How Much Mulch Do I Need? Mulch vs Gravel, Bags vs Bulk

The short answer

One cubic yard of mulch covers about 108 square feet at 3 inches deep. To work out how much mulch you need, multiply each bed’s length by its width, multiply by the depth in inches, divide by 12 to get cubic feet, and divide by 27 to get cubic yards. A common 2 cubic foot bag covers 8 square feet at 3 inches.

Depth is the number people get wrong. University extension services recommend 2 to 4 inches of mulch, not “as much as looks good”. This guide covers the right depth, how gravel compares, and the one sum that tells you whether to buy bags or order a truckload.

How deep should mulch be?

The University of Illinois Extension recommends a 2 to 4 inch layer on well-drained sites. It narrows that by material: composted mulches at 2 to 3 inches, coarse mulches like wood chips and bark at 3 to 4 inches. UC IPM (University of California) gives similar guidance: fine-textured mulch near 2 inches, coarse mulch up to about 4 inches, and nothing deeper than 4 inches, because thick layers keep the soil too wet and starve roots of oxygen.

Two rules from the same sources matter as much as the number:

• Keep mulch off trunks. Illinois Extension says to keep mulch at least 4 inches away from a tree trunk. Mulch piled against bark (the “volcano” look) is one of the most common causes of tree decline it lists.
• Measure what is already there. If last year’s mulch is still 2 inches deep, you only need to add an inch or two. Illinois Extension suggests raking and breaking up matted mulch to refresh it rather than piling on more.

Material	Depth to plan for	Source
Compost or fine mulch	2–3 in	Illinois Extension, UC IPM
Wood chips, shredded bark	3–4 in	Illinois Extension, UC IPM
Any organic mulch	Never more than 4 in	UC IPM

Mulch vs gravel: which one for this bed?

Both suppress weeds and both look tidy. They behave very differently over time.

Organic mulch (wood chips, bark, pine needles, compost) breaks down. Illinois Extension lists what that buys you: it holds moisture, insulates roots, improves soil structure and adds fertility as it decomposes. The cost is that it has to be replenished as it breaks down, so you will buy some every year or two.

Gravel and rock do not break down, so you buy them once. UC IPM points out the trade-offs:

• Weeds still come. Inorganic mulches are more stable but harder to keep weed-free, and hand-weeding or hoeing in rock is very difficult.
• Heat. Light rock reflects enough light and heat to harm sensitive plants, and dark rock holds heat and can stress them.
• Fabric underneath. UC IPM suggests landscape fabric under rock, so the stones do not sink and soil (with weed seed) does not work up between them.
• No soil benefit. Rock adds no organic matter.

A practical split many yards end up with: organic mulch in planting beds and around trees, where the soil benefit matters, and gravel along foundations, under downspouts, on paths and in spots that are never replanted.

Worked example 1: mulching two beds

A 40 × 3 ft bed runs along the front of the house, and there is an 8 × 12 ft island bed in the lawn. Both get 3 inches of shredded bark.

• Foundation bed: 40 × 3 × 3 ÷ 12 = 30 cubic feet
• Island bed: 8 × 12 × 3 ÷ 12 = 24 cubic feet
• Total: 54 cubic feet ÷ 27 = 2.0 cubic yards
• In 2 cubic foot bags: 54 ÷ 2 = 27 bags

Bags or bulk?

The trick is to turn the bag price into a price per cubic yard, then compare it with the bulk price.

Using example prices of $4.50 for a 2 cubic foot bag, $38 a yard for bulk and a $75 delivery fee (example prices only; check your local yard):

• Bag price per yard: $4.50 × 27 ÷ 2 = $60.75
• Each yard bought in bulk saves $60.75 − $38 = $22.75
• Break-even: $75 delivery ÷ $22.75 = 3.3 cubic yards

Below about 3.3 yards, bags win. Above it, bulk wins. For this job:

• Bags: 27 × $4.50 = $121.50
• Bulk: 2 × $38 + $75 = $151.00

Bags are cheaper here, by about $30.

