Mulch Calculator

What it calculates and who it’s for

This calculator turns a garden bed into cubic feet, cubic yards, and bags of mulch. It is for someone covering a new bed, refreshing a bed that already has some mulch, or ringing a tree.

You can measure a rectangle, a circle, a tree ring, or type an area you already know in square feet. Depth starts at 3 inches, inside the 2 to 4 inch range from the University of Illinois Extension sheet Proper Mulching Techniques (accessed October 6, 2026). A tree ring keeps a bare circle around the trunk. A top-up subtracts the depth that is already there.

The results are material counts. They leave out price.

How to use

The form opens on a 20 ft by 10 ft rectangle, 3 inches deep, with no mulch there yet, and a 2 cubic foot bag. Change any box and the results update.

1. Under Bed, choose Rectangle, Circle, Tree ring, or Enter sq ft.
2. Under Size, enter the measurement that shape uses. A rectangle uses Length / diameter and Width. A circle uses Length / diameter as the diameter. A tree ring uses Outer diameter. Enter sq ft uses Area. The other size boxes stay on the form and are skipped.
3. Under Depth, enter the depth you want when you are done. For a top-up, enter Existing depth. Leave that box at 0 on bare soil.
4. Under Bags, set Bag size to the cubic feet printed on your bag.
5. Under Advanced, set Trunk clearance if you chose a tree ring. Other shapes skip that box. It starts at 0.33 ft, which is 3.96 inches.
6. Read Bags to buy and Cubic yards. Bags to buy is the shopping count. Cubic yards is the number to give a bulk supplier.

The formula

Two unit facts come from NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement, accessed October 6, 2026. That appendix states that 1 foot equals 12 inches, and that 27 cubic feet equal 1 cubic yard. It does not give a mulch depth or a bag size.

Area

• Rectangle: area (sq ft) = length (ft) × width (ft).
• Circle: area (sq ft) = π × (diameter ÷ 2)². The Length / diameter box is the diameter in feet. π is the usual circle constant, about 3.14159265.
• Enter sq ft: area is the number in the Area box.
• Tree ring: outer radius (ft) = outer diameter (ft) ÷ 2. The bare circle (sq ft) = π × clearance radius². Mulched area (sq ft) = π × (outer radius² − clearance radius²).

Volume

• Depth to add (inches) = depth you want − existing depth.
• Cubic feet = mulched area × depth to add ÷ 12.
• Cubic yards = cubic feet ÷ 27.
• Bags to buy = cubic feet ÷ bag size, rounded up to the next whole bag. A number that is already whole stays there. 8.01 bags becomes 9. 25 bags stays 25.
• Bags in one yard = 27 ÷ bag size. This line is the conversion for the bag you typed. It is not rounded up.

The chart uses the same mulched area and the same bag size at 2 inches, 3 inches, and 4 inches, with existing depth treated as 0. Those three depths are the extension range. A top-up changes Bags to buy and leaves the chart on the bare-soil comparison.

Worked example

These are the numbers already filled in when the page opens.

A rectangle 20 ft long and 10 ft wide, 3 inches deep, existing depth 0, bag size 2 cubic feet.

1. Area = 20 × 10 = 200 sq ft.
2. Depth to add = 3 − 0 = 3 inches.
3. Cubic feet = 200 × 3 ÷ 12 = 50.
4. Cubic yards = 50 ÷ 27 = 1.851851…, shown as 1.85.
5. Bags to buy = 50 ÷ 2 = 25 exactly, so the count stays 25.
6. Bags in one yard = 27 ÷ 2 = 13.5.
7. Left clear at trunk = 0, because this shape is a rectangle.

The chart reads 17 bags at 2 inches, 25 bags at 3 inches, and 34 bags at 4 inches. The 3 inch point matches Bags to buy, because the bed starts bare.

The same bed as a top-up. Existing depth 2 inches, and you still want 3 inches when you are done.

1. Depth to add = 3 − 2 = 1 inch.
2. Cubic feet = 200 × 1 ÷ 12 = 16.666…, shown as 16.67.
3. Cubic yards = 16.666… ÷ 27 = 0.61728…, shown as 0.62.
4. Bags to buy = 16.666… ÷ 2 = 8.333…, which rounds up to 9 bags.

The chart still reads 17, 25, and 34, because it ignores the mulch already in the bed.

A tree ring. Outer diameter 6 ft, trunk clearance 0.33 ft, depth 3 inches, existing depth 0, bag size 2 cubic feet. The outer radius is 3 ft.

