Home & Construction⏱ 4 min read
How to Calculate Mulch for Garden Beds
Too little mulch fails to suppress weeds or retain moisture; too much smothers roots. Here is the formula for any bed shape, the right depth by use case, and how many bags to buy.
Mulch is one of the highest-value gardening inputs — it suppresses weeds, retains moisture, moderates soil temperature, and feeds soil organisms as it breaks down. But the quantity calculation trips people up because bags are sold in litres, not m2.
The Volume Formula
Volume (m3) = Area (m2) x Depth (m)
Convert bag sizes to m3:
60-litre bag = 0.06 m3
70-litre bag = 0.07 m3
1,000-litre bulk bag = 1.0 m3
Example: 12m2 of border, 75mm depth
Volume = 12 x 0.075 = 0.9 m3
In 60-litre bags: 0.9 / 0.06 = 15 bags
Recommended Depths by Use
ApplicationRecommended DepthNotes
Annual weed suppression50-75mmMinimum for effective suppression
Moisture retention focus75-100mmMore effective in dry summers
Tree/shrub base75-100mmKeep clear of trunk — leave 10cm gap
Paths and walkways100-150mmDeeper layer withstands foot traffic better
Playground safety surface200-300mmDifferent calculation — use ROSPA guidance
Mulch Types and Settling Factor
All organic mulches settle (compress over time).
Add 15-20% to your volume calculation to account for settling.
Wood chip: settles ~15%, lasts 2-3 years
Bark (fine): settles ~20%, lasts 1-2 years
Composted bark (large chips): settles ~10%, lasts 3-4 years
Straw: settles ~25%, degrades quickly (annual reapplication)
Gravel/slate (inorganic): no settling, permanent
Adjusted calculation for bark mulch:
0.9 m3 needed x 1.20 (20% settling factor) = 1.08 m3
In 60-litre bags: 1.08 / 0.06 = 18 bags
Bulk Bag vs Individual Bags
Loose bulk bag (typically 800-900 litres = 0.8-0.9 m3):
Cost: £60-£90 delivered
Equivalent individual bags (60L): ~14-15 bags at £5-8 each = £70-£120
For areas requiring more than 10-12 bags, bulk bags are usually cheaper.
Minimum delivery requirements vary — check with supplier.
Bulk bags require a vehicle with a tail-lift for delivery
and space to store until used.