Now scale it up. Mulching 600 square feet at 3 inches is 150 cubic feet, or 5.56 cubic yards. That is 75 bags ($337.50). If the yard loads in half-yard steps, bulk means paying for 6 yards: 6 × $38 + $75 = $303.00. Bulk wins once the job passes the break-even.

The general formula, which the mulch, gravel and topsoil calculator runs for your own prices:

Break-even (cubic yards) = delivery fee ÷ (bag price × 27 ÷ bag size in cubic feet − bulk price per yard)

Worked example 2: gravel instead of mulch along the house

Suppose the 40 × 3 ft foundation bed gets pea gravel instead, 2 inches deep. (Two inches is the calculator’s starting depth for gravel, a planning default rather than a standard. Use the depth your project needs.)

• 40 × 3 × 2 ÷ 12 = 20 cubic feet, or 0.74 cubic yards
• Pea gravel is often sold in 0.5 cubic foot bags (the size of Vigoro bagged pea gravel at Home Depot): 20 ÷ 0.5 = 40 bags
• By weight, at a typical 1.4 to 1.7 tons per cubic yard for dry gravel: 1.04 to 1.26 tons

Small gravel bags are expensive per yard. At an example $5.50 for a 0.5 cubic foot bag, the bag price works out to $5.50 × 27 ÷ 0.5 = $297 a cubic yard. Against an example bulk price of $45 a yard and the same $75 delivery fee, the break-even is $75 ÷ ($297 − $45) = 0.3 cubic yards. Almost any gravel job beats that:

• Bags: 40 × $5.50 = $220
• Bulk, rounded up to 1 yard in half-yard steps: $45 + $75 = $120

That is the biggest practical difference between the two materials. Mulch bags are reasonably priced for small jobs; gravel bags rarely are, unless the job is tiny or the yard will not deliver small loads. Ask about a minimum order before you count on bulk.

Gravel is also sold by the ton at most yards, because trucks are loaded by weight. The 1.4 to 1.7 range is typical for dry gravel and crushed stone, not a fixed figure. Ask the supplier what their material weighs per yard and use that.

Coverage at a glance

One cubic yard is 27 cubic feet. Here is how far one yard, and one 2 cubic foot bag, goes:

Depth	One cubic yard covers	One 2 cu ft bag covers
1 in	324 sq ft	24 sq ft
2 in	162 sq ft	12 sq ft
3 in	108 sq ft	8 sq ft
4 in	81 sq ft	6 sq ft

Common mistakes

• Using feet for depth. Depth is usually given in inches. 3 inches is 0.25 ft, not 3 ft. Divide inches by 12 before multiplying.
• Going too deep. More than 4 inches of organic mulch can keep soil waterlogged and short of oxygen. Thicker is not better.
• Mulching over old mulch without measuring. Check the depth with a trowel first and add only the difference.
• Piling mulch against trunks and stems. Keep it at least 4 inches back from trees.
• Comparing bag and bulk prices directly. A $4 bag and a $35 yard are not comparable until you convert the bag to a price per yard.
• Forgetting bulk is rounded up. Many yards load in half-yard or full-yard steps, and some have a minimum order.
• Putting gravel where you will replant. Rock with fabric under it is hard work to remove later.

When to call a pro or the supplier

• Large or hard-to-reach areas. A dump truck needs room to tip. If the pile has to land on the street, check local rules about materials on the roadway, and plan how you will move several yards by wheelbarrow.
• Drainage problems. If water pools against the foundation, gravel on top will not fix it. Grading and downspout extensions are the real fix, and a landscaper can assess the slope.
• Driveways and load-bearing paths. These need a compacted base, which is a separate material and depth. Work it out as its own layer, and ask the supplier what base they recommend for your soil.
• Sick or stressed trees. If a tree is declining and the mulch has been piled against it for years, a certified arborist can check the root flare.

To get your own numbers for several beds at once, in bags, cubic yards and gravel tons, with the bags-versus-bulk comparison worked out for your prices, use the mulch, gravel and topsoil calculator.