By Tim King · Updated 7 October 2026

Mulch Calculator

Cubic Yards, Cubic Metres and Bags

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Units

Material

Length (metres)

Width (metres)

Depth (e.g. 75mm)

Waste allowance (%, 10 if left blank)

Result

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How the Mulch & Topsoil Calculator works

This mulch calculator gives the cubic yards or cubic metres of mulch or topsoil a bed needs, how many bags that is and roughly what it weighs. A cubic yard is 27 cubic feet, so it covers 324 square feet at 1 inch deep or 108 square feet at 3 inches. Type the length and width of the bed and the depth, choose the material, and the volume appears as you type, with a 10% waste allowance unless you change it.

Mulch calculator: why the depth matters most

Mulch is one of the cheapest ways to improve a garden. A 50–75 mm layer of bark or compost holds moisture in the soil and keeps the root zone cooler in summer. It also smothers annual weeds. The Royal Horticultural Society recommends a layer at least 5 cm thick, enough to block light from weed seeds. Much over 100 mm and it can starve roots of air and shed light rain. Depth decides whether the job works, and it is the figure most often got wrong.

The formula

Volume = length × width × depth × (1 + waste ÷ 100).

Depth accepts a unit with the number, so 75mm, 7.5cm and 0.075 all mean the same in metric mode. In imperial mode, 3in and 0'3" both mean a quarter of a foot, and lengths like 12'6" work. Imperial volumes convert at 0.0283168 m³ per cubic foot. The result is shown in cubic yards, at 0.764555 m³ each. Bag counts divide the volume in litres by the usual retail bag for the material and round up. Weight uses a loose bulk density: bark 0.3 kg per litre, straw 0.1, compost 0.6, topsoil 1.2 and potting mix 0.5.

Worked examples

The first example: a border 6 m long and 1.5 m wide, mulched with bark at 75 mm. 6 × 1.5 × 0.075 = 0.675 m³, and adding 10% waste gives 0.74 m³. That is about 15 bags of 50 L, weighing roughly 0.22 tonnes.

The second example is imperial: a bed 20 ft by 4 ft at 3 inches. That is 20 cubic feet, or 0.566 m³, and 0.62 m³ with waste. The page shows 0.81 yd³ and about 13 bags of 50 L. US bags are usually 2 cubic feet, about 57 L, so allow 11 of those.

How many bags of mulch or soil for your bed

The table assumes the default 10% waste. It shows bark mulch at a 75 mm finished depth and topsoil at 150 mm, which is a typical depth for turfing or a new border.

Mulch and topsoil needed by bed area, 10% waste

Bed area	Mulch at 75 mm	50 L bark bags	Topsoil at 150 mm	25 L soil bags	Soil weight
1 m²	0.08 m³	2	0.17 m³	7	0.20 t
2 m²	0.17 m³	4	0.33 m³	14	0.40 t
3 m²	0.25 m³	5	0.49 m³	20	0.59 t
4 m²	0.33 m³	7	0.66 m³	27	0.79 t
5 m²	0.41 m³	9	0.83 m³	34	0.99 t
6 m²	0.49 m³	10	0.99 m³	40	1.19 t
8 m²	0.66 m³	14	1.32 m³	53	1.58 t
10 m²	0.83 m³	17	1.65 m³	67	1.98 t
12 m²	0.99 m³	20	1.98 m³	80	2.38 t
15 m²	1.24 m³	25	2.48 m³	99	2.97 t
18 m²	1.48 m³	30	2.97 m³	119	3.56 t
20 m²	1.65 m³	34	3.30 m³	133	3.96 t
25 m²	2.06 m³	42	4.12 m³	165	4.95 t
30 m²	2.48 m³	50	4.95 m³	198	5.94 t
40 m²	3.30 m³	67	6.60 m³	265	7.92 t
50 m²	4.12 m³	83	8.25 m³	330	9.90 t
60 m²	4.95 m³	99	9.90 m³	396	11.88 t
75 m²	6.19 m³	124	12.38 m³	496	14.85 t
100 m²	8.25 m³	165	16.50 m³	660	19.80 t
150 m²	12.38 m³	248	24.75 m³	991	29.70 t

How to read the result

Volume needed is the amount to order, in m³ or yd³, with the waste already added.
Including waste repeats the allowance used.
Bags gives the number of retail bags of the stated size, rounded up.
Approximate weight is in tonnes. Damp soil weighs up to a third more.

Choosing the waste allowance

The default 10% suits a rectangular bed that has been raked level. Use 15–20% for curved or uneven beds. Use about 25% for fresh bark or sugar cane mulch you want at a settled depth, because loose organic mulch compacts noticeably in the first month. For 75 mm settled, spread close to 100 mm.

Tips and pitfalls

Keep mulch off stems and trunks. Leave a 5–10 cm gap; piled mulch invites rot.
Weigh the load before you tow it. A cubic metre of topsoil is about 1.2 tonnes, often more than a small car trailer's payload. The same volume of bark is 300 kg.
Bulk beats bags above about a third of a cubic metre. A cubic metre is 20 bags of bark or 40 bags of soil. Yards sell in half-cubic-metre steps, so round up.
Match the depth to the job. Straw and sugar cane go on at about 100 mm. A new lawn wants 100–150 mm of topsoil, a compost top-up 25–50 mm.

For a raised bed with sides, the raised bed calculator works out fill by frame size. For paths and drives, use the gravel calculator, and for making your own mulch, the compost calculator.

Sources: NIST unit conversion.

Checked against: 100 sq ft at 3 in deep = 25 cu ft = 0.93 cu yd

FAQ

How much mulch do I need for a 10 square metre garden bed?

At 75mm deep, 10 m² needs 0.75 m³ before waste, or about 0.83 m³ with the default 10% allowance. That is roughly seventeen 50 L bags, or a small trailer load from a landscape yard. Each extra 25mm of depth adds another quarter of a cubic metre.

How many bags of mulch are in a cubic metre?

Twenty 50 L bags make a cubic metre, or about thirty-four 30 L bags. Topsoil usually comes in 25 L bags, so a cubic metre is forty of them, and at over a tonne it is a lot of lifting — a bulk delivery is a better idea beyond about a third of a cubic metre.

What waste allowance does this calculator use?

Ten per cent by default, added to the calculated volume before it is converted into bags and weight. Increase it to 15 or 20% for beds with curved or irregular edges, and to about 25% for fresh bark or sugar cane mulch, which settles noticeably in its first few weeks.

How deep should I spread mulch?

Between 50 and 75mm for bark and wood chip. Less lets weeds through; more than about 100mm can starve roots of water and air. Straw and sugar cane mulch are spread thicker, around 100mm, because they compress quickly. Keep any mulch clear of plant stems and tree trunks.

How much does a cubic metre of topsoil weigh?

Roughly 1.2 tonnes when loose and slightly damp, which is why topsoil comes in smaller 25 L bags than mulch. Compost is about 0.6 tonnes per cubic metre and bark mulch only around 0.3 tonnes. Check the payload limit of a trailer before collecting soil yourself.

Can I use this calculator in feet and yards?

Yes. Switch the units to imperial and enter length and width in feet, including feet and inches such as 12'6", and the depth in inches such as 3in. The volume is then shown in cubic yards, with the bag count and weight calculated the same way.