How the calculation works

The amount you need is the area you are covering multiplied by the depth, added up over all your areas.

Rectangle: area = length × width. Circle: area = π × (diameter ÷ 2)². If you already know the area, type it in.
US units: cubic feet = square feet × depth in inches ÷ 12. Divide by 27 to get cubic yards.
Metric: cubic metres = square metres × depth in centimetres ÷ 100. One cubic metre is 1,000 litres.
Bags: the volume is divided by the bag size and rounded up to a whole bag.
Bulk: cubic yards (or cubic metres) × price, plus delivery. If your supplier only sells in steps, such as half yards, the amount is rounded up to the next step first.
Break-even: delivery fee ÷ (what bags cost per cubic yard − bulk price per cubic yard). Above roughly that amount, bulk is cheaper.

Worked example

A 20 × 10 ft bed is 200 square feet. Spreading bark mulch 3 inches deep takes 200 × 3 ÷ 12 = 50 cubic feet, which is 1.85 cubic yards. That is 25 bags of 2 cu ft, or 17 bags of 3 cu ft.

Now say 2 cu ft bags cost $4 each, and a landscape yard charges $40 per cubic yard plus $50 delivery. Bags cost 25 × $4 = $100. Bulk costs 50 ÷ 27 × $40 = $74.07, plus $50 delivery, so $124.07. Bags win by $24.07. Here bagged mulch works out at $54 per cubic yard, $14 more than bulk, so bulk pays back the delivery fee at about 50 ÷ 14 = 3.6 cubic yards.

In metric, a 6 × 3 m bed is 18 m². At 7.5 cm deep that is 18 × 0.075 = 1.35 m³, or 1,350 litres: 27 bags of 50 L or 17 bags of 80 L.

How deep to spread each material

Picking a material in the calculator fills in a suggested depth, which you can change. These are the depths the extension services listed below recommend. Where they disagree, the range covers both.

Job	Suggested depth	What the sources say
Mulch: bark or wood chips	2–4 in (5–10 cm)	Penn State Extension: 2–4 in (5–10 cm) and no more; use less on poorly drained soil Penn State Extension: 1–2 in (2.5–5 cm) for fine or double-shredded mulch Iowa State Extension: 3–4 in (7.5–10 cm) on well-drained sites Iowa State Extension: 2–3 in (5–7.5 cm) on heavy soils
Compost: yearly top-up for garden beds	0.25–3 in (0.6–7.5 cm)	Oregon State Extension: 0.25–1 in (0.6–2.5 cm) each year WSU Extension: 2–3 in (5–7.5 cm) each year to raise organic matter
Compost: dig into a new garden bed	3–6 in (7.5–15 cm)	Oregon State Extension: 3–4 in (7.5–10 cm) mixed into the soil WSU Extension: 4–6 in (10–15 cm) in the first year on rocky or compacted soil
Compost: top-dress a lawn	0.25–0.5 in (0.6–1.3 cm)	Oregon State Extension: 0.25–0.5 in (0.6–1.3 cm) after core aeration Iowa State Extension: 0.25 in (0.6 cm) at most per application
Compost: mix in before seeding a lawn	1–2 in (2.5–5 cm)	Oregon State Extension: 1–2 in (2.5–5 cm) while preparing the soil Penn State Extension: 1–2 in (2.5–5 cm) worked into sandy or clay soils
Topsoil: base for a new lawn	4–6 in (10–15 cm)	Penn State Extension: 4–6 in (10–15 cm) settled depth, as a minimum WSU Extension: 6 in (15 cm) as a minimum, over a well-drained base

Mulch: more is not better. Penn State Extension warns that more than 4 inches may harm a tree's roots and says to keep all mulch away from the trunk. Iowa State Extension says to keep it at least 6 inches from tree trunks, because mulch piled against them invites cankers, root rot and rodents.
Compost on a lawn: Iowa State advises using only fine, screened compost and no more than ¼ inch at a time, so the grass blades stay visible. Oregon State suggests spreading it after mowing short and core aerating, in late spring or early fall.
Compost in beds: Oregon State suggests mixing it into the top 8 to 12 inches of soil. The amount is where sources differ most: Oregon State suggests ¼ to 1 inch a year for an existing bed, while Washington State suggests 2 to 3 inches a year to build up organic matter.
Topsoil for a new lawn: Penn State's 4 to 6 inches is the firmed, settled depth, and it suggests mixing about 2 inches of the new topsoil into the top 2 to 3 inches of the ground below so the layers blend. Washington State calls 6 inches over a well-drained base the minimum for a good lawn. Use the "Extra" box to allow for loose soil settling.

Bags or bulk?

Bags are easy to carry and need no delivery, and you can buy exactly as many as you need. Bulk material is sold by the cubic yard or cubic metre and may come with a delivery fee or a minimum order. A cubic yard is 13.5 bags of 2 cu ft or 9 bags of 3 cu ft, so once the job runs to a few cubic yards it is worth asking a local landscape supplier for a price. Enter both prices above to see which wins for your job, and where the break-even point is.

Common questions

How much area does a cubic yard of mulch cover?: One cubic yard is 27 cubic feet. Spread 3 inches deep it covers 108 square feet, at 2 inches 162 square feet and at 4 inches 81 square feet. It takes 13.5 bags of 2 cu ft (so buy 14) or 9 bags of 3 cu ft to make a cubic yard. In metric, a cubic metre spread 7.5 cm deep covers about 13.3 square metres and fills 20 bags of 50 L.
How much compost do I need to top-dress a lawn?: A 1,000 sq ft lawn top-dressed ¼ inch deep needs 20.8 cubic feet (0.77 cubic yards) of compost: 21 bags of 1 cu ft or 11 bags of 2 cu ft. A 100 m² lawn at 0.6 cm needs 600 litres, or 12 bags of 50 L. Oregon State Extension suggests ¼ to ½ inch after core aeration, and Iowa State Extension advises no more than ¼ inch at a time.
When is bulk cheaper than bags?: When bulk costs less per cubic yard than bags and the job is big enough to pay for the delivery fee. With the example prices above ($4 per 2 cu ft bag, $40 per cubic yard plus $50 delivery), bagged mulch costs $54 per cubic yard, so bulk saves $14 on each cubic yard and pays back the delivery at about 3.6 cubic yards. Rounding up to whole bags can tip it a little sooner: at 3.5 cubic yards, 48 bags cost $192 and bulk costs $190. Enter your own prices and the calculator works out your break-even point.
How do I measure a curved bed or a ring around a tree?: Split an odd-shaped bed into rectangles and circles and add each one as its own area, or work out the area another way and choose "I know the area". For a ring around a tree, choose Circle and enter the distance across the ring: one 6 ft across covers 28.3 sq ft. For several identical rings, set "How many".

Sources

Last checked and updated: October 31, 2026.