Mulch Calculator
How much mulch a bed takes at the depth you actually want — in cubic yards for a bulk load, and in the 2 and 3 cubic foot bags garden centres stock. Rectangles, circles, triangles, or a whole garden of beds at once.
A rectangular bed or border
An awkward bed is usually two rectangles and a triangle. Measure them separately and use the multi-bed mode.
The long side of the bed, measured along the edging.
The depth of the border from the lawn edge to the wall or fence.
Two to three inches is the working range. Under two lets enough light through to germinate weed seed; over three or four keeps oxygen from the roots and holds damp against the stems.
Five percent covers what settles, spills off the barrow, and disappears into an edge that is never quite straight. Kept as its own line below so you can argue with it.
200.00 sq ft of bed at 3 inches is 50.00 cu ft, and a 5% allowance brings it to 52.50 cu ft — or 18 bags of 3 cu ft.
- Area to cover
- 200.00 sq ft
- Mulch at 3 in
- 50.00 cu ft
- Waste allowance (5%)
- 2.50 cu ft
- Order this much
- 52.50 cu ft
- In cubic yards
- 1.94 cu yd
- Before waste
- 1.85 cu yd
- Bulk or bagged
- Under 2 cu yd — bags usually win
- Next year’s top-up
- 0.62 cu yd
| Bag size | Bags needed | Mulch bought |
|---|---|---|
| 2 cu ft | 27 | 54.00 cu ft |
| 3 cu ft | 18 | 54.00 cu ft |
- Never pile mulch against a trunk.A cone of mulch heaped up the bark — a “mulch volcano” — holds moisture against living tissue until it rots, and invites roots to circle the stem and slowly strangle the tree. Spread it flat and pull it back three to six inches from the trunk so the flare at the base stays visible.
- Under about two cubic yards, the delivery fee on a bulk load usually costs more than the bags save. Price both before you decide.
- Organic mulch decomposes into the soil, so the first year is the expensive one. Budget roughly 0.62 cu yd a year afterwards to restore the inch that rots away.
Area, depth, and two different sellers
Mulch is an area problem wearing a volume disguise. Work out the square footage of the bed, turn the depth from inches into feet by dividing by twelve, and multiply. Everything after that is a conversion into a unit somebody sells in — cubic yards at a landscape yard, bags at a garden centre.
V = A × (D ÷ 12) • Acircle= π × (⌀ ÷ 2)² • Atriangle= (b × h) ÷ 2 • yd³ = ft³ ÷ 27- A
- Bed area in square feet. Several beds are simply added together before the depth is applied
- D
- Finished mulch depth in inches, hence ÷ 12 to reach feet
- b, h
- Triangle base and the height measured square to it — not the length of the sloping side
- ÷ 27
- Cubic feet to cubic yards: three feet in each of three directions
Beds are summed before anything is divided into bags. Running each bed through separately and adding the bag counts would round every one of them up to its own whole bag, which on a five-bed garden buys four bags of nothing.
Bag counts are the ordered volume divided by what is printed on the bag, rounded up. The waste allowance is kept on its own line rather than folded into the total, because five percent for spillage and settling is a guess, and a guess you can see is a guess you can change.
A 20 × 10 foot border, three inches deep
The bed is 20 × 10 = 200 square feet. Three inches is 3 ÷ 12 = 0.25 feet, so the mulch is 200 × 0.25 = 50 cubic feet. A five percent allowance adds 2.5 cubic feet, giving 52.5 cubic feet to order — which is 1.94 cubic yards, or 27 bags of 2 cubic feet, or 18 bags of 3 cubic feet.
That figure lands almost exactly on the crossover. Just under two cubic yards is 18 bags to load into a car, and a bulk supplier delivering less than two yards will charge a delivery fee that often exceeds what the bulk price saved. Price both — this is the one job size where the answer genuinely depends on your local yard.
Now scale it. Three borders of that size are 600 square feet and, before any allowance, 150 cubic feet — 5.56 cubic yards, or 75 of the small bags. That is two pallets, two car trips, and 75 plastic sacks to dispose of. At this size bulk is not a close call.
