How much mulch or gravel do I need?
By Dexaloop · Updated 27 October 2026 · Checked against the sources listed below
To work out how much mulch or gravel you need, multiply the area (in square feet) by the depth (in feet) to get cubic feet, then divide by 27 to get cubic yards. A 20 x 6 foot bed mulched 3 inches deep needs 120 x 0.25 = 30 cubic feet, which is 1.11 cubic yards or 15 bags of 2 cubic feet. The two decisions that change the answer most are the depth you choose and, for gravel, whether the supplier sells by volume or by weight. This guide covers both.
Step 1: choose the depth
Depth matters more than people expect. Going from 2 to 3 inches adds 50% to the order, and too much can harm plants. Clemson Cooperative Extension warns that "excessive amounts of these fine-textured mulches can suffocate plant roots." Its recommendations, and the RHS's, are below.
| Material | Recommended depth | Source |
|---|---|---|
| Wood chips, pine bark, pine needles | Up to 2 to 3 inches | Clemson |
| Coarse pine bark nuggets | Up to 4 inches | Clemson |
| Grass clippings, shredded leaves | Never deeper than 2 inches | Clemson |
| Small rocks or gravel, for weed control | 1 inch | Clemson |
| Mulch in general | At least 5 cm (2 in) | RHS |
| Biodegradable mulches | At least 5 cm, ideally 7.5 cm (3 in) | RHS |
Coarse materials can go deeper because air still moves through them. Fine materials pack down and are kept shallow.
Step 2: measure the area
Measure each bed in feet and work out its area: length x width for rectangles, and pi x radius x radius for circles such as tree rings. For curved beds, split them into rough rectangles and add them up, or measure the longest and widest points and accept a small overestimate.
Clemson says newly planted trees need "a circle of mulch 3 to 4 feet in diameter," with the mulch kept off the trunk itself.
Step 3: area x depth, then divide by 27
Turn the depth into feet first by dividing inches by 12. Then multiply by the area to get cubic feet. NIST lists 27 cubic feet in one cubic yard, so divide by 27 for cubic yards.
Clemson offers a handy shortcut that comes from the same numbers: "One cubic yard will cover a 324-square-foot area with an inch of mulch." (27 cubic feet x 12 inches = 324.) So cubic yards = area x depth in inches / 324.
Worked example: mulch
- Front bed: 20 x 6 ft = 120 sq ft at 3 inches. 120 x 3 / 324 = 1.11 cubic yards, or 30 cubic feet.
- Two new trees: each ring 4 ft across, so radius 2 ft. Area = 3.1416 x 2 x 2 = 12.57 sq ft each, 25.13 sq ft for both. At 3 inches: 25.13 x 0.25 = 6.28 cubic feet.
- Total: 30 + 6.28 = 36.28 cubic feet = 1.34 cubic yards.
In bags of 2 cubic feet that is 36.28 / 2 = 18.14, so 19 bags. If a bulk supplier sells in half yards, you would compare 1.5 cubic yards delivered against 19 bags.
In metric the sum is simpler: 10 m² at the RHS's 7.5 cm is 10 x 0.075 = 0.75 m³, which is 750 liters, or 15 bags of 50 liters.
Gravel: volume is the same, weight is not
The volume sum for gravel is identical. The extra step is that many quarries and yards sell stone by the ton, and the weight of a cubic yard of stone is not one fixed number. It depends on the rock, how wet it is, its grading, and above all whether it is loose in a truck or compacted on the ground.
Wisconsin's Department of Transportation publishes conversion factors for compacted aggregates. A selection from its research report:
| Material (compacted) | Tons per cubic yard |
|---|---|
| Dense graded base, 3/4 inch | 1.75 to 2.1 |
| Breaker run | 1.60 to 1.9 |
| Select crushed material | 1.70 to 1.9 |
| Granular backfill | 1.50 to 1.7 |
Loose stone has more air in it than compacted stone, so a cubic yard in the truck weighs less and settles into less than a cubic yard on the ground. Penn State's road aggregate bulletin gives a field example of the same effect: its surface aggregate can be placed 8 inches deep and compacted to 6 inches.
