Have you ever been in the middle of a project and suddenly realized you majorly miscalculated some of your supplies?
In my 15 years working with landscape professionals, nursery growers, college programs, and friends and family, the most common mistake I see is a gross underestimation of proper mulch application.
In most cases, it ends in a “well, that’s good enough” attitude and a thin layer of mulch that looks great for a few weeks. Then, the weeds invade, and they quickly obscure the landscape everyone worked so hard to plan (here’s lookin’ at you, mom).
In This Article
Mulch calculations are simple: measure the square footage of the area you need to cover with mulch and decide how thick you want to apply it. Then, plug the numbers into this formula:
(Square footage x depth in inches)/12 = Cubic feet of mulch required
For mulch applied 3”- 4” deep, this means 2cu’ of mulch will cover 7sq’ of soil.
Your specific mulch needs may vary depending on the material you decide to use and how you intend to use the area you’re mulching.
How Much is a Cubic Foot of Mulch?
A cubic foot is 12” long x 12” wide x 12” tall. Mulch is measured in cubic feet, as opposed to square feet, because we have to account for depth when mulch is spread over an area. Bags of mulch are sold by the cubic foot, while bulk orders are sold by the cubic yard (27 cubic feet).
How Deep Should I Spread Mulch?
Mulch calculations are based on the depth of the material.
For example, if you were to spread your mulch 12” thick, one cubic foot would cover one square foot. Obviously, this is far too deep, but you get the idea.
If you were to spread the mulch 6” thick, one cubic foot would cover two square feet. A 3” depth would cover four square feet, and so on.
The real question is, how deep should mulch be?
Well, assuming you’re not going to be making those stupid mulch volcanoes (sorry, mediocre landscape companies!), you should plan on spreading mulch 3”- 4” thick.
Why Does Mulch Need to be so Deep?
Mulch provides a host of benefits for your garden bed:
- Weed suppression
- Moisture retention
- Decomposition into organic matter
- Home for beneficial insects
Plus, it just looks nice.
However, mulch can only provide these benefits if it is thick enough to do the job.
What About Weed Fabric?
Weed fabric is either woven cloth or plastic that is spread out on top of the soil under the mulch. The purpose of weed fabric is to almost completely suppress weeds.
While weed fabric is an excellent mulch in this regard, it prevents organic mulches from breaking down and building rich soil for your garden bed. It can also disrupt the life cycles of important beneficial garden insects.
The upside to weed fabrics is it suppresses weeds so that you can spread your mulch out thinner. This helps reduce your overall need for mulch material.
Weed fabric is best for the following applications:
- Permanent landscapes with a rich foundation soil
- Garden beds that will have fabric removed each year to add amendments
- Large, bare areas in landscape beds
- When used with other inorganic mulches
In these cases, it is not as important for the mulch to build up the underlying soil because the need to reduce weed growth far outweighs the need to add organic matter.
How do I Calculate How Much Mulch I Need?
Before you take measurements, you need to decide which material you will be using and the primary purpose for using mulch.
Put simply, mulch is just a protective layer of material. Mulch can be organic or inorganic. Examples include:
- Organic Mulches
- Wood Chips
- Straw (not hay!)
- Pine Needles
- Grass Clippings
- Dead Leaves
- Inorganic Mulches
- River Rock
- Weed Fabric
Loose, organic mulches, like straw or pine needles, may need to be spread thicker than dense materials like grass clippings or newspapers.
Inorganic mulches are often paired with weed fabric and combined with the dense nature of these materials and the likelihood that they will be for pathways or other areas without many plants, you can choose the depth based mostly on aesthetics.
Regardless of the material you choose, the calculations for how much you need remain the same.
Measuring Oddly-Shaped Beds
Most landscapes are fluid, curving, aesthetic shapes rather than nice, neat rectangles. Unless you’re Pythagoras, you should divide the bed into approximate rectangles and triangles and estimate square footage.
Round up your measurements to whole numbers (instead of 19.75 x 11.125, just say 20×12), and this will ensure you have enough to finish the job.
How Much Does 2 Cubic Feet of Mulch Cover?
The most common mulch material is wood chips, and the most common way to purchase wood chips is in 2cu’ bags.
A 2 cubic foot bag will cover:
- 2sq’ @ 12” deep
- 4sq’ @ 6” deep
- 6sq’ @ 4” deep
- 8sq’ @ 3” deep
- 12sq’ @ 2” deep
- 24sq’ @ 1” deep
So, in most landscape/garden beds, 2 cubic feet of mulch will cover 7 square feet of soil.
This means for a 100sq’ garden bed, you can estimate 14-15 2cu’ bags.
Of course, for larger projects, you can always use a mulch calculator.
These calculations also work for applying compost.