Why Seed Packets Give Two Numbers
Every seed packet lists two spacing measurements: an in-row spacing and a between-row spacing. These are not the same number, and they serve different purposes. In-row spacing is the biological minimum — the distance a plant's roots, canopy, or fruit-production pattern actually requires. Between-row spacing is the agricultural minimum — the distance wide enough for you, or a tractor, to walk between rows.
In a raised bed, you never walk in the bed. You reach in from the side. That erases the need for between-row spacing almost entirely. A bed 4 feet wide is reachable from both sides without stepping in — at most 2 feet of reach. So the "between rows" number from the seed packet is irrelevant to raised bed planning. The in-row number is the one that matters.
The formula the calculator uses
rows = floor(bed_width_in ÷ row_spacing_in)
plants = floor(bed_length_in ÷ in_row_spacing_in)
total = rows × plants
The floor function means you never count a plant that doesn't have its full spacing. Plants at bed edges need that margin for root development and airflow — the calculator doesn't add them back.
Rows vs. Intensive: Why the Method Changes Everything
The difference between row spacing and intensive spacing is not a rounding error. For head lettuce (10" in-row, 12" between rows) in a 4×8 bed:
- Row spacing: 4 rows × 9 plants = 36 plants
- Intensive grid: floor(48÷10) × floor(96÷10) = 4 × 9 = 36 plants — same here because the math happens to match. But for Bush beans (4" in-row, 18" between rows): rows give floor(48÷18) × floor(96÷4) = 2 × 24 = 48 plants. Intensive grid gives floor(48÷4) × floor(96÷4) = 12 × 24 = 288 plants. That's 6× more.
The difference is real because bush beans have a 4" biological requirement. The 18" between-row number was sized for a walking path, not for the plant. In a bed you reach from the side, a 4"×4" grid is fine for beans — you harvest from the perimeter and never need a path through the planting.
The Square Foot Gardening Cheat Sheet
Mel Bartholomew's Square Foot Gardening method simplifies all of this to four numbers based on how many plants fit in one square foot of bed. The numbers are:
- 1 per sq ft: crops needing 12" spacing (tomatoes, peppers, broccoli)
- 4 per sq ft: crops needing 6" spacing (lettuce, basil, Swiss chard)
- 9 per sq ft: crops needing 4" spacing (beets, spinach, onions)
- 16 per sq ft: crops needing 3" spacing (carrots, radishes)
Where does 1/4/9/16 come from? It's the square root of 144 (square inches in a square foot) divided by the spacing, squared: 144 ÷ 12² = 1, 144 ÷ 6² = 4, 144 ÷ 4² = 9, 144 ÷ 3² = 16. The SFG method rounds each crop to the nearest clean number. It is a practical simplification, not an exact formula, but it is accurate enough for planning and avoids the mental math of working in fractional inches.
SFG is optimized for productivity over simplicity of setup. Each square foot becomes its own mini-bed that gets replanted as soon as it empties. Used with the Succession Planting Calculator, it becomes a machine for continuous harvest across the whole season.
The Overcrowding Tax
Spacing isn't just a matter of how many plants physically fit. It determines what each plant produces.
A tomato planted at 18" instead of 24" will grow — but it will direct energy toward height rather than fruit as its canopy merges with neighbors and it races for light. The lower third of the plant will stay shaded and damp, which is the exact environment powdery mildew and early blight need to establish. By mid-summer you're removing diseased leaves faster than the plant is setting new fruit.
The mathematics of this are clear: in a 4×8 bed, 4 tomato plants at proper 24" intensive grid spacing each have 576 square inches of root space. Squeeze in 8 plants and each has 288 square inches — half the root zone, half the nutrient uptake capacity. Studies from university extension programs consistently show that tomatoes grown at proper spacing outproduce overcrowded plants by 30–60% total yield, even though there are half as many plants. The fewer, properly-spaced plants are a better use of your bed.
Edge Margins for Large Crops
The calculator uses floor math, which means the outermost plant always has at least one full spacing of distance to the bed edge. For small crops (carrots, radishes), this margin is 1–2 inches — negligible. For large crops (tomatoes at 24"), the outermost plants sit 12" from the frame's interior wall.
That 12" margin is appropriate. The frame wall reflects heat, can harbor slugs, and reduces airflow on the side facing it. Large plants against the frame also make it harder to access the bed for harvesting and training. For any crop with spacing over 12", the floor math is giving you the right answer even if the empty perimeter looks wasteful. Fill that perimeter with a border of quick-maturing crops instead — radishes, lettuce, or scallions that will finish before the main crop's canopy expands to shade them.