Mulch Calculator

Enter your dimensions above and the calculator returns the answer in the units suppliers actually sell in — cubic yards, cubic feet, tons and bags.

Typical mulch depth by application

Mulch is sold by the cubic yard in bulk with delivery, or in 2 cubic foot bags from a garden centre; bulk is dramatically cheaper above about 2 yards.

What the numbers assume

The result uses a bulk density of 25 lb per cubic foot and a default waste allowance of 5%. Both are typical values, not measurements of your material — bagged products and different aggregate sources vary by 10% or more. Change the waste field if your subgrade is uneven or you are pouring against forms with complex edges.

More than 3 inches of mulch smothers roots and holds too much moisture against trunks — pushing it into a cone around a tree is the single most common way people kill one.

Costs shown are material only, based on a national average price you can edit. They exclude delivery, short-form fees, equipment rental and labour.

What mulch is doing under the surface

Mulch gets sold on how it looks, but the reason a mulched bed outperforms a bare one has nothing to do with colour.

It breaks the evaporation chain. Bare soil wicks moisture up and hands it to the air; a two-to-three-inch layer interrupts that, so water stays in the root zone instead of leaving by mid-afternoon. That is the difference between watering twice a week and watering daily in July.

It buffers temperature. Soil under mulch stays cooler in summer and holds warmth longer into autumn, which keeps root growth going for weeks after top growth stops — and at the first hard frosts, when bare beds heave and push shallow-rooted perennials out of the ground.

It feeds the soil. As the bottom of the layer decomposes, earthworms and fungal networks pull organic material down into the profile, which is how a bed's structure and water-holding capacity improve year after year. The mulch you can still see in spring is the part that has not been eaten yet.

Weed control follows from the same layer: most small-seeded weeds need light to germinate, and two inches of anything blocks it. It suppresses weeds rather than sterilising a bed.

Depth is a range, and your soil picks the number

Two to three inches is the answer for most beds, and that it is a range rather than a single figure is not a cop-out. The right number depends on three things you can check in five minutes.

Soil type. Heavy clay holds water and drains slowly, so two inches is enough; three inches keeps the crowns of shallow-rooted plants wetter than they want. Sandy soil drains fast and can take the top of the range.

Mulch texture. Shredded hardwood mats down and blocks light at two inches. Coarse bark nuggets have gaps between the pieces and need the full three inches. A fine mulch at four inches forms a crust that sheds water instead of absorbing it, which is worse than no mulch.

Where the bed is starting from. A new bed over bare soil wants two to three inches. A bed mulched for five years already has decomposed organic matter under the surface, so measure down to actual soil with a trowel before adding anything. If you are reading four inches of dark crumbly material above the native soil, the bed needs raking, not another yard.

One distinction: mulch covers a bed that already exists. If you are building the growing medium itself, the topsoil calculator is the right page. If the bed has to be raised first, the fill dirt calculator comes first.

The trunk cone is the mistake that kills trees

Mulch piled against a trunk — the volcano look — kills trees three separate ways, and none of the damage shows in the first year.

Bark is not built to stay wet. Trunks shed water; a mound holding moisture against the bark keeps it damp through the growing season and invites fungal cankers exactly where the tree's living tissue sits under the surface. The damage is hidden until the canopy thins a few summers later.

Rodents use the mound as a winter nest. Voles and mice move into deep mulch and feed on the bark under the snow line, chewing all the way around the trunk. Anything that girdles a tree — removes a ring of bark all the way around — kills it, because the tissue carrying sugars down to the roots has been cut. It dies over two or three years.

Roots grow up into the mound. Mulch is moist, warm and airy, so fine feeder roots colonise it — then the mound is pulled back or dries out, and those roots die. The tree spent energy on a layer that was never permanent.

The fix is free. Keep mulch two to four inches clear of the trunk so the root flare — where the trunk widens into the roots — stays visible, and spread the rest flat, no deeper than three inches, out to the drip line. Do the same for shrubs. Pull an existing mound back rather than digging it out; if the bark underneath is soft or loose, get an arborist.

Bulk versus bagged, with the arithmetic shown

A bag of mulch is two cubic feet, and 13.5 bags make a cubic yard. At the $3.95 default that is $53.33 per yard in bags — round up to the 14 bags you actually carry out of the store and it is $55.30. The bulk default here is $38 per cubic yard, a US national average for material only; the methodology page shows where that and the 25 lb per cubic foot density come from.

So bulk is about $15 cheaper per yard, roughly 29%, before delivery — but delivery changes the arithmetic at small volumes, because many yards will not send a truck for less than two or three yards.

• One yard, bagged: $55.30. One yard, bulk: $38 if you collect it. A yard of mulch weighs about 675 lb, so that is one pickup trip.
• Two yards, bagged: $110.60. Two yards, bulk: $76 plus delivery. Once the fee passes about $35, bags are cheaper unless you are hauling it yourself.
• Five yards, bagged: $276.50 across 68 bags, roughly 2,700 lb to load and stack. Five yards bulk: $190 plus delivery. Here bulk becomes a question about your back.

Two things people do not expect. Bulk mulch sits in a wet outdoor pile and goes out damper and heavier than the bagged product, which is why 13.5 bags weigh around 540 lb rather than 675 — loading a trailer, check weight rather than volume. And bagged only wins below about a yard, or when you cannot take a delivery. If your yard charges $60 a yard instead of $38, that is regional variation, not an error; if a density or bag yield here looks wrong, the contact page is how it gets fixed.

A worked bed: ten by ten feet at three inches is 100 × 0.25 = 25 cubic feet, or 0.93 cubic yards. Add the 5% waste allowance and you are at 0.97 — one yard, or 14 bags. The material calculator runs the same bed in any other material.

What a bed really needs over three years

Mulch is a consumable, not a one-time purchase. An organic mulch layer loses about 10 to 15% of its depth in the first season as it composts down and settles, so a three-inch application measures closer to two and a half by the following spring. That lost inch is not waste; it is the part that fed the soil, and why a yearly refresh is normal, not a sign you did something wrong.

A three-year plan for that same hundred-square-foot bed:

• Year one, new bed: three inches — one yard, or 14 bags.
• Year two, refresh: an inch and a half, 12.5 cubic feet, 0.46 cubic yards — half a yard, about seven bags.
• Year three: the same again, unless the bed has built up.

Two yards over three years for a hundred square feet. People end up with eight inches of mulch and plants struggling in it because they reapply the year-one three inches every spring without checking what is already there. If the bed measures over three inches, skip the year.

The other thing to watch at bulk volume is sour mulch. A pile that has sat wet and compacted goes anaerobic, and it announces itself — a sharp smell of vinegar, ammonia or sulfur instead of bark. Sour mulch is acidic and can scorch the leaves of annuals and young perennials. It is not a health hazard: spread it thin and let it air for a week or two before using it near anything tender. If it smells like a sewer, refuse the load.

And if the yearly refresh is the part you dislike, the alternative does not decompose. That is the trade the river rock calculator sets out: two to three times the cost per yard, no annual top-up, and no organic matter going back into the soil. Neither choice is wrong; they do different jobs.