Table of Contents
How much soil do you need for a raised bed? How far apart should tomatoes be planted? Those two questions start more garden projects than any other. This guide turns the common measurements you face into quick calculations and rules of thumb. You’ll learn how to go from bed dimensions to cubic feet, how depth affects mulch and compost needs, simple seed math, fertilizer rates per square foot, and container volumes. There’s also a surprising fact and one famous mistake gardeners can learn from.
1Raised beds: how to calculate soil volume
Raised beds are simple shapes—rectangles, sometimes triangles or circles—so their soil needs follow basic geometry. For a rectangular bed, volume in cubic feet = length (ft) × width (ft) × depth (ft). If depth is given in inches, divide by 12. To get cubic yards, divide cubic feet by 27. One quick trick: multiply the bed area in square feet by the depth in inches, then divide by 12 to get cubic feet. That mental shortcut avoids converting depths first.
Example: a 4×8×12in raised bed
A 4 ft × 8 ft bed with 12 in (1 ft) of soil: 4 × 8 × 1 = 32 cubic feet. If you buy soil by the cubic yard, 32 ÷ 27 ≈ 1.19 cubic yards. If soil sold by bags states cubic feet per bag, divide 32 by the bag size to get number of bags.
Deep beds and layered filling
For deep beds made with layers (coarse material first, finished topsoil above), calculate total volume the same way, then subtract the approximate volume of bulky coarse fill if you’re filling the top with purchased soil. For example, if 30% of a 48 cu ft bed is logs/sticks for hugelkultur, you’ll need about 33.6 cu ft of soil (48 × 0.7).
Conversions and mental math
Metric: volume (m³) = length (m) × width (m) × depth (m). 1 m³ ≈ 35.3147 ft³. Handy garden trick: area (sq ft) × depth (in) ÷ 12 = ft³. That saves a calculator when ordering soil quickly.
2Mulch depth and coverage
Mulch is sold by volume or bag coverage. Coverage depends on area and desired depth. Use the same volume math: volume = area × depth. Depth is often inches for mulch—1 inch cover over 100 sq ft is about 100 × (1/12) = 8.33 cubic feet. The depth you choose matters: 1 inch for light coverage, 2–3 inches for weed suppression, 4 inches for coarse bark. Over-mulching can suffocate plant crowns, so match depth to material.
Common coverage numbers
Quick reference: 1 inch over 100 sq ft = 8.33 ft³. 2 inches over 200 sq ft = 33.33 ft³ (200 × 2/12). If a mulch bag covers 2 cu ft, you’d need ~4 bags for 8.33 cu ft.
Ordering tips
Bulk mulch is sold by cubic yard. Convert your needed cubic feet to cubic yards by dividing by 27 (8.33 ft³ ÷ 27 ≈ 0.31 yd³ for 1 inch over 100 sq ft). Round up a bit for settling and uneven ground.
Surprising fact
You might be surprised: 1 inch of mulch across 500 sq ft needs about 41.7 cubic feet—almost one and a half cubic yards. Knowing that helps avoid under-ordering the day before planting.
3Plant spacing and seed quantity
Plant spacing guides tell how far apart to put plants for air flow and yield. For many vegetables, spacing is a trade-off between maximizing yield and maintaining plant health. For seeds, seed packet info gives seeds per foot or per area; convert to rows or beds using simple area math. Seed math usually goes: desired plants ÷ germination rate = seeds to plant (adjust for thinning). For direct-sow crops, plan for losses and thin to final spacing.
Common spacing rules
Tomatoes: 18–36 inches apart depending on variety. Peppers: 12–18 inches. Leafy greens (lettuce, spinach): 6–12 inches. Use closer spacing for baby salad mixes, wider spacing for full heads.
Seed quantity example
If you want 20 mature beets and germination is 80%: seeds needed ≈ 20 ÷ 0.8 = 25 seeds. If seeds are sown in a row and packet says 30 seeds per foot, you can sow one foot and expect ~24 germinations—thin as needed.
Layout and bed planning
Plan beds by area: if you want 12 tomato plants at 2 ft spacing in a single row, you need 22 ft of row (11 spaces × 2 ft). Alternatively, in a 4×8 bed using staggered spacing you may fit 6–8 plants depending on variety.
4Fertilizer and amendment rates per area
Many fertilizer recommendations are given per 1000 sq ft or per 100 sq ft. To get a per-square-foot rate, divide the given rate by the area listed. For example, 5 lb per 1000 sq ft = 0.005 lb per sq ft (5 ÷ 1000). Apply amendments by weight or volume depending on product. Garden lime and rock phosphate are often measured in pounds per 100 sq ft; compost is usually in cubic feet or cubic yards.
Example: applying a common fertilizer
If a bag says apply 2 lb per 100 sq ft, and your bed is 32 sq ft, required fertilizer = 2 × (32 ÷ 100) = 0.64 lb. Use a kitchen scale for small amounts or measure by tablespoons when the bag gives volume equivalents.
From bulk to small beds
For compost: if you need 1 inch of compost over a 100 sq ft bed, that’s 8.33 cu ft. A yard of compost (27 cu ft) covers roughly 324 sq ft at 1 inch depth (27 × 12 = 324 sq ft·in).
Safety and accuracy
Fertilizers are concentrated; over-application causes plant burn. When converting rates, double-check units (lb, oz, kg) and area basis before spreading.
5Container volumes and small-space gardening
Containers come in liters, gallons, or cubic feet. For a cylinder pot: volume = π × (radius)² × height. For rectangular planters: length × width × height. Convert liters to cubic feet by dividing liters by 28.3168. A quick garden trick: for rectangular containers, area (sq ft) × depth (in) ÷ 12 gives cubic feet just like beds. That makes it easy to compare potting mix bag sizes.
Example: a 12-inch deep 2 ft × 1 ft planter
Area = 2 ft × 1 ft = 2 sq ft. Depth = 12 in = 1 ft. Volume = 2 × 1 = 2 cu ft. If potting mix bag is 0.75 cu ft, you’ll need ~3 bags (2 ÷ 0.75 ≈ 2.67).
Converting gallon sizes
US liquid gallons to cubic feet: 1 US gallon = 0.133681 ft³. Many nursery pots are sold by gallon (e.g., 5 gal pot ≈ 0.6684 ft³). For metric: 1 liter = 0.001 m³; a 20 L bag = 0.02 m³.
Planting density in containers
Match container volume to plant root needs: large tomatoes often need 10–20 gal (≈1.33–2.67 ft³). Herbs and salad greens do fine in smaller pots; follow spacing guidelines but reduce numbers for shared containers.
Pro Tips
- 1Quick trick: area (sq ft) × depth (in) ÷ 12 = cubic feet
- 2To get cubic yards, divide cubic feet by 27
- 3For fertilizer: convert per-1000-sq-ft rates by dividing by 1000 for per-sq-ft amounts
- 41 inch over 100 sq ft ≈ 8.33 cubic feet (use this for mulch and compost estimates)
Conversions in the garden are mostly simple area × depth or area ÷ spacing problems. Once you memorize a couple of shortcuts—area × depth(in) ÷ 12 = cubic feet and cubic feet ÷ 27 = cubic yards—you can order soil, mulch, or compost with confidence. Try the related converters on this site for quick checks: they’ll convert between cubic feet and cubic yards, square feet and square meters, and liters and gallons so you don’t have to recalc by hand before heading to the nursery.
