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
- 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
- Decide on depth
- Standard recommendation: 5-8cm (2-3 inches)
- For weed suppression: 8-10cm (3-4 inches)
- For paths: 5cm (2 inches)
- Calculate volume
- Add 10% extra for settling and waste
Worked Examples
Example 1: Rectangular Flower Bed (Metric)
- Bed dimensions: 4m × 2m
- Desired depth: 7cm
Calculation:
- Area = 4m × 2m = 8m²
- Volume = (8 × 7) ÷ 100 = 0.56 cubic metres
- 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:
- Radius = 10 ÷ 2 = 5 feet
- Area = 3.14 × 5² = 78.5 sq ft
- Volume = (78.5 × 3) ÷ 324 = 0.73 cubic yards
- Add 10%: 0.73 × 1.1 = 0.80 cubic yards
You need: 0.80 cubic yards of mulch
Recommended Mulch Depths for Different Applications
| Application | Depth (cm) | Depth (inches) | Notes |
|---|---|---|---|
| Flower beds | 5-8cm | 2-3″ | Standard coverage |
| Vegetable gardens | 5-10cm | 2-4″ | Keep away from stems when mulching vegetables |
| Around shrubs | 8-10cm | 3-4″ | Thicker layer for better weed suppression |
| Trees | 8-10cm | 3-4″ | Never pile against trunk |
| Paths | 5-8cm | 2-3″ | Compacts over time |
| Weed prevention | 10cm | 4″ | Maximum depth for blocking weeds like bindweed |
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 5cm depth: covers 1m²
- At 7cm depth: covers 0.7m²
- At 10cm depth: covers 0.5m²
- 70 litres (0.07 cubic metres)
- At 5cm depth: covers 1.4m²
- At 7cm depth: covers 1m²
- At 10cm depth: covers 0.7m²
- 100 litres (0.1 cubic metres)
- At 5cm depth: covers 2m²
- At 7cm depth: covers 1.4m²
- At 10cm depth: covers 1m²
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: 5-8cm 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 7cm depth:
- 10m² bed = 10 bags
- 20m² bed = 20 bags
- 50m² garden = 50 bags
- 100m² 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.
