How Much Mulch Do I Need? (Calculator & Buying Guide)

Getting the right amount of mulch is crucial for effective garden coverage. Too little won’t suppress weeds or retain moisture properly, while too much can suffocate plant roots and waste money.

In this guide, I’ll show you exactly how to calculate how much mulch you need, explain the calculations behind it, and provide a practical buying guide to help you get the best value when purchasing bark, wood chips, or other mulch materials.

Quick Mulch Calculator

Basic Formula:

Cubic metres needed = (Area in m² × Depth in cm) ÷ 100

Or for imperial measurements:

Cubic yards needed = (Area in sq ft × Depth in inches) ÷ 324

Step-by-Step Calculation

1. Measure your area
   • For rectangular beds: Length × Width
   • For circular beds: π × radius² (radius = diameter ÷ 2)
   • For irregular shapes: break into smaller rectangles/circles and add together
2. Decide on depth
   • Standard recommendation: 5-8 cm (2-3 inches)
   • For weed suppression: 8-10 cm (3-4 inches)
   • For paths: 5 cm (2 inches)
3. Calculate volume
4. Add 10% extra for settling and waste

Worked Examples

Example 1: Rectangular Flower Bed (Metric)

• Bed dimensions: 4m × 2m
• Desired depth: 7 cm

Calculation:

1. Area = 4m × 2m = 8 m²
2. Volume = (8 × 7) ÷ 100 = 0.56 cubic metres
3. Add 10%: 0.56 × 1.1 = 0.62 cubic metres

You need: 0.62 cubic metres of mulch

Example 2: Circular Bed (Imperial)

• Diameter: 10 feet
• Desired depth: 3 inches

Calculation:

1. Radius = 10 ÷ 2 = 5 feet
2. Area = 3.14 × 5² = 78.5 sq ft
3. Volume = (78.5 × 3) ÷ 324 = 0.73 cubic yards
4. Add 10%: 0.73 × 1.1 = 0.80 cubic yards

You need: 0.80 cubic yards of mulch

Recommended Mulch Depths for Different Applications

Coverage Guide: How Far Will One Bag Go?

Most bagged mulch comes in these standard sizes:

UK Bagged Mulch

• 50 litres (0.05 cubic metres)
  • At 5 cm depth: covers 1 m²
  • At 7 cm depth: covers 0.7 m²
  • At 10 cm depth: covers 0.5 m²
• 70 litres (0.07 cubic metres)
  • At 5 cm depth: covers 1.4 m²
  • At 7 cm depth: covers 1 m²
  • At 10 cm depth: covers 0.7 m²
• 100 litres (0.1 cubic metres)
  • At 5 cm depth: covers 2 m²
  • At 7 cm depth: covers 1.4 m²
  • At 10 cm depth: covers 1 m²

US Bagged Mulch

• 2 cubic feet bag
  • At 2″ depth: covers 12 sq ft
  • At 3″ depth: covers 8 sq ft
  • At 4″ depth: covers 6 sq ft
• 3 cubic feet bag
  • At 2″ depth: covers 18 sq ft
  • At 3″ depth: covers 12 sq ft
  • At 4″ depth: covers 9 sq ft

Mulch Buying Guide

Bags vs Bulk: Which Should You Buy?

Buy bagged mulch when:

• You need less than 1 cubic metre (1.3 cubic yards)
• You have limited storage space
• You want convenience (no delivery wait)
• You’re mulching small, scattered areas

Buy bulk mulch when:

• You need more than 2 cubic metres (2.6 cubic yards)
• You want to save money (typically 30-50% cheaper per cubic metre)
• You have space for delivery and can spread it quickly
• You’re mulching large continuous areas

Cost Comparison Example (UK)

For 3 cubic metres of bark mulch:

• Bagged (70L bags at £5 each): 43 bags = £215
• Bulk delivery: £70-90 + £30-50 delivery = £100-140 total
• Savings: £75-115 (35-53%)

Where to Buy Mulch

Garden centres:

• Pros: Wide selection, can see/feel before buying, advice available
• Cons: Most expensive option, limited to bagged products

DIY stores (B&Q, Wickes, Home Depot, Lowe’s):

• Pros: Competitive pricing, convenient locations
• Cons: Limited organic options, mostly bagged

Local landscape suppliers:

• Pros: Best bulk prices, delivery available, fresh product
• Cons: Minimum order quantities, delivery charges

Online retailers:

• Pros: Home delivery, often good prices on bulk orders
• Cons: Can’t inspect quality beforehand, delivery delays possible

Quality Checks Before Buying

• Check for weed seeds: Particularly important with straw mulch — look for “weed-free” certification
• Smell test: Mulch should smell earthy and fresh, not sour or ammonia-like
• Check particle size: Fine mulch breaks down faster; coarse lasts longer
• Avoid dyed mulches near edible plants: Stick to natural for vegetable gardens
• Check moisture content: Very wet mulch weighs more (you’re paying for water)

Common Mulching Mistakes That Waste Money

• Overestimating depth needed: 5-8 cm is usually sufficient; going deeper wastes mulch
• Not accounting for settling: Always add 10% extra to your calculation
• Buying too far in advance: Some mulches (particularly compost materials) degrade in storage
• Not measuring properly: Guessing areas often leads to over-purchasing
• Mulching areas with active weed problems: Remove weeds like horsetail or ground elder first or you’ll trap them under mulch

Quick Reference: Bags Needed for Common Garden Sizes

For 70-litre bags at 7 cm depth:

• 10 m² bed = 10 bags
• 20 m² bed = 20 bags
• 50 m² garden = 50 bags
• 100 m² garden = 100 bags (consider bulk delivery at this size)

Final Tips

• Measure accurately: Use a tape measure, not estimates
• Buy slightly extra: It’s better to have 5-10% too much than run short mid-project
• Store leftovers properly: Keep bags sealed and dry for next season
• Consider delivery timing: Order bulk mulch when you can spread it within a week
• Don’t skimp on quality: Cheap mulch with weed seeds will cost more in labour later

With these calculations and guidelines, you can confidently order exactly the right amount of mulch for your garden project without waste or shortages.