Mulch Calculator — Bags or Cubic Metres for Garden Beds
A layer of mulch retains moisture, suppresses weeds, and improves soil health — but ordering too little means a second trip, and too much is wasted money. Enter your bed dimensions and target depth to get the exact volume and bag count.
How we calculate
Volume = length × width × depth (m) × (1 + waste %). Bags = ⌈volume ÷ bag volume⌉.
Worked example
4 m × 2 m bed at 75 mm deep: 4 × 2 × 0.075 = 0.6 m³. With 10% waste: 0.66 m³. At 60 L/bag (0.06 m³): ⌈0.66 ÷ 0.06⌉ = 11 bags.
Common mistakes
- Applying mulch too thick — more than 100 mm can suffocate plant roots and cause rot.
- Piling mulch against plant stems — leave a gap to prevent crown rot.
- Forgetting irregular-shaped beds — measure length × width and subtract any large unpanted patches.
- Buying fine bark for paths — coarse or chunky mulch resists compaction better underfoot.
Frequently asked questions
How many bags of mulch do I need for a garden bed?+
A 4 m × 2 m bed at 75 mm depth needs about 0.66 m³ of mulch — roughly 11 bags at 60 L each, or 17 bags at 40 L each. Always check the bag volume label and use this calculator to adjust for your exact dimensions.
How deep should I apply mulch?+
Apply 50–75 mm for weed suppression and moisture retention. Do not exceed 100 mm — too thick a layer can prevent rain penetrating to the soil and may cause root rot. For light top-dressing, 25–40 mm is sufficient.
How often should I top up mulch?+
Organic mulch (bark, wood chips, straw) breaks down over time. Plan to top up annually in spring, adding 25–50 mm to restore the layer. Inorganic mulch (gravel, pebbles) rarely needs topping up.
Should I remove old mulch before adding new mulch?+
Not usually. A thin layer of decomposed mulch improves soil structure and feeds worms. Only remove old mulch if it has become compacted into a water-repelling mat or shows signs of disease. Otherwise, top up over the existing layer.