How Much Mulch Do I Need?
Buy too little mulch and you are back at the garden centre; too much and it is sitting in a wet heap on the drive. Here is how to get the amount right - and how deep to spread it.
By the Calculate My Reno Team / Published
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Mulch is one of the cheapest, highest-impact jobs in the garden: it suppresses weeds, holds moisture, and tidies up beds in an afternoon. The only tricky part is buying the right amount.
The formula
Mulch is spread to an even depth, so the amount you need is just the bed area times the depth:
volume = length × width × depth
Keep the depth in the same units as the area when you do it by hand, or let the mulch calculator handle the conversion - it returns the volume in cubic yards or cubic metres and tells you how many bags to buy.
How deep should it go?
Depth is where most people go wrong - too thin and weeds push through, too thick and you smother the plants.
| Job | Depth |
|---|---|
| Bark / wood-chip on beds | 50–75 mm (2–3 in) |
| Around trees (clear of the trunk) | 75–100 mm (3–4 in) |
| Weed suppression on bare soil | 75 mm (3 in) |
| Topping up last year’s mulch | 25–50 mm (1–2 in) |
Worked example
A 4 m × 2.5 m border, mulched 50 mm deep, using 50-litre bags:
- Area: 4 × 2.5 = 10 m².
- Volume: 10 × 0.05 = 0.5 m³ = 500 litres.
- From 50-litre bags: 500 ÷ 50 = 10 bags.
For an irregular bed, split it into rectangles, add a bed for each in the mulch calculator, and it totals the volume for you.
Bags or bulk?
- Bags suit small beds - easy to carry, store and handle, and you buy exactly what you need.
- Bulk (delivered by the cubic yard or metre) is much cheaper per unit for big areas, but you need somewhere to tip it and a barrow to move it.
The rough switchover is around ten bags: beyond that, bulk usually wins on price. The calculator gives you both a bag count and a bulk volume so you can compare.
A few ordering tips
- Bulk mulch settles, so order a touch generous rather than risk a second delivery for a few litres.
- Mulch breaks down over a season or two - budget to top beds up about once a year.
- Filling raised beds or laying a gravel path instead? See how to fill a raised garden bed and our gravel driveway guide.
Frequently asked questions
How deep should mulch be?
Spread mulch 50–75 mm (2–3 inches) deep on beds and borders - enough to suppress weeds and hold moisture without smothering roots. Around trees, go 75–100 mm (3–4 inches) but keep it clear of the trunk. Deeper than that wastes material and can starve roots of air.
How many bags of mulch are in a cubic yard?
About 13–14 bags at 2 cubic feet each, or 9 bags at 3 cubic feet. In metric, a cubic metre is roughly twenty 50-litre bags. Once you need more than about ten bags, ordering in bulk by the cubic yard or metre is usually cheaper per unit.
When is the best time to mulch?
Mid- to late spring is ideal - the soil has warmed but is still moist, so mulch locks that moisture in for summer. Autumn is the second-best time, insulating roots over winter. Avoid mulching over cold, dry or frozen soil.