What this calculator does
Find out how much mulch your garden bed needs, how many bags that is, and what it will cost. Mulch is sold two ways. A landscape yard sells it loose by the cubic yard (or cubic meter). A garden center sells it in bags marked with how much they hold. Either way, the amount you need depends on two things: the size of the bed, and how thick a layer you want.
Describe the bed as a rectangle, or as a circle like a ring around a tree. Then enter the depth, the size of the bags you're looking at and their price. You get the amount for a bulk order and the number of bags, rounded up to a whole bag. You also see the area one bag covers and both costs. A table shows how many bags the same bed would take at other depths.
Where it's used
Why this math matters outside the classroom:
- Garden beds. A 20 × 4 ft bed mulched 3 in deep needs 20 ft³, which is about 0.74 cubic yards.
- Bags or bulk. That same 20 ft³ is 10 bags of 2 ft³, which helps decide whether a bulk delivery is worth it.
- Tree rings. A ring of mulch 6 ft across and 3 in deep around a young tree holds about π × 3² × 0.25 ≈ 7.1 ft³.
- Play areas. Schools work out the cubic yards of wood chips needed to cover a play area to a chosen depth.
- Inches to feet. Students change a depth of 3 in to 0.25 ft before multiplying, then divide cubic feet by 27 for cubic yards.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.G.A.2, 6.RP.A.3.d, 7.G.B.4. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose US units (feet, inches, cubic-foot bags) or metric units (meters, centimeters, liter bags).
- Pick a rectangle or a round bed, and enter its measurements.
- Enter how deep you want the mulch once it's spread, and the amount printed on one bag.
- Enter the bag price and a bulk price per cubic yard (or meter), or clear them to skip the cost.
- Read the bulk amount for a delivery, the bag count and the two costs. Open the depth table to compare thinner or thicker layers.
What the inputs mean
- Bed diameter
- For a ring around a tree, measure the full width of the ring. The calculator ignores the small space the trunk takes up, which leaves you a little spare.
- Mulch depth
- How thick the layer is once it's spread. A thin layer under 2 inches does little to stop weeds. Piling it much deeper than 4 inches can keep air and water from reaching the soil.
- Bag size
- The amount printed on the bag, usually 2 or 3 cubic feet in the US. Check the label, because bag sizes vary between products.
- Price per bag (optional)
- The starting prices ($4 a bag, $35 a cubic yard in bulk) are stand-ins so you see a first result. They aren't market prices. Enter what your store or yard charges.
- Bulk price per cubic yard (optional)
- Landscape-yard price, before delivery. Clear it to skip.
- Bulk price per cubic meter (optional)
- Landscape-yard price, before delivery. Clear it to skip.
Formula and method
area = length × width or area = π × (diameter ÷ 2)² volume = area × depth cubic yards = cubic feet ÷ 27 bags = volume ÷ bag size, rounded up area one bag covers = bag size ÷ depth bag cost = bags × price per bag; bulk cost = cubic yards × price per yard 1 ft³ = 0.028316846592 m³; 1 m³ = 1,000 L
In the formulas, π (pi) is about 3.14, and the diameter is the full width of a round bed. The calculator treats the bed as a flat layer that's the same thickness everywhere. So the volume (the amount of space the mulch fills) is the area times the depth. You enter the depth in inches or centimeters. The calculator divides it by 12 or 100 first, so every measurement is in feet or meters.
In the US, bulk mulch is sold by the cubic yard, which holds 27 cubic feet. The bag count is the volume divided by the bag size, rounded up, since you can't buy part of a bag. The calculator first clears away tiny rounding leftovers from computer math. That way, an exact answer such as 4 bags is never pushed up to 5.
Worked example
A border bed is 20 ft long and 5 ft wide. You want 3 inches of mulch, and the store sells 2 ft³ bags.
- Area: 20 × 5 = 100 ft².
- Volume: 100 × (3 ÷ 12) = 25 ft³.
- Cubic yards: 25 ÷ 27 = 0.93 yd³.
- Bags: 25 ÷ 2 = 12.5, which rounds up to 13 bags. At 3 inches deep, each bag covers 2 ÷ 0.25 = 8 ft².
- Cost in bags at $4 each: 13 × $4 = $52. In bulk at $35 per cubic yard: 0.926 × $35 = $32.41 before delivery.
Result: The bed needs about 0.93 cubic yards (0.71 m³) of mulch. That's 13 bags of 2 ft³ for $52, or about $32 of bulk mulch plus delivery.
Understanding the result
For small beds, bags are usually easier. Once you need dozens of bags, compare the bag price with a bulk delivery of the cubic-yard amount. Loose mulch is often far cheaper per yard.
Fresh, fluffy mulch settles over the first few weeks. If you want the layer to stay near your chosen depth, it's sensible to buy a little more than the result.
Assumptions and limitations
- The mulch is the same depth across the whole bed.
- The amount on the bag label is the loose amount you'll spread.
- Round beds are full circles. The space taken up by a tree trunk or plants isn't subtracted.
- It doesn't handle odd-shaped beds directly. Split them into rectangles and circles, work out each one, and add up the results.
- Mulch packs down and breaks down over time, so topping it up every year or two is normal. That top-up isn't included.
- Delivery yards may sell in set amounts, such as half yards, so round up to what they offer.
- Costs use the prices you enter. Delivery fees, landscape fabric, edging and tax aren't included.
