Thoughtful Calc

Mulch Calculator

How much mulch to buy, in bags or cubic yards, at a depth that really keeps weeds down.

How do you want to measure?
Use this for several beds or odd shapes: measure each part as a rectangle and add them up.
Measure straight across the whole circle. For a ring around a tree, measure across it through the trunk.
Go by the name on the bag or your supplier's list. Chunky pieces are nuggets or chips; stringy mulch that mats together is shredded.
Depth depends on the mulch. Recommended: 3 in for nuggets and chips, 2.5 in for shredded, 2 in for fine. Weed-blocking: 4, 3, and 2 in. On soil that stays wet, 2 in at most (1 in for fine). Your answer shows the depth used.
The total depth you want once it's spread. We still add 10% for settling, and subtract any mulch already there.
Push a finger down to bare soil in a few spots and measure the old layer. Enter 0 for a new bed.
Pick the second if water sits in puddles long after rain or the soil is sticky clay.
The size in cu ft is printed on the bag. Pick Bulk for a truckload from a landscape yard, sold by the cubic yard.
Add your prices (optional; we use typical prices otherwise)

Why our numbers are more realistic

Many mulch calculators leave the depth to you or default to 2 inches, and almost none ask what's already in the bed. We checked them against university extension guidance and a field trial on weeds. Here's where we part ways:

Typical calculatorThis calculatorWhy it matters
One depth (often 2 in) for every mulch 3 in for coarse mulch like bark nuggets, with a 4 in weed-blocking option University research: wood chips 4–6 in deep control weeds, and "shallow mulch layers will enhance, rather than prevent weed growth." In the first year of a city park trial, beds with 2 in of chips had about 12% weed cover, and beds with 4 in had about 7.5%.
Ignores mulch already in the bed Subtracts what's there University guidance counts old mulch toward the total depth. Piling too much new mulch on top wastes money and can starve roots of air.
One depth for every soil and mulch Thinner for fine mulch and on soil that stays wet University guidance: fine mulch goes in 1–2 in layers, and no more than 2 in on compacted or poorly drained soil.
Exact cubic feet, no allowance Adds 10% for uneven spreading and settling, then rounds up to whole bags Mulch settles after it's spread: in university trials every mulch tested lost depth in the first year. Spreading is never perfectly even, either. Calculators that use the exact volume leave you a little short; most guides add 10–25%, and we use 10%.
Length and width only A round bed or tree ring option, by diameter A circle covers less than the square around it. Measuring a 6 ft tree ring as 6 × 6 ft gives 36 sq ft instead of about 28, which orders 27% too much.

How this works

Cubic feet = bed area × depth to add (in inches ÷ 12), plus 10%. Bags = cubic feet ÷ bag size, rounded up. Cubic yards = cubic feet ÷ 27.

  • Pine bark nuggets, mini nuggets, and wood chips are coarse (pieces ½ in or larger): 3 in recommended, 4 in for weed-blocking. University guidance recommends no more than 3 in total for most beds, and a bit deeper for coarse mulch, and research on coarse chips found 4–6 in controls weeds. Our 4 in is the cautious end.
  • Single-shredded hardwood: 2.5 in, or 3 in for weed-blocking.
  • Fine mulch, including double-shredded: 2 in. University guidance recommends 1–2 in layers, since fine particles pack down and can keep air from roots.
  • Clay, compacted, or wet soil: no more than 2 in total for any mulch (1 in for fine mulch), even with weed-blocking on.
  • The depth to add is the target minus what's already there.

Fabric, stone and rubber

This calculator is for wood and bark mulch, and that's what we'd use in a planting bed. Here's why.

Landscape fabric under mulch

  • It works for a season or two. Then leaves and mulch break down on top of it, and weeds root in that layer and grow down through the fabric. University guidance calls it "not a long term solution" for permanent beds.
  • Plant roots grow into it, so pulling it up later tears them. It also gets in the way any time you dig or replant there, and it clogs with soil and slows water getting down.
  • Wood mulch on bare soil can be topped up for years without harming plants. For a weedy bed, start with cardboard under the mulch, or pull the weeds first.
  • Fabric is reasonable where you won't plant or dig: under a gravel path or a rock bed.
  • Avoid anything that never breaks down, like plastic sheeting, in beds or under a lawn.

Wood mulch or stone

  • Wood mulch keeps soil cooler in summer, holds moisture and feeds the soil as it breaks down. University guidance says rock offers "no benefit for plant growth or soil health".
  • Rock holds heat and warms the ground around it, which can stress plants in hot weather.
  • Stone lasts for years and stays put on slopes. Weed seeds can still start in the gaps.
  • Stone makes sense on slopes that wash out, in desert-style plantings, and, in wildfire country, in the first 5 ft beside the house, so embers have nothing to catch on next to the wall.

Rubber mulch

  • We'd skip it in beds. It never feeds the soil, and university research found it controls weeds worse than wood chips.
  • Tire rubber can be up to 2% zinc by weight. Zinc can build up in soil to levels that harm plants, especially in acidic soil.
  • It catches fire more easily and is harder to put out than most mulches tested. In one test of eight mulches, composted wood chips burned least, and shredded rubber, pine needles and shredded cedar burned worst. Keep those three away from the house.
  • Rubber playground surfacing is a different use. The U.S. Consumer Product Safety Commission says it knows of no specific chemical hazard from tire rubber in playground surfacing at this time, and suggests limiting time on it on very hot days. That says nothing either way about rubber in garden beds.

Common questions

How much mulch is in a bag, and how many bags make a yard?

Usually 2 cu ft; 1.5 and 3 cu ft bags are common too. A cubic yard (27 cu ft) is 18 bags of 1.5 cu ft, 13½ of 2 cu ft, or 9 of 3 cu ft. At the 3 in we recommend, with our 10% for settling, a 2 cu ft bag covers about 7 sq ft. Many bag labels print 12 sq ft, but that's at 2 in with nothing extra, which is too thin to keep weeds down.

Can I spread mulch right over grass?

Not if you want the grass gone. Established grass is much tougher than seedling weeds, and a normal layer of mulch usually won't stop it; it grows back up through. Kill it or remove it first: dig it out, or smother it under cardboard, a tarp, or even your unopened mulch bags until it's dead, then spread the mulch.

Can mulch be too deep?

Yes. Fine mulch packs into a mat that keeps water and air from roots. Any mulch piled against stems or trunks holds moisture on the bark and invites rot and rodents. So the calculator keeps fine mulch at 2 in, caps everything at 2 in on soil that stays wet, and never adds mulch to a bed that's already deep enough.

Should I replace mulch every year?

No, just top it up. Enter what's left in "Mulch already there" and you'll get only what to add. If the old layer has matted into a crust, rake it loose first.

What this doesn't cover

  • Pine straw. It makes a good mulch, but it's sold by the bale, and bales vary in size. Sellers we checked say a bale covers about 40 sq ft at 3 in, less for short needles. It settles to about half its depth in the first year (university research), so expect to top it up.

Sources

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Numbers last reviewed: October 9, 2026.