1. Bare circle = π × 0.33² ≈ 0.342119 sq ft, shown as 0.3.
2. Mulched area = π × (3² − 0.33²) ≈ 27.932214 sq ft, shown as 27.9.
3. Cubic feet uses the full area, before that rounding: 27.932214 × 3 ÷ 12 ≈ 6.983054, shown as 6.98.
4. Cubic yards ≈ 0.258632, shown as 0.26.
5. Bags to buy ≈ 6.983054 ÷ 2 ≈ 3.49, which rounds up to 4 bags.

The chart for that ring reads 3 bags, 4 bags, and 5 bags at 2, 3, and 4 inches.

How to read your results

Bags to buy is the shopping line. It is a whole number, rounded up. Cubic yards is the line for a bulk delivery, rounded to the nearest hundredth. Cubic feet is the same volume, also to the nearest hundredth. Trailing zeros are dropped, so 1.00 shows as 1, 3.70 shows as 3.7, and 13.50 shows as 13.5.

Bags to buy uses the full cubic feet, before that display rounding. A job that needs 16.666… cubic feet shows 16.67 cubic feet and 9 bags, because 8.333 bags rounds up.

Bags in one yard answers “how many bags are in a yard” for the bag size you typed. At 2 cubic feet per bag it is 13.5. It does not round up. A job that is exactly one cubic yard (27 cubic feet) at that bag size shows 13.5 bags in one yard and 14 bags to buy. Fourteen bags is 28 cubic feet, which covers the yard. Thirteen bags is 26 cubic feet, which is short. One way to get exactly one yard on this form: choose Enter sq ft, type 54, set Depth to 6 inches, and leave Existing depth at 0.

Mulched area is the ground that gets mulch. On a tree ring it is the ring after the trunk gap is removed. Left clear at trunk is 0 for a rectangle, a circle, and a typed area. On a tree ring it is the bare circle.

Depth to add is Depth minus Existing depth. If those two numbers match, you are already at the depth you asked for: cubic feet, cubic yards, and bags are 0. The Illinois sheet says that when enough mulch is already there, refresh it with a rake instead of adding another layer.

The chart is three bag counts for this mulched area at 2, 3, and 4 inches on bare soil. It does not subtract Existing depth. On the opening example, the middle point matches Bags to buy. On a top-up, Bags to buy is the smaller number and the chart stays on the full-depth comparison.

If your supplier sells whole yards only, round Cubic yards up yourself. This tool leaves yards at the hundredth. Read the bag for the cubic feet it holds, and confirm the load with the supplier before you buy.

Assumptions and limits

Every assumption is listed here, with the reason it is in the tool.

• Rectangle, circle, tree ring, or a known area. Most beds are rectangles, so that shape is the start. A circle uses the Length / diameter box because the form has one diameter field. Enter sq ft is there when you already measured the bed. A tree ring is there because people ask how to mulch around a tree.
• Unused boxes stay visible. The form cannot hide length, width, outer diameter, area, or trunk clearance when the shape does not use them. A rectangle skips a bad value in those unused boxes. Switch the shape and the matching boxes start to count.
• Size limits. Length or circle diameter is 1 to 200 ft. Width is 1 to 100 ft. A typed area is 1 to 20,000 sq ft, the same cap as a 200 ft by 100 ft rectangle. Outer diameter is 2 to 30 ft.
• Depth starts at 3 inches and allows 1 to 6. The Illinois sheet recommends 2 to 4 inches on a well-drained site. Composted material is a 2 to 3 inch layer. Coarse mulch is a 3 to 4 inch layer. Three inches sits in the middle of 2 to 4, at the top of the composted range and the bottom of the coarse range. The box is wider than 2 to 4 so you can follow a product label that differs. Layers thicker than the sheet’s range can mat, block water and air, hold too much moisture, and give rodents a place to live.
• Existing depth starts at 0 and allows 0 to 6 inches. Zero means a new layer on bare soil. A top-up subtracts what is already there from the depth you want. The sheet says to check the depth before you add more, and to rake a matted layer when enough mulch is already in place. If Existing depth is higher than Depth, the form stops, because that would mean taking mulch out.
• No extra waste percent. The sheet does not give an overage for spills, settling, or an uneven bed. Bags to buy already rounds up to the next whole bag. Ask the supplier if the bed is lumpy or the mulch is fluffy.
• Bag size starts at 2 cubic feet and allows 0.5 to 3. Two cubic feet is a size people ask about. It was not copied from a bag label in a store. Read your bag and type the cubic feet it actually holds.
• Bags in one yard is 27 divided by that bag size. One cubic yard is 27 cubic feet, from the NIST table. At 2 cubic feet per bag, one yard is 13.5 bags.
• Purchase bags round up. Yards do not. A partial bag still means buying a whole bag. Cubic yards stay at two decimal places so a bulk order can be read closely.
• Trunk clearance starts at 0.33 ft and allows 0.25 to 1 ft. The sheet says to keep mulch at least 4 inches away from the trunk, and to pull mulch back if it is piled against the bark. 0.33 ft is 3.96 inches, just under 4. Type 0.34 or more when you want a full 4 inch gap. The low end of the box, 0.25 ft, is 3 inches, so you can see a tighter gap. That lower end is below the sheet’s 4 inch line. The gap cannot be zero: clearance has to be at least 0.25 ft, and it has to be smaller than the outer radius or the ring has no area left to cover.
• A 6 ft outer diameter is the start for a tree ring. The sheet says to carry mulch out toward the dripline, and that a 3-foot radius ring (6 ft across) is the maximum for most trees. The box allows up to 30 ft across so a wider bed still calculates. A wider ring is a quantity, not a recommendation to exceed that 3-foot radius.
• The chart is a bare-soil comparison at 2, 3, and 4 inches. It uses your area and your bag size. It does not use Existing depth.
• Counts only. There is no price.