And the year after: the beds still cover 600 square feet, but they only need the inch that decomposed. That is 600 × (1 ÷ 12) = 50 cubic feet, or 1.85 cubic yards — a third of the first-year order. Mulch is cheaper to keep than to start.
Two to three inches, and why not more
Mulch suppresses weeds by blocking light, and light is the whole mechanism. Below about two inches, enough gets through that the seed already in your soil germinates anyway and the layer is decorative rather than functional. That is the floor.
The ceiling is less obvious and does more damage. Past three or four inches, mulch stops being a blanket and starts being a lid. Feeder roots sit in the top few inches of soil and need air; a deep, packed layer keeps oxygen out and keeps water in, and roots suffocating in permanently damp soil look exactly like roots dying of drought. Fine shredded bark packs tightest and should stay nearer two inches. Coarse wood chips hold their structure and tolerate three or four.
Depth is measured on what is finished, not what is delivered. Fresh mulch settles noticeably in its first few weeks, which is part of what the waste allowance covers.
Never pile mulch against a trunk
This is the most common and most expensive mulching error, and it is entirely avoidable. Mulch heaped in a cone up the trunk — a “mulch volcano” — holds moisture against bark that evolved to be dry. Bark kept wet decays, and decayed bark is an open door for insects and disease that intact bark keeps out.
The slower failure is worse. A tree reads the top of the mulch as the soil line and sends roots up into it. Those roots have nowhere to spread, so they circle the trunk instead. Over five or ten years they thicken and constrict the stem they grew from — girdling roots — cutting off the flow between canopy and roots. The tree declines for years, and by the time the cause is visible the damage cannot be undone.
The correct shape is a flat layer, not a cone: spread mulch out towards the drip line, where the feeder roots actually are, and pull it back three to six inches from the trunk. The flare where the trunk widens into the roots should stay visible. If you cannot see the flare, there is too much mulch in the wrong place.
The two-cubic-yard crossover
Bagged mulch is priced per bag and bulk mulch per cubic yard, and per cubic foot the bulk price is normally a good deal lower. What stands in the way on a small job is the delivery fee, which is roughly the same whether the truck brings one yard or six. That fixed cost is what makes the crossover exist: somewhere around two cubic yards, the saving on the material overtakes the fee.
Labour points the same way. Two cubic yards is 27 three-cubic-foot bags or 54 of the two-cubic-foot ones — a pallet to load, unload, open one at a time, and then dispose of. A bulk load arrives as one pile on the driveway and goes straight into the barrow.
Bags still win in three situations: a job under a yard, where the delivery fee dominates; anywhere a truck cannot tip a pile; and a bed you are topping up in stages, since an unopened bag keeps and a pile on the driveway does not.
What this calculator assumes
- Depth is uniform across the whole bed, and the ground is reasonably flat. Real beds are deeper in the hollows, which is one of the things the waste allowance covers.
- Bag counts are always rounded up to a whole bag. Nobody sells four-fifths of a bag, and coming up short means a second trip.
- The 2 and 3 cubic foot bag sizes are what bagged mulch is commonly sold in. Sizes vary by brand and region — check the bag, because a 1.5 cubic foot bag at the same price is a third less mulch.
- The two cubic yard bulk crossover is a pricing rule of thumb, not a constant. It moves with your local delivery fee and with the spread between bagged and bulk prices where you live. It is a prompt to compare, not a quote.
- The annual top-up assumes about one inch of organic mulch decomposes each year — faster for fine shredded material and in warm, wet climates, slower for coarse chips. Inorganic mulches such as stone or rubber do not decompose and need no top-up at all.
- Nothing here deducts the space taken by existing plants, which is normally small and comfortably inside the allowance. Nor does it price the mulch, recommend a type, or address landscape fabric, dyes, or whether wood mulch is right for your beds.
Mulch calculator FAQ
How many bags of mulch are in a cubic yard?
