Square Foot Garden Calculator
Bed size to grid squares, plant count, and soil volume.
What it does
Square foot gardening trades rows for grids. Each 1-foot square holds 1, 4, 9, or 16 plants by size, so a 4 by 8 bed becomes 32 managed cells instead of vague rows. Counting replaces guessing.
The tool above multiplies bed sides into squares, multiplies by per-square density, and adds 6-inch soil volume. Imperial mode works in feet and cubic feet; metric mode works in meters and liters on 30-cm cells.
The Square Foot Gardening Foundation publishes the method with planting charts by size class, and extension services teach the same grid with Mel's Mix: equal compost, peat, and vermiculite. Density runs from one broccoli to sixteen carrots per square.
Build narrow enough to reach the middle, fill once, then replant squares as they empty. Succession turns one bed into three seasons; the math reruns per crop while the grid stays put.
How to use
- Pick imperial (ft) or metric (m).
- Enter bed length and width.
- Pick plant size: 1, 4, 9, or 16 per square.
- Read squares, plant total, and soil volume.
- Grid the bed into 1-ft squares.
- Fill 6 in with Mel's Mix and plant.
Formula
- L
- Bed length in ft (or m metric).
- W
- Bed width in ft (or m metric).
- per-square
- Plants per square: 1, 4, 9, or 16.
Grid cells are 1 ft (30 cm). Soil volume assumes 6 in (15 cm) of mix: squares x 0.5 ft3 or area x 150 L. Narrow beds under 1 ft break the grid; keep widths reachable at 4 ft or less.
Worked example
Example: 4 by 8 ft bed
4 by 8 ft bed of medium plants like beans.
- Bed is 4 x 8 ft.
- Squares: 4 x 8 = 32.
- Medium plants (9 per square): 32 x 9 = 288 plants.
- Soil 6 in deep: 32 x 0.5 = 16.0 ft3.
- Mix equal compost, peat, and vermiculite.
Reference table
| Size | Per square | Examples |
|---|---|---|
| Extra large | 1 | Broccoli, peppers, tomatoes |
| Large | 4 | Lettuce, chard, marigold |
| Medium | 9 | Beans, beets, spinach |
| Small | 16 | Carrots, radishes, onions |
Limits
- Grid method only. Row gardening spaces differently.
- Densities suit listed crops; giants need more room.
- Soil math assumes 6 in depth and fresh mix.
- Sun, water, and season still rule yields.
Common mistakes
- Planting rows inside grid beds.
- Skipping the grid and eyeballing spacing.
- Filling with native dirt instead of mix.
- Building wider than an arm's reach.
- Leaving squares empty after harvest.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 by 4 ft bed of small (16/square)?
Squares are 16. Plants are 16 x 16 = 256. Soil is 16 x 0.5 = 8.0 ft3.
1.2 by 1.2 m bed of small?
Squares are 4 x 4 = 16. Plants are 256. Soil is 1.44 x 150 = 216 L.
Questions
How many plants fit one square?
Extra large 1 per square (broccoli, peppers), large 4 (lettuce, chard), medium 9 (beans, beets, spinach), small 16 (carrots, radishes, onions). One square is 1 ft (30 cm) per side.
What soil goes in the bed?
Equal parts blended compost, peat moss (or coco coir), and coarse vermiculite, 6 inches deep. Skip native dirt: the mix drains, holds water, and stays loose for roots.
How much Mel's Mix do I buy?
Area times depth: squares times 0.5 ft3 (or m2 times 150 L). A 4 by 8 takes 16 ft3, about eight 2-ft3 bags. Top up yearly as it settles.
How do I mark the grid?
Lay 1-ft (30-cm) grids with string or slats, then plant each square by its spacing. Grids turn thinning and succession into counting, not guessing.
Does this suit beginners?
Yes, and it helps most there: raised depth warms early, grids space evenly, and Mel's Mix forgives clay or sand below. Keep beds 4 ft wide so all squares stay reachable.
How does succession planting work?
Replant each harvested square within days: radish to beans to garlic across seasons. The plant count math reruns per crop, so succession triples one bed's year.