How to Calculate How Much Mulch I Need for My Project

Mulch protects your plants, keeps moisture in the soil, and suppresses weeds, whether you have a tree-lined walkway or a garden full of veggies. But how many bags of mulch do you need? Thankfully, there's a simple formula you can use to work out how much mulch you should be using. In this guide, we’ll help you figure out how much mulch to get for your landscaping project.

How Much Mulch Do I Need in Cubic Yards?

Every cubic yard of mulch will cover around 162 square feet at a depth of 2 inches or 108 square feet at a depth of 3 inches. Your first step in figuring out how many cubic yards of mulch you need for your project is to determine what area of the space you want to cover. You can use the formulas below based on your project.

• For square or rectangular beds: Work out the size of the area you want to cover by multiplying its width and length.

• For round beds: Measure the radius, the distance from the center of the circle to the edge of the bed. Multiply that by itself and then by 3.14.

• For beds with a varied shape: For all other shapes, including non-standard designs, you can take the dimensions in two directions at the widest parts and multiply them together. You’re likely to overestimate how much mulch you need, but that’s better than not having enough.

Now that you know the surface area in square feet, you multiply that figure by the depth you want your mulch to be in inches and divide by 324. That tells you the number of cubic yards you need for your desired coverage. Just round up to the nearest whole number to make sure your local mulch delivery company delivers enough. You can also use the table below, which shows how much mulch you'll need by area and depth, rounded up to the nearest cubic yard.

How Many Bags of Mulch Do I Need?

If you’re planning on picking up the mulch yourself instead of hiring a pro to deliver it for you, chances are you’ll be buying it in bags instead of in bulk. The average bag of mulch contains 1.5 cubic feet of mulch, which can cover around 9 square feet at a depth of 2 inches or 6 square feet at a depth of 3 inches. You can use the formulas above to determine your coverage area and then divide by 9 or 6 to figure out how many bags of mulch you need for a depth of 2 inches or 3 inches, respectively.

You can also use the table below to get an estimate of how many bags of mulch you need without doing any math.

What Is the Maximum Depth of Mulch by Type?  

Generally speaking, you should never put down more than 4 inches of mulch. Most garden beds benefit the most from a mulch depth of 2 to 3 inches.

The type of mulch you choose determines the maximum depth you can use safely around plants. Too much, and your plants don't get enough air or water. Too little, and the mulch doesn't do its job to keep weeds at bay, maintain proper soil moisture, and insulate the ground.

Dense Mulch

Fine, dense, small-particle mulch, like shredded hardwood and mulched leaves, has a maximum safe depth of 3 inches, but a depth of 1 to 2 inches is usually best unless you live in an arid area. Because it creates such a dense layer, airflow is limited, so if you go any deeper than 3 inches, you risk starving your plants of oxygen, even in drier climates.

Popular types of organic and inorganic mulches that should be kept to a depth of 3 inches or less include the following:

• Shredded hardwood

• Shredded softwood

• Rotted manure

• Gravel

• Tumbled glass

• Slate chips

• Coconut coir matting

• Rubber mulch

• Mulched leaves

A dense, heavy layer of mulch can cause dry soil, too, which is the opposite of what you want when putting down mulch. Instead of percolating through the mulch, picking up nutrients as it goes and working its way to the soil and your plant roots, the water can't get through easily and might never make it down into the soil.

The dense, moist layer with very limited airflow that results can encourage fungal pathogens and pests. This is especially an issue if you don’t remove old mulch before laying down the new layer.

Lighter Mulch

Larger and lighter mulch types, such as straw, pine bark nuggets, and other coarse mulches, can go up to 4 inches deep. These coarser types of mulch allow more airflow and don't form such a dense, heavy layer, so even at a depth of 4 inches, there's still plenty of oxygen and water getting through.

Lighter mulch types that can safely be used around plants at a depth of 4 inches or under include the following:

• Straw

• Pine needles

• Coarse bark chips

• Shells

• Seaweed

• Grass clippings

How Much Mulch To Use by Project

The volume of mulch to use depends on how you plan to use it. Want to grow fruits and veggies? Great, go for something you can easily dig through. Need something for under the kids' swing set? Grab something longer-lasting that’s less likely to track inside. Below, we’ll include some recommendations for which mulch to use and when.

Vegetables, Annuals, and Biennials

In the vegetable garden, a 2- to 3-inch layer of straw is sufficient, and you'll need to replace it annually. In areas where you won't be digging much but still want your plants to thrive, such as adding mulch to flower beds, you can go for coarse wood mulch at up to 3 inches deep. 

Mulch for Your Hardscape

If you want to suppress weeds in an area where you're not actively planting, such as a walkway, or you want to create a safe but natural surface in a kids’ play area, you can use longer-lasting coarse bark chips at a depth of up to 4 inches.

Mulch for Trees and Perennials

Trees and established perennials can tolerate a layer of coarse mulch up to 3 inches deep, or a fine-textured mulch up to 2 inches deep. It makes sense to add a longer-lasting mulch around these types of plants. 

This is because you won't be digging around them much, so you don't have to worry about trying to work through a deep coarse mulch layer, and you'll only have to top it up or replace it every three to five years.

Should You Get Mulch in Bulk or By the Bag?

Can’t decide whether it’s better to get mulch by the bag or delivered in bulk? First, figure out your coverage area using the formulas above and the depth of mulch you want. Calculate based on both cubic yards and bags.

If you need less than 0.5 cubic yards or fewer than 25 bags based on your calculations, it’s a better idea to buy mulch in bags, as most mulch delivery companies have minimum quantities they’ll deliver, starting at 0.5 cubic yards (the equivalent of about 25 bags of mulch).

If you need 50 or fewer bags and have a pickup truck or trailer you can tow to transport the mulch, consider saving on delivery fees by buying the mulch in bags and putting in a little elbow grease to load and unload them. If you need more than 50 bags or don’t have a truck available, hiring a local delivery company is your best option.