A short ton is 2,000 pounds (NIST). If a supplier quotes in metric tonnes instead, ask, because the two are not the same.
Worked example: gravel driveway topping
A 40 x 10 ft parking pad needs 4 inches of compacted 3/4 inch base.
- Volume compacted: 400 sq ft x (4 / 12) ft = 133.3 cubic feet = 4.94 cubic yards.
- Buying by weight: 4.94 x 1.75 = 8.6 tons, up to 4.94 x 2.1 = 10.4 tons using the Wisconsin factors.
- Buying by loose volume: you need more than 4.94 cubic yards, because loose stone shrinks as it is compacted. As a rough illustration only, Penn State's 8 inches compacting to 6 is a ratio of 8 / 6, which would mean 4.94 x 8 / 6 = 6.6 loose cubic yards. Your supplier's figure for its own stone comes first.
The spread is wide because stone varies. Give the supplier your compacted volume and ask what their product weighs, rather than relying on a single generic figure.
For small jobs in bags, the RHS gives a rule of thumb: "A 25kg (55lb) bag of gravel will cover 0.6sq m (0.7sq yd) to a depth of 5cm (2in)."
Common mistakes
- Forgetting to convert inches to feet. 3 inches is 0.25 feet, not 3.
- Dividing by 9 instead of 27. 9 converts square feet to square yards, not cubic.
- Mulching too deep or piling it against trunks.
- Using a compacted figure for a loose order, or the reverse.
- Assuming every gravel weighs the same. Ask for the product's own weight per yard.
- Rounding bags down. 18.14 bags means buying 19.
Measure each bed, pick a depth from the table, and let the mulch calculator below do the cubic yards and bag counts. For stone by the ton, use the gravel calculator listed with it.
Work out your mulch: tools.dexaloop.com/mulch-calculator/. Free, no sign-up, and it runs on your device.
Questions
How many square feet does a cubic yard of mulch cover?
Clemson Extension says one cubic yard covers 324 square feet at 1 inch deep. Divide 324 by your depth in inches: about 162 square feet at 2 inches, 108 at 3 inches and 81 at 4 inches.
How deep should mulch be?
Clemson says most wood, bark and pine needle mulches should be no more than 2 to 3 inches deep, up to 4 inches for coarse bark nuggets, and no more than 2 inches for grass clippings or shredded leaves. The RHS says at least 5 cm, ideally 7.5 cm, for biodegradable mulches.
How many tons is a cubic yard of gravel?
It depends on the stone, how wet it is and whether it is loose or compacted. Wisconsin DOT's factors for compacted aggregates run from about 1.5 to 2.2 tons per cubic yard. Ask your supplier for the figure for the exact product.
How many bags of mulch make a cubic yard?
A cubic yard is 27 cubic feet, so it takes 13.5 bags of 2 cubic feet or 9 bags of 3 cubic feet. Check the volume printed on the bag.
Should mulch touch the tree trunk?
No. Clemson says not to pile mulch against the trunk and to keep it 2 to 3 inches away from the stems of woody plants.
Sources
- Clemson Cooperative Extension, Home and Garden Information Center: Mulch
- Royal Horticultural Society: Mulches
- Royal Horticultural Society: Gravel gardens
- Wisconsin DOT research report 0092-22-05: Weight-volume relationships and conversion factors for soils and aggregates of Wisconsin
- Penn State Center for Dirt and Gravel Road Studies: Driving Surface Aggregate technical bulletin
- NIST Handbook 44, Appendix C: General tables of units of measurement
This guide is general information, not legal, tax or financial advice. Rules change; check the official source for your own case. Found a mistake? Email support@dexaloop.com.