The two numbers that decide everything
Mulch volume is area times depth, and the only trick is that the area is in square feet while the depth is in inches. Divide the inches by twelve first. A bed of 500 square feet at 3 inches is 500 x 0.25, which is 125 cubic feet. A cubic yard is 27 cubic feet, so that is 4.63 cubic yards. At two cubic feet a bag it is 63 bags, since 27 divided by 2 gives 13.5 bags to the cubic yard.
That last conversion is the one worth memorising: 13.5 bags per cubic yard. It is what turns a bag price into something comparable with a bulk price. Bagged mulch that looks cheap on the shelf is frequently two or three times the delivered bulk price per yard once you have done the multiplication, and you still have to load, haul and split open every bag.
Depth against weeds, depth against roots
Depth is a genuine trade-off rather than a "more is better" setting, and the two ends of it pull in opposite directions.
| Depth | What it does | Where it fits |
|---|---|---|
| 1 inch | Cosmetic. Weed seed germinates through it. | Topping up a bed that still has last year's layer |
| 2 inches | Holds moisture, some weed suppression | Established beds with dense planting |
| 3 inches | The usual working depth — blocks light to most weed seed | Most beds, most of the time |
| 4 inches | Strong suppression, slower to break down | New beds, weedy ground, coarse bark that settles |
| Deeper than 4 | Sheds light rain, stays sodden below, harbours rodents | Rarely a good idea around plants |
Weed suppression works by cutting light, not by making a physical barrier, which is why the depth matters and why fine, dense mulch suppresses better than coarse nuggets at the same depth. It also does nothing against perennial weeds already rooted underneath — those come back through any depth you can reasonably put down. Dig them out first.
Do not pile it against the trunk
The cone of mulch heaped against a tree trunk is the most common and most damaging thing done in a front garden. Bark above ground is not built to sit wet. Held against moist mulch it stays damp, softens, and gives insects and rot an entry point, and the tree responds by putting out roots into the mulch itself, which then circle the trunk and slowly strangle it. The damage takes years to show and by then it is not fixable.
Keep the mulch a few inches clear of the trunk, all the way around, and leave the root flare visible — the place where the trunk stops being a cylinder and spreads into the roots. If you cannot see the flare, there is too much material against the tree. The same applies at a smaller scale to shrub stems and to the crowns of perennials.
Bags or bulk
Bags win on small jobs, on access, and on storage: you can buy four, put them in the boot, and leave the rest for next month. Bulk wins on everything else. The crossover in practice is usually somewhere around one to two cubic yards, which is 14 to 27 bags, at which point you are making several trips and paying a large premium for the plastic.
What tips the decision is delivery and access. Bulk arrives as a pile wherever the truck can tip it, which may be the middle of your driveway for a week, and moving it from there is wheelbarrow work. If the beds are behind a house with a narrow side passage, factor in that every yard is roughly nine to fourteen barrow loads, and the delivered price stops looking quite so decisive.
What this does not tell you
It gives you volume, not a planting decision. Whether to mulch at all over particular plants, whether the material suits the soil chemistry, and whether a fresh wood product will lock up nitrogen at the surface as it breaks down are all questions about your specific bed and your specific plants. The weight figures used here are a broad range for bark-type mulch and nothing more: rubber, stone, compost and pine straw are entirely different materials by weight and behave differently once down. Ask the supplier what their material weighs before you plan a load.
Questions people ask
How many bags of mulch in a cubic yard?
Thirteen and a half, if the bags are the standard two cubic feet. A cubic yard is 27 cubic feet, and 27 divided by 2 is 13.5 — so you round to 14 in practice. For three cubic foot bags it is 9 per yard, and for the 1.5 cubic foot bags common in rubber and some hardwood mulches it is 18. Check the bag before you do the sum, because the size is not standardised across brands and a bag that looks the same on the shelf can hold a third less.
How deep should mulch be?
Three inches covers most situations. Two is enough on an established bed that is already densely planted and mostly shaded at soil level. Three to four is what you want on newer beds, on bare ground between plants, and anywhere weed pressure is the reason you are mulching, because suppression works by cutting light and thin layers let light through. Going past four inches starts causing problems rather than solving them: rain runs off before it soaks in, the layer underneath stays permanently wet, and rodents nest in it.
How much does a cubic yard of mulch weigh?
Roughly 400 to 800 pounds for bark and wood mulch, and that is a real range rather than a hedge — the same product can double in weight after a week of rain, and different materials sit at different points in the band. Compost and soil-based products are much heavier again, often over 1,000 pounds a yard. This matters when you are hauling it yourself: a half-ton pickup is at its limit somewhere around one to two yards of wet mulch, and bulk suppliers load by the bucket without weighing what goes in. Ask the yard what their material runs at before you decide how many trips it is.
Should I put landscape fabric under mulch?
It is not the free win it is sold as, and this calculator takes no position on it beyond the arithmetic. Fabric does block perennial weeds coming up from below for a few years. What it also does is stop the mulch breaking down into the soil, which is one of the main benefits of mulching, and once soil and organic debris accumulate on top of the fabric — which takes two or three seasons — weeds root in that layer instead and the fabric does nothing except make them harder to pull. Under stone it is more defensible than under bark. Under bark in a planted bed, most experienced gardeners have stopped bothering.