Complete User Guide: Garden Bed Calculator

What it does

• Calculates surface area for rectangle, circle, triangle, and L-shaped beds
• Determines soil volume needed in cubic feet and gallons
• Estimates number of soil bags required (standard 2 cu ft bags)
• Projects plant capacity by spacing method (intensive, standard, wide)
• Estimates lumber and material costs for building raised beds

How to use

Step 1: Select bed shape

• Rectangle/Square (most common)
• Circle (aesthetic, curved gardens)
• Triangle (corners, unique spaces)
• L-Shaped (wraparound beds, maximizing space)

Step 2: Enter dimensions

• Rectangle: Length and width in feet
• Circle: Diameter in feet
• Triangle: Base and height in feet
• L-Shaped: Two section dimensions (length/width each)

Step 3: Set depth

• Minimum 6″ for shallow-rooted plants
• 12″ recommended for most vegetables
• 14-18″ for deep-rooted crops (tomatoes, carrots)

Step 4: Calculate

• Get area, soil volume, bags needed, plant capacity, and material estimates

Understanding the outputs

Surface Area (sq ft): Total planting space available; determines how many plants fit

Cubic Feet (Soil): Volume of soil mix needed to fill bed to specified depth

Soil Bags: Number of standard 2 cu ft bags required (bulk soil often cheaper for large beds)

Plant Capacity:

• Intensive (4/sq ft): Lettuce, herbs, radishes, onions
• Standard (2/sq ft): Most vegetables, balanced spacing
• Wide (1/sq ft): Large plants like tomatoes, peppers, cabbage

Common bed sizes reference

4×4 ft (16 sq ft):

• Beginner-friendly, fits small spaces
• 12″ deep = 16 cu ft soil (~4.5 bags)
• Holds 9-16 plants depending on spacing
• Easy to reach all areas (2 ft max reach)

4×8 ft (32 sq ft):

• Most popular size, versatile
• 12″ deep = 32 cu ft soil (~9 bags)
• Holds 18-32 plants
• Standard lumber lengths (8 ft boards)

3×6 ft (18 sq ft):

• Narrow spaces, along walkways
• 12″ deep = 18 cu ft soil (~5 bags)
• Holds 10-18 plants
• Great for intensive planting

Depth guidelines by plant type

6-8″ deep (shallow):

• Lettuce, spinach, arugula
• Radishes, green onions
• Most herbs (basil, cilantro, parsley)
• Flowers with shallow roots

10-12″ deep (standard):

• Beans, peas
• Beets, turnips
• Kale, chard, broccoli
• Peppers, eggplant
• Most vegetables thrive here

14-18″ deep (deep):

• Tomatoes (indeterminate)
• Carrots, parsnips
• Potatoes
• Squash, cucumbers
• Sweet potatoes

24″+ deep (extra deep):

• Asparagus
• Rhubarb
• Artichokes
• Fruit bushes

Soil mix recommendations

Mel’s Mix (Square Foot Gardening):

• ⅓ peat moss or coconut coir
• ⅓ vermiculite (or perlite)
• ⅓ blended compost (5+ types)
• Benefits: Lightweight, excellent drainage, nutrient-rich

Budget Mix:

• 50% topsoil or garden soil
• 25% peat moss or coir
• 25% compost
• Benefits: Cost-effective for large beds

Premium Mix:

• 40% compost
• 30% peat moss/coir
• 20% vermiculite
• 10% worm castings
• Benefits: Maximum fertility, water retention

Never use: Garden soil alone (too dense, poor drainage, compacts over time)

Material selection

Wood (most common):

• Cedar: Naturally rot-resistant, lasts 15-20 years, $4-6/linear foot
• Redwood: Similar to cedar, beautiful, $5-7/linear foot
• Pine (untreated): Budget option, 3-5 years, $2-3/linear foot
• Composite: 20+ years, no rot, $6-10/linear foot

Avoid: Pressure-treated wood (before 2003 contained arsenic), railroad ties (toxic chemicals)

Other materials:

• Concrete blocks: Durable, heavy, permanent, ~$2-4 each
• Stone/brick: Beautiful, expensive, labor-intensive
• Galvanized metal: Long-lasting, can heat up in sun
• Recycled plastic: Eco-friendly, durable, lightweight

