Mulch Calculator
A 20 by 10 ft bed covered 3 inches deep takes 50 cubic feet of mulch, which is 28 two-cubic-foot bags once the form's 10% waste allowance is added. This mulch calculator turns bed length and width in feet, depth in inches, the bag size printed on the label, and a waste percentage into the bed area, the volume in cubic feet and cubic yards, and a bag count rounded up to whole bags. Use the cubic yard figure to compare bagged mulch with a bulk delivery quote. The tool treats the bed as one rectangle at one depth, so split curved or L-shaped beds into rectangles and add the results. For a top-up, enter only the depth you are adding, not the finished depth.
Quick answer
The calculator treats the bed as a rectangle, so it multiplies length by width to find square footage before any volume math starts.
What this tells you
- •The calculator treats the bed as a rectangle, so it multiplies length by width to find square footage before any volume math starts.
- •Depth is entered in inches because mulch is usually spread in 2-inch to 4-inch layers, then converted to feet so volume stays in cubic feet.
- •A cubic yard always equals 27 cubic feet, which makes it easy to compare bag totals with bulk delivery quotes from a yard supplier.
- •Waste allowance adds a buffer for settling, uneven edges, bed curves, and small spills that happen during transport and spreading.
- •Bag count is always rounded up because you cannot buy part of a bag and coming up short often costs more than one extra bag.
How to Use
- 1Measure the longest length and width of the bed in feet. If the bed has curves, measure a practical rectangle that covers most of the area, then trim the estimate later if needed.
- 2Enter the mulch depth in inches. Extension guidance generally runs 2 to 4 inches, with finer mulch kept thinner. If old mulch is still in place, measure it and enter only the depth you are adding.
- 3Type the bag size printed on the packaging. Many store bags are 1.5, 2, or 3 cubic feet, so matching that label matters for the bag count.
- 4Add a waste allowance if the bed shape is irregular, the ground is uneven, or you want a safety margin for touch-ups after spreading.
- 5Click Calculate to review the raw cubic feet, cubic yards, waste-adjusted totals, and final bag count before you compare bag and bulk pricing.
How It Works
Formula
Area (ft²) = Length × Width
Depth (ft) = Depth (in) ÷ 12
Cubic Feet = Length × Width × Depth (ft)
Cubic Yards = Cubic Feet ÷ 27
Total with Waste = Cubic Feet × (1 + Waste% ÷ 100)
Bags Needed = ceil(Total with Waste ÷ Bag Size)The mulch calculator follows the same steps a contractor would use on paper. First it finds the bed area in square feet. Next it converts depth from inches to feet because cubic feet require all dimensions in feet. Multiplying length, width, and depth gives the raw mulch volume. Dividing by 27 converts that number to cubic yards for bulk ordering. If you add waste, the tool multiplies the raw volume by 1 plus the waste percentage as a decimal. Last, it divides the waste-adjusted cubic feet by the bag size and rounds up to the next whole bag. A useful shortcut comes from the same math: cubic yards = square feet × depth in inches ÷ 324.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Refresh a 20 ft by 10 ft planting bed at 3 inches deep
The bed area is 200 square feet. Three inches equals 0.25 feet, so the raw volume is 20 × 10 × 0.25 = 50 cubic feet. Adding 10% waste brings the total to 55 cubic feet, and 55 ÷ 2 = 27.5, which rounds up to 28 bags.
Top off a small 12 ft by 8 ft bed with 1.5 cu ft bags
This bed covers 96 square feet. At 2 inches deep, the mulch depth is 2 ÷ 12 = 0.1667 feet, so the raw volume is about 16 cubic feet. With no waste added, 16 ÷ 1.5 = 10.67, so you need 11 bags.
Mulch a 30 ft by 4 ft border strip at 4 inches deep
The strip covers 120 square feet. Four inches is 0.3333 feet, which gives 120 × 0.3333 = 40 cubic feet before waste. A 5% allowance raises the total to 42 cubic feet, and 42 ÷ 2 gives exactly 21 bags.
Compare bag count and yardage for a wider front bed
An 18-by-15-foot bed has 270 square feet of area. A 3.5-inch layer equals 0.2917 feet, so the raw volume is 78.75 cubic feet or about 2.92 cubic yards. After adding 8% waste, the total reaches 85.05 cubic feet, which means 28.35 three-cubic-foot bags, rounded up to 29.
Estimate a long side yard bed before comparing bulk quotes
This project covers 240 square feet. At 3 inches deep, the raw requirement is 60 cubic feet, which equals 2.22 cubic yards before waste. With a 12% buffer, the order rises to 67.2 cubic feet or 2.49 cubic yards, so you can compare 34 bags against a bulk quote of roughly 2.5 yards.
Top up a foundation bed that still has old mulch
Lena's shredded bark bed runs 26 ft along the front of the house and 6.5 ft out from the wall, or 169 square feet. She pushes a trowel through the old layer in several spots and finds about 1.5 inches left. Her target is 3 inches, the low end of USU Extension's 3 to 4 inch range for organic mulch, so she enters 1.5 inches, the depth she is adding. That is 21.13 cubic feet, or 23.24 with 10% waste, which comes to 12 two-cubic-foot bags or 8 three-cubic-foot bags. Entering the full 3 inches would have returned 46.48 cubic feet and 24 bags, twice what the bed needs. She keeps the new layer 2 to 3 inches back from the shrub stems, as Clemson Extension advises.