Thirteen and a half 2 cubic foot bags, or exactly nine 3 cubic foot bags. A cubic yard is 27 cubic feet — three feet in each of three directions — so the arithmetic is 27 ÷ 2 and 27 ÷ 3. In practice you buy 14 of the small ones, because half a bag is not sold. That is also the quickest way to sanity-check a quote: if a landscape yard offers a cubic yard for less than nine bags cost at the store, the bulk load is cheaper before you have even counted the labour of hauling a pallet.
How deep should mulch be?
Two to three inches on an established bed. Under two inches, enough light reaches the soil for weed seed to germinate, which defeats most of the point. Over three or four inches, the layer starts working against you: it keeps oxygen from the shallow feeder roots, holds water against stems and crowns, and can shed light rain before it ever reaches the soil. Fine-textured mulches like shredded bark pack down and should sit at the lower end of that range; coarse wood chips breathe better and can go to three or four inches. Around shallow-rooted shrubs and in poorly drained soil, less is safer.
Why should I never pile mulch against a tree trunk?
Because bark is not a root, and it is not built to stay wet. A cone of mulch heaped up the trunk — the shape is common enough that arborists call it a mulch volcano — keeps moisture against living bark until it decays, which opens the tree to rot and to insects and pathogens that could not otherwise get in. Worse, the tree responds to the false soil line by sending roots up into the mulch, where they circle the trunk instead of spreading out. Years later those girdling roots strangle the stem they grew from, and by then nothing can be done. Spread mulch flat across the bed and pull it back three to six inches from the trunk so the flare at the base of the tree stays visible.
Is bulk mulch cheaper than bags?
Above roughly two cubic yards, almost always. Two cubic yards is 54 cubic feet, which is 27 three-cubic-foot bags — a full pallet to load, drive home, unload, open, and dispose of. Bulk mulch is priced per cubic yard, typically a fraction of the per-cubic-foot price of bagged, and the delivery fee stops dominating the invoice once the load is that size. Below a yard or two the fee usually swallows the saving, and bags win on convenience anyway: you buy what you need, and an extra bag costs an extra bag rather than a second delivery.
How often does mulch need replacing?
Organic mulch needs topping up roughly once a year, and that is a feature rather than a fault. Bark, wood chips and shredded hardwood decompose into the soil, feeding it and improving its structure — the price of that is about an inch of depth a year in a temperate garden, faster where it is warm and wet or where the mulch is finely shredded. So the first year costs the most, because you are buying the full two or three inches, and every year after buys back the inch that rotted away. Do not simply add three fresh inches on top of what is left; measure what is actually there first, or the bed creeps deeper each year until it is smothering the plants.
How much mulch do I need for a 20 by 10 foot bed?
At three inches, 50 cubic feet — a little under two cubic yards, or 25 bags of 2 cubic feet. The sum is 20 × 10 = 200 square feet, and three inches is 3 ÷ 12 = 0.25 feet, so 200 × 0.25 = 50 cubic feet. Adding a five percent allowance for spillage and settling takes it to 52.5 cubic feet, which is 27 small bags or 18 of the 3 cubic foot bags, and 1.94 cubic yards. That is right on the line where bulk starts to make sense — worth pricing both.
Does mulch actually stop weeds?
It suppresses them; it does not eliminate them. A two to three inch layer blocks enough light that most weed seed already in the soil never germinates, which is the mechanism — and it is why a thin layer does so little. What mulch cannot do is stop seed blowing in and germinating in the mulch itself, or stop established perennial weeds pushing through from below. Pull those before you spread, not after.
Sources and review notes
- International Society of Arboriculture, “Proper Mulching Techniques” — the arborists’ consumer guidance on mulch depth, keeping mulch off the trunk, and what girdling roots do to a tree
- University of Minnesota Extension — mulching the yard and garden, including depth by mulch texture and how organic mulches break down over a season
- NIST Special Publication 811 — the definitions behind the two conversions on this page: 12 inches to the foot, 27 cubic feet to the cubic yard
The geometry here is exact and has nothing behind it to go stale. The figures that move are the bag sizes, which are a retail convention rather than a standard, the two cubic yard bulk crossover, which depends on your local delivery fee, and the one-inch annual decomposition rate, which varies with climate and with how finely the mulch is shredded. Check all three locally before you order.