Mulch Calculator

How to estimate mulch for a bed

Mulch is sold by volume and spread by depth, so the whole calculation is area times depth. The only trap is units: your bed is in feet, your depth is in inches, and the mulch is in cubic feet or cubic yards.

cubic feet = bed area (sq ft) × depth (in) ÷ 12
cubic yards = cubic feet ÷ 27Dividing by 12 turns inches into feet. Dividing by 27 turns cubic feet into cubic yards, because a cubic yard is 3 × 3 × 3 feet.

Measuring the bed

Rectangles are easy: length times width. For a curved bed, break it into rectangles and add them up, or measure the average width and multiply by the length. Being off by a foot on a border bed changes the answer by less than a bag.

For a circle around a tree, use radius times radius times 3.14. A 6 foot circle has a 3 foot radius, so 3 × 3 × 3.14 is about 28 square feet.

How deep, and why not deeper

Two to three inches. That range is not a guess. UMass Extension recommends 2 to 3 inches and adds "never more". The University of Florida's Florida-Friendly Landscaping program gives the same 2 to 3 inches and says to top up to maintain it. Michigan State horticulture puts 3 inches as the starting point for the main benefits.

Use 2 inches on established beds where there is already a layer, and 3 on new plantings where you are starting from bare soil. Four inches is defensible if weed suppression is the whole point and nothing is planted close, but it is the ceiling, not the target.

Deeper is actively worse. Piled mulch holds moisture against stems and trunks, and a thick mat can shed water instead of letting it through. The volcano of mulch around a tree trunk is the classic version of this mistake.

Bags or bulk

This is the real buying decision, and it turns on volume rather than price per unit.

A cubic yard is 27 cubic feet. At the common 2 cubic foot bag, that is 13.5 bags, so 14. Bag sizes are not standard: 1.5, 2, and 3 cubic feet all sit on the same shelf, and the number of bags changes by nearly half between the smallest and largest. Always read the cubic feet on the bag rather than counting bags from someone else's estimate.

Under about two cubic yards, bags win on convenience. You can fit them in a car, store the extras, and open only what you need. Past two cubic yards, you are opening 27 or more bags and dealing with the plastic, and bulk delivery is usually both cheaper per yard and far less work. That crossover is a handling judgment, not a spec, so treat it as a nudge and price both.

What one cubic yard actually covers

Useful to carry in your head, because it turns a delivery quote into a picture of your yard. One cubic yard spreads to roughly 160 square feet at 2 inches, 108 square feet at 3 inches, and 81 square feet at 4 inches. That is the same division as above, run backwards: 27 cubic feet divided by the depth in feet.

So a single yard covers a bed about 10 by 10 feet at a comfortable 3 inches. If your beds add up to more ground than that, you are into multiple yards, and it is worth measuring properly rather than guessing at the counter.

A quick cost check

Take a 20 by 10 foot bed at 3 inches. That is 200 square feet, 50 cubic feet, about 1.9 cubic yards, or 25 bags at 2 cubic feet. At $4 a bag that is $100. At $45 a delivered cubic yard it is about $85 plus whatever the delivery fee is. Close enough that convenience decides it, which is exactly the point where the crossover sits.

Common estimating mistakes

• Forgetting to divide by 12. Multiplying 200 square feet by 3 gives 600, not 50. Depth has to be in feet before it multiplies cleanly into cubic feet.
• Assuming every bag is 2 cubic feet. It is the most common size, not the only one, and a 1.5 cu ft bag needs a third more bags for the same bed.
• Adding a full layer over old mulch. Measure what is already there and top up to your target depth instead.
• Ordering exactly. Beds are rarely perfect rectangles, and settling costs you a little depth. Rounding up to the next bag or quarter yard is normal.
• Using this for rock or rubber. The volume math holds, but those are often sold by weight, so the bag count will not transfer.

When to hand it off

Mulch is not dangerous work, so the honest boundary here is about volume rather than safety. Past three or four cubic yards you are moving several tons of material by wheelbarrow, and a delivery plus a couple of hours of help is usually worth it.

One real caution: bagged mulch left sealed and wet can go anaerobic and turn sour, which will damage plants. If a bag smells like vinegar or ammonia when you open it, spread it out to air before it goes anywhere near a plant.