Mulch Calculator
Estimate mulch needed for beds. Choose rectangle or circle, set depth and output unit, then price by bags or bulk to get volume, bags, and cost.
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What the mulch calculator does
Mulch keeps beds tidy, holds in moisture, and slows down weeds, but a bed soaks up more of it than you would think. This works out how much you need. You give it the shape and depth of the bed, and it returns the volume in your choice of units, plus the number of bags or a bulk cost if you add pricing.
It handles a rectangle or a circle, so a border and a round tree ring are both covered. Below is how it works and how to choose a depth.
How to use it
- Pick the shape. Rectangular for a border or bed, circular for a ring around a tree.
- Enter the dimensions and the depth. Each with its own unit, mixed however your measurements come.
- Choose an output unit, then a pricing method if you want a cost: by the bag, where you enter the bag size and price, or in bulk by the cubic yard.
- Press Calculate for the breakdown, or Reset to clear it.
How the volume is worked out
Mulch is sold by volume, so volume is what the calculator is after. It is the area of the bed times how deep you spread the mulch:
Volume = area × depth
For a rectangle the area is length times width, and for a circle it is pi times the radius squared. From the volume, choosing bag pricing divides by your bag size to count the bags, and choosing bulk multiplies the cubic yards by your price per yard.
How deep to lay mulch
Depth is the number that drives the whole order, and for most beds 2 to 3 inches is the sweet spot. That is deep enough to hold moisture and keep weeds down, without being so thick that it stops water and air reaching the soil. Topping up an existing bed usually needs less, since you are only making up what has broken down.
One thing worth getting right, especially around trees: keep the mulch pulled back from the trunk and stems rather than piled against them. A mound heaped up the trunk traps moisture against the bark and invites rot, so a flat, even layer with a gap around the base is what you are after.
A worked example: a 20 ft by 4 ft bed
Say you are mulching a 20 foot by 4 foot border to a depth of 3 inches.
The area is 20 × 4 = 80 square feet, and at 3 inches (0.25 ft) deep that is 80 × 0.25 = 20 cubic feet, which is about 0.74 cubic yards. In bags of 2 cubic feet, the common size, that is 10 bags.
So that border is either ten bags from the garden centre or three quarters of a yard delivered, which is the choice the next section gets into.
Bags or bulk
Bags suit small beds and tidy jobs, where they are easy to carry, store, and spread, and you only buy what you need. Bulk mulch, delivered by the yard, is cheaper per unit and far less hauling for a big planting, as long as you have somewhere for the pile to sit while you barrow it around.
Either way, it is worth rounding up a little. A bed that comes up a bag short looks patchy until you make a second trip, and mulch settles and breaks down over a season, so a touch extra is rarely wasted.
Questions people ask
How much mulch do I need?
Work out the bed area, multiply by the depth for the volume. A 20 by 4 foot bed at 3 inches deep needs about 0.74 cubic yards, or around 10 bags of 2 cubic feet.
How deep should mulch be?
About 2 to 3 inches for most beds. That holds moisture and suppresses weeds without smothering the soil. Topping up an old bed needs less.
How many bags of mulch are in a yard?
About 13 or 14 if the bags hold 2 cubic feet, since a cubic yard is 27 cubic feet.
Should mulch touch the tree trunk?
No. Keep it pulled back from the trunk with a gap around the base. Mulch piled against the bark traps moisture and can cause rot.
References
A quick note on where these figures come from. The volume is pure geometry, the area times the depth, with 27 cubic feet making up a cubic yard. The depth guidance, and keeping mulch back from trunks, follow the mulching recommendations published by university cooperative extension services. The unit conversions follow the US National Institute of Standards and Technology guide.
- University cooperative extension, landscape mulching guidance (depth and application). https://extension.umn.edu/planting-and-growing-guides/using-mulch-yard-and-garden
- National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811
Mahendra Thapaliya is a graduate student in Structural Engineering at the University of Bologna, with research interests in structural systems, FEM, earthquake engineering, and numerical modeling. At Eon Tools, he reviews construction tools.