Mulch Calculator
Mulch is cheap per yard and expensive per bag. This tells you both numbers, so you know whether the trunk of your car is enough or whether delivery is the cheaper choice.
Reviewed · Method: measured area + industry waste allowance, rounded up to real package sizes · How the numbers are calculated
Enter your dimensions and hit Calculate my mulch. You will get the quantity, the buyable quantity, a shopping list, and a budget range.
Figure. Mulch is a volume problem: bed area times depth, converted to cubic yards or bags.
Figure. Three inches suppresses weeds and holds moisture. Four inches and above starts to smother roots.
Three inches, and not more
Two inches is enough to suppress most annual weeds. Three inches is the depth that holds moisture through a hot week and still lets air reach the soil. Four inches and above starts to work against you: oxygen cannot move through the layer, water sheds off instead of soaking in, and roots stay near the surface where they dry out fastest. Deep mulch also invites fungus and can rot the crown of perennials. The one exception is a bed you are trying to smother completely, where a heavier layer under a cardboard sheet is a deliberate choice.
Bags versus bulk
A two cubic foot bag covers 8 square feet at three inches deep. A cubic yard is 27 cubic feet, which is thirteen and a half of those bags, and covers about 108 square feet at the same depth. Bulk mulch delivered by the yard is usually a third to a half the cost per cubic foot of bagged material. The trade-off is a minimum order, a delivery fee, and a pile in the driveway that you have to move. Under three cubic yards, bags are often the practical answer. Above five, bulk almost always wins.
When to spread it
Early spring, after the soil has warmed but before weeds have established. Spreading in late autumn is also defensible in cold climates, because it moderates the freeze-thaw cycle that heaves shallow-rooted plants. What you should not do is spread fresh mulch over dry soil in midsummer and walk away, because the layer will shed the next rain and the bed will be drier under the mulch than it was without it. Water the bed first, then spread.
Pre-emergent, and the timing trap
Pre-emergent herbicide stops weed seeds from germinating, and it has to be applied before they do. Putting it on top of fresh mulch is usually a waste, because the chemical needs to reach the soil surface. Either apply it under the mulch, or choose a mulch that already contains it and accept the shorter effective life. The calculator includes a quantity based on bed area so you do not have to guess the rate.
Why three inches is the number
Below 2 in, weed seeds germinate through the layer and moisture evaporates almost as fast as on bare soil. Above 4 in, water stops reaching the soil in light rain, roots grow up into the mulch, and the layer stays wet against stems.
Three inches is the depth that does both jobs: blocks light and holds water. For a refresh on existing mulch, half that depth is enough because you are topping up, not building a new layer.
Beds, borders and the shapes that fool you
Curved beds are easy to overestimate. Measure the longest length and the widest width, multiply, then take 80% of that figure for a rounded bed. For a border strip, measure length times the strip width and add 10% for the curve.
Trees and shrubs inside the bed are subtracting area, but the mulch still goes around them, so do not deduct. Ring the trunk by hand and keep the layer flat rather than mounded.
Worked example: 30 x 8 ft bed at 3 in deep
| Bed area | 30 x 8 = 240 sq ft |
| Volume | 240 x 0.25 = 60 cu ft |
| Cubic yards | 60 / 27 = 2.22 cu yd |
| 2 cu ft bags | 60 / 2 = 30 bags |
| Coverage per yard | about 108 sq ft at 3 in |
| Budget band | $62 low, $107 mid, $189 high |
Mistakes that cost real money
Spreading too thick to save a trip
More than 4 in of mulch smothers the root zone, holds too much moisture against stems and invites fungus. Three inches suppresses weeds and retains water; six inches kills shrubs slowly and looks like a mistake.
Piling mulch against trunks
Mulch volcanoes kill trees. Keep it a hand's width away from the trunk and never let it touch the bark. Moisture trapped against bark is the entry point for rot and borers.
Buying bags for a large bed
A cubic yard is 13.5 bags of 2 cu ft. For anything over about 2 cu yd, bulk is roughly half the price per yard and far less work than carrying 30 bags around the house.
Mulching over weeds
Mulch suppresses weed seeds, it does not kill existing weeds. Sheet-mulch with cardboard or pull first, or you are giving the weeds a nice dark blanket to grow through.
Forgetting that mulch settles
Fresh mulch settles 20 to 30% in the first season as it compacts and breaks down. A 3 in layer is 2 in by autumn, which is why beds need topping up annually at roughly half the original depth.
Frequently asked questions
How many bags of mulch equal a cubic yard?
Thirteen and a half 2 cubic foot bags make one cubic yard, since a yard is 27 cubic feet. In practice buy fourteen, because bags are rarely filled to exactly the stated volume and settlement in transit takes a little more off the top.
How deep should mulch be around trees?
Two to three inches, and keep it at least six inches away from the trunk. Mulch piled against the bark holds moisture on the trunk, which invites decay and can girdle a young tree. The volcano shape you see in some landscapes is actively harmful.
Does mulch need to be removed every year?
No. Organic mulch breaks down and feeds the soil, so you top it up rather than replace it. Check the depth with a finger before adding more; if it is already at three inches, you only need a thin refresh to restore colour.
Which mulch lasts longest?
Cedar and cypress resist decay best and hold colour for two to three seasons. Dyed hardwood lasts well and is usually the cheapest per yard. Straw and grass clippings break down within a single season and are better thought of as soil amendments than as mulch.
Can I use mulch instead of landscape fabric?
For weed suppression in a planted bed, a three inch layer of mulch works about as well as fabric and is better for the soil. Fabric is worth it under gravel paths and around permanent shrubs, where you do not want to top up every year.