Garden Bed Calculator

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

With over 15 years of hands-on experience in controlled-environment agriculture, Angelina leads our lab audits. Her focus is on bridging the gap between high-end agricultural tech and the home grower, ensuring every recommendation is backed by real-world data and yield performance.