in per plant
• At 10-inch centers: 10 × 10 = 100 sq in per plant

Note: The middle of the range (9 inches) is usually recommended. For quicker coverage, use the low end (8 inches). The high end (10 inches) risks incomplete coverage if some plants fail.

Step 4:

• 8-inch centers: 6,912 ÷ 64 = 108 plants
• 9-inch centers: 6,912 ÷ 81 = 85.3 → 86 plants
• 10-inch centers: 6,912 ÷ 100 = 69.1 → 70 plants

Example B: French Marigolds in a 4×4-Foot Bed

Step 1: 4 ft × 4 ft = 16 sq ft.

Step 2: 16 × 144 = 2,304 sq in.

Step 3: Suggested spacing for French marigold (Tagetes patula) is 12–15 inches.

• 12-inch centers: 12 × 12 = 144 sq in
• 13-inch centers: 13 × 13 = 169 sq in
• 14-inch centers: 14 × 14 = 196 sq in
• 15-inch centers: 15 × 15 = 225 sq in

Step 4:

• 12-inch centers: 2,304 ÷ 144 = 16 plants
• 13-inch centers: 2,304 ÷ 169 = 13.6 → 14 plants
• 14-inch centers: 2,304 ÷ 196 = 11.8 → 12 plants
• 15-inch centers: 2,304 ÷ 225 = 10.2 → 11 plants

Important: If your calculation yields a fraction (e.g., 40.4 or 80.7), always round up to the next whole number. That one extra plant may make the difference between a filled-out design and visible gaps.

If you're growing vegetables, our online Garden Planner automatically calculates spacing by crop. Technology is amazing!

Source: 2001 Early Spring Garden Guide. Originally published 2011, updated.