Building cost estimates

4×4 bed (16 sq ft, 12″ deep):

• Lumber: $20-40 (pine) or $60-80 (cedar)
• Soil: $70-90 (bagged) or $130-190 (bulk delivered)
• Hardware: $10-15 (screws, brackets)
• Hardware cloth: $8-12
• Total: $110-250

4×8 bed (32 sq ft, 12″ deep):

• Lumber: $30-60 (pine) or $120-180 (cedar)
• Soil: $140-180 (bagged) or $260-380 (bulk)
• Hardware: $15-25
• Hardware cloth: $16-24
• Total: $200-450

Location planning

Sun requirements:

• Full sun (6-8 hours): Most vegetables, tomatoes, peppers, squash
• Partial sun (4-6 hours): Lettuce, kale, herbs, root vegetables
• Avoid: Deep shade, under trees

Site considerations:

• Level ground (or willing to level)
• Good drainage (avoid low spots that pool water)
• Near water source (within hose reach)
• Away from large tree roots (competition)
• Visible from house (easier monitoring)
• South-facing for maximum sun (northern hemisphere)

Plant spacing guide

1 plant per square foot:

• Tomatoes
• Peppers
• Eggplant
• Cabbage
• Broccoli
• Cauliflower

4 plants per square foot:

• Lettuce
• Swiss chard
• Kale
• Spinach

9 plants per square foot:

• Beets
• Turnips
• Bush beans
• Kohlrabi

16 plants per square foot:

• Carrots
• Radishes
• Onions (sets)
• Garlic (cloves)

Watering raised beds

Beds dry faster than ground gardens:

• Exposed sides increase evaporation
• Warmer soil = more water loss
• Need 30-50% more water than in-ground beds

Watering strategies:

• Deep watering 1-2× weekly (better than shallow daily)
• Mulch surface with 1-2″ straw/bark (retains moisture)
• Soaker hoses or drip irrigation (most efficient)
• Morning watering (reduces disease)
• Check moisture 2″ below surface

Maintenance tips

Spring:

• Add 1-2″ compost layer
• Mix in slow-release fertilizer
• Check for board rot/damage

Summer:

• Water consistently (don’t let soil dry completely)
• Mulch to retain moisture
• Side-dress with compost mid-season

Fall:

• Plant cover crops or mulch heavily
• Add leaves/compost
• Harvest final crops

Winter:

• Protect soil from erosion
• Plan crop rotation for next year
• Repair any damage

Common mistakes to avoid

❌ Too wide: Beds over 4 ft wide make reaching center difficult
❌ Too shallow: 6″ depth insufficient for most vegetables
❌ Poor soil: Using pure topsoil or garden soil (compacts, poor drainage)
❌ No drainage: Beds need to drain; avoid impermeable liners
❌ Ignoring sun: Vegetables need 6+ hours sun; shade = poor yields
❌ Overcrowding: Plants need space; follow spacing guidelines
❌ Under-watering: Raised beds dry quickly; check moisture daily

Troubleshooting

Problem: Soil settling/compacting

• Solution: Top off with compost annually; mix in perlite/vermiculite

Problem: Poor drainage/waterlogged

• Solution: Add drainage holes in bottom; mix in more perlite; raise bed slightly

Problem: Weeds growing

• Solution: Use landscape fabric underneath; mulch surface; hand-pull early

Problem: Boards rotting quickly

• Solution: Use rot-resistant wood (cedar); line inside with plastic (don’t cover drainage); elevate slightly off ground

Problem: Plants yellowing/stunted

• Solution: Add nitrogen (compost/fertilizer); check for pests; ensure adequate depth

This calculator and guide help you plan and build productive raised garden beds with confidence!

Angelina Everly

Angelina Everly leads the editorial desk at Live Green Gardens, blending practical plant care, hands-on product testing, and approachable outdoor styling. She focuses on step-by-step how-tos, buyer’s guides, and small-space makeovers that work in real life and real budgets. When she’s not comparing pruning shears or setting up a drip kit, you’ll find her creating cozy corners with planters, solar lights, and pollinator-friendly picks—always with clear pros/cons and safety notes so you can buy once and garden happy.