Mulch per 100 sq ft by depth, in cubic yards and bags
How far one cubic yard spreads at each depth, and what a 10 by 10 ft bed needs. The cubic yard columns show the volume with and without the 10% waste allowance the form starts with, and the bag columns include that waste. Scale the cubic yards to your area, then run the tool for an exact bag count, because bag counts round up and do not scale.
| Depth | 1 cu yd covers | Cu ft, no waste | Cu yd, no waste | Cu yd with 10% waste | 2 cu ft bags with 10% waste | 3 cu ft bags with 10% waste |
|---|---|---|---|---|---|---|
| 1 inch | 324 sq ft | 8.33 | 0.31 | 0.34 | 5 | 4 |
| 2 inches | 162 sq ft | 16.67 | 0.62 | 0.68 | 10 | 7 |
| 2.5 inches | 129.6 sq ft | 20.83 | 0.77 | 0.85 | 12 | 8 |
| 3 inches | 108 sq ft | 25.00 | 0.93 | 1.02 | 14 | 10 |
| 3.5 inches | 92.6 sq ft | 29.17 | 1.08 | 1.19 | 17 | 11 |
| 4 inches | 81 sq ft | 33.33 | 1.23 | 1.36 | 19 | 13 |
| 5 inches | 64.8 sq ft | 41.67 | 1.54 | 1.70 | 23 | 16 |
| 6 inches | 54 sq ft | 50.00 | 1.85 | 2.04 | 28 | 19 |
Computed with the formula above. The coverage column is 324 divided by the depth in inches, because 27 cubic feet spread 1 inch deep covers 324 square feet, and entering that area at 0% waste returns exactly 1.00 cubic yard. The other columns use a 10 by 10 ft entry: cubic feet = 100 x depth / 12, cubic yards = cubic feet / 27, and bags = cubic feet x 1.10 / bag size, rounded up. UC IPM and Clemson Extension both warn against more than 4 inches of mulch. The 5 and 6 inch rows are there for fresh pine straw, which University of Georgia Extension says settles from 4 to 5 inches to about 2.
How deep to spread mulch, and how that changes the order
Mulch is sold two ways, and the conversion between them is fixed. A cubic yard is 3 ft by 3 ft by 3 ft, or 27 cubic feet, so it equals 13.5 two-cubic-foot bags or 9 three-cubic-foot bags. Spread 1 inch deep, a yard covers 324 square feet. Utah State University Extension's fact sheet Using Mulches in Utah Landscapes and Gardens rounds that to about 3 cubic yards per 1,000 square feet for each inch of depth. The exact figure is 3.09, so the rounded rule runs about 3% short on a large order. Bag volume is a labeled net quantity, and NIST Handbook 133 section 3.10 sets the procedure weights and measures inspectors use to check packages of mulch and soil labeled by volume.
Depth advice comes from university extension services, and it changes with the material. USU Extension's Water-Wise Landscaping guide recommends 3 to 4 inches of organic mulch. Penn State Extension says 2 to 4 inches is enough on well-drained soil and less on poorly drained soil. UC IPM's Pest Notes on weeds in landscapes say coarse mulch can go up to 4 inches, fine-textured mulch packs more tightly and should stay near 2 inches, and mulch deeper than 4 inches may injure plants. Clemson Extension also links more than 4 inches to oxygen and moisture problems for tree roots.
Where the mulch stops matters as much as how deep it goes. Penn State Extension's rule of thumb is to pull mulch back until the root flare shows, about 3 to 5 inches from young trees and 8 to 10 inches from mature ones. UC IPM says to keep organic mulch 6 inches or more from trunks to avoid promoting root and crown diseases, and Clemson Extension advises 2 to 3 inches of clearance around shrub stems to prevent stem rot. A cone of mulch piled against a trunk, often called a mulch volcano, keeps the bark wet and spends material the bed does not need.
Loose mulch shrinks after it is spread, but published figures are rare. University of Georgia Extension says 4 to 5 inches of fresh pine straw settles to about 2 inches, roughly half. For pine straw, enter the fluffed depth you will lay down, not the settled depth you want, or the order comes out about half of what the bed takes. The sources cited here give no settling figure for shredded bark or wood chips, so cover those with the waste allowance. USU Extension also advises against packing mulch down, especially when it is wet.
Most beds are topped up rather than mulched from bare soil. Push a trowel through the old layer in a few places, measure what is left, and enter only the difference between that and your target. On a 20 by 10 ft bed, going from 2 to 3 inches adds 16.67 cubic feet and takes the order from 17 to 25 two-cubic-foot bags before waste, so that one inch costs 8 bags. Entering the finished depth instead of the added depth can double the order, as the foundation bed example above shows.
Common mistakes
- Entering depth in feet instead of inches, which can make the estimate 12 times too large.
- Entering the finished depth when topping up an old layer, which can double the order.
- Skipping waste on curved beds, around tree rings, or on sloped areas where coverage is harder to spread evenly.
- Using the wrong bag size from memory instead of checking the package label before ordering.
- Rounding the bag count down, which often leaves thin spots or forces a second store trip.
- Entering the settled depth for pine straw, which loses about half its depth as it settles.
- Piling mulch against trunks and stems, which wastes material and holds moisture against the bark.
- Treating an irregular bed as a perfect rectangle without trimming the result or splitting the project into smaller shapes.
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