What This Calculator Does
The Mulch Coverage Calculator tells you how much ground a single bag of mulch will cover at a given depth, and how many bags you need to mulch your entire bed. Mulch is sold by volume (cubic feet), but you spread it across an area (square feet) at a chosen depth — this tool bridges those two units so you don't over- or under-buy.
How to Use It
Enter the area you want to cover in square feet (length × width of the bed), choose your spreading depth in inches, and pick your bag size. Most retail bags are 2 cubic feet. The calculator returns the coverage per bag, the exact number of bags, the rounded-up whole bags to purchase, and the total volume of mulch.
The Formula Explained
Because depth is given in inches and volume in cubic feet, depth is first converted to feet by dividing by 12. One bag's coverage equals its volume divided by that depth in feet: \(\text{coverage} = \text{bagVolume} \div (\text{depth} \div 12)\). The number of bags is then your total area divided by the coverage per bag, rounded up to whole bags.
$$\text{Bags} = \left\lceil \frac{\text{Area (sq ft)}}{\dfrac{\text{Bag Size (cu ft)}}{\text{Depth (in)}/12}} \right\rceil$$
Worked Example
Suppose you have a 200 sq ft bed and want 3 inches of mulch using 2 cu ft bags. Depth in feet = \(3 \div 12 = 0.25\) ft. Coverage per bag = \(2 \div 0.25 = 8\) sq ft. Bags needed = \(200 \div 8 = 25\) bags exactly, requiring 25 bags and 50 cubic feet of mulch.
$$\text{Bags} = \left\lceil \frac{200}{\dfrac{2}{3/12}} \right\rceil = \left\lceil \frac{200}{8} \right\rceil = 25$$
FAQ
How deep should mulch be? Two to three inches is typical for most beds; 3–4 inches helps suppress weeds and retain moisture.
Why round up? You can't buy a partial bag, so always round up to ensure full coverage with a little extra.
Does it account for settling? No — mulch compacts slightly over time. If you want a buffer, add roughly 10% to the bag count.