Keep mulch off the trunk. Mulch piled against the bark, sometimes called a mulch volcano, can soften the bark and lead to insects, disease, and roots that girdle the trunk. The sheet says that directly. This page is not a fire-clearance calculator and not a playground-surface calculator. Playground depth and fire rules are separate questions. Confirm the order with your supplier, follow the product label, and follow local rules.

FAQ

How many bags of mulch are in a yard?

As many as 27 divided by the cubic feet in your bag. One cubic yard is 27 cubic feet. At the starting 2 cubic foot bag, one yard is 13.5 bags. Bags in one yard shows that 13.5. Bags to buy rounds the whole job up, so a job that is exactly one yard shows 14 bags to buy.

How deep should mulch be?

The Illinois Extension sheet recommends 2 to 4 inches on a well-drained site. Composted material is 2 to 3 inches. Coarse mulch is 3 to 4 inches. The depth box starts at 3 inches and allows 1 to 6, so a different product label still fits. Thicker layers can mat and harm the planting. This page passes on that range. It does not pick a depth for a special soil or climate.

How deep should mulch be around trees?

Use the same 2 to 4 inch layer, and keep it off the trunk. The sheet says at least 4 inches away from the bark, and it says a 3-foot radius (6 ft across) is the maximum ring for most trees. Choose Tree ring, leave Outer diameter at 6 or type your own, and set Trunk clearance. The box starts at 0.33 ft (3.96 inches). Type 0.34 or more for a full 4 inch gap. Do not pile mulch against the trunk.

How much mulch is in a bag?

Read the bag. Bag size is the cubic feet printed on it. The box starts at 2 and accepts 0.5 to 3. Two cubic feet is a size people ask about. It is not copied from a manufacturer’s label.

How many 2 cu ft bags of mulch are in a yard?

27 ÷ 2 = 13.5 bags. Fourteen of those bags hold 28 cubic feet, which is a little more than a yard. Thirteen hold 26 cubic feet, which is short of a yard. Set Bag size to 2 and read Bags in one yard. It shows 13.5.

How much mulch prevents weeds?

The Illinois sheet lists weed control as one benefit of mulch, and it recommends a 2 to 4 inch layer. It does not give a depth that guarantees a weed-free bed. More than 4 inches can harm plants. This tool counts material for the depth you enter. A weed-free bed is not one of the results.

Bags for common bed sizes

The table below is the printed companion to the chart. Every row is bare soil, a 2 cubic foot bag, and Existing depth 0. Bags are rounded up. Cubic yards are shown the way the tool shows them, with trailing zeros dropped.

Bed	2 inches	3 inches	4 inches
100 sq ft	0.62 cu yd, 9 bags	0.93 cu yd, 13 bags	1.23 cu yd, 17 bags
200 sq ft	1.23 cu yd, 17 bags	1.85 cu yd, 25 bags	2.47 cu yd, 34 bags
300 sq ft	1.85 cu yd, 25 bags	2.78 cu yd, 38 bags	3.7 cu yd, 50 bags

The 200 sq ft row at 3 inches is the worked example: 1.85 cubic yards and 25 bags. The 100 sq ft row at 2 inches is the same 16.67 cubic feet as the top-up example, so it is also 9 bags.

The live chart redraws the 2, 3, and 4 inch bag counts for whatever area and bag size you enter. A tree ring’s chart uses the ring after the trunk gap is removed. A top-up changes Bags to buy and leaves the chart on the bare-soil counts. For either of those, use the form. This table cannot show them.