How much mulch, compost or topsoil do you need?
Area times depth, converted to litres, divided by the bag.
10 bags
0.6 cubic metres, which is 600 litres, over 12 square metres.
- 6 m by 2 m is 12 square metres.
- 12 square metres at 5 cm deep is 0.6 cubic metres, which is 600 litres.
- 600 litres divided by a 60 litre bag is 10 bags, rounded up.
Working it out on paper
The sum is short enough to do by hand, and it is the same sum the tool does.
- Multiply the length by the width to get the area in square metres.
- Divide the depth in centimetres by 100 to get it in metres.
- Multiply the two together for the volume in cubic metres.
- Multiply by 1000 for litres, then divide by the bag size and round up.
| Area | 5 cm | 7.5 cm | 10 cm | 30 cm |
|---|---|---|---|---|
| 2 square metres | 100 | 150 | 200 | 600 |
| 5 square metres | 250 | 375 | 500 | 1500 |
| 10 square metres | 500 | 750 | 1000 | 3000 |
| 20 square metres | 1000 | 1500 | 2000 | 6000 |
| 50 square metres | 2500 | 3750 | 5000 | 15000 |
What the numbers mean
Bulk material is a volume problem, not an area problem, and that is why so many people order a third of what they need. A border that looks enormous when you are standing on it is usually twelve or fifteen square metres, and a proper mulch over it takes the best part of a cubic metre. A cubic metre is a thousand litres. Sixty litre bags are the common size in a garden centre, so that same border is fifteen or sixteen bags, which does not fit in a car.
Depth is where the decision actually is. Five centimetres is the least that will do the job a mulch is for, which is keeping light off weed seeds and keeping moisture in the ground. Seven and a half centimetres is the figure to aim at if you want it to last the year without topping up. Ten is a bark path or a very deep mulch over bare ground. If you are filling a raised bed rather than dressing a border, you are into thirty centimetres and more, and the volume climbs fast: a standard two point four by one point two metre bed at thirty centimetres deep is nearly nine hundred litres, or eighteen bags.
Bags, bulk bags and loose
Bagged compost is sold by the litre and priced per bag. Bulk bags, sometimes called tonne bags or jumbo bags, hold roughly seven hundred to eight hundred litres, though the figure is rarely printed and is worth asking about, because the difference between seven hundred and a thousand is three bags of compost. Loose deliveries are sold by the cubic metre and are the cheapest way to buy anything above about half a cubic metre, provided you have somewhere to put it and a wheelbarrow.
Add roughly ten per cent to anything you are using to fill rather than to dress. Loose material settles, and compost settles a good deal more than soil does, so a bed filled level in October is a bed with a dip in it by March. The calculator gives you the exact figure you asked for and no more, because a tool that quietly adds a margin is a tool you cannot check.
What not to do
Do not pile mulch against the stem of a tree or a shrub. Leave a hand's width of clear ground around the base, because bark held against bark stays wet and rots. Do not mulch dry ground either: the whole point is to keep moisture in, and a mulch laid over dry soil keeps the rain out just as effectively. Water first, then mulch, and do it in autumn or spring when the ground is already damp.
Questions from the margin
How deep should a mulch be?
Five centimetres is the thinnest that suppresses weeds, and seven and a half will last a full season. Ten is a bark path. Anything less than five looks tidy for a fortnight and then the weeds come through it.
How many litres are in a cubic metre?
A thousand, exactly. That is the conversion the whole calculation turns on, and it is why bulk compost is sold by the cubic metre and bagged compost by the litre.
Is a bulk bag cheaper than bags?
Usually, from about half a cubic metre upwards. A bulk bag holds roughly 0.7 to 0.8 cubic metres, which is 700 to 800 litres, or somewhere between twelve and sixteen sixty litre bags.
Should I order extra?
Add about ten per cent if the ground is uneven or you are filling a bed rather than topping one, because loose material settles. The calculator does not add it for you, so the figure you see is the figure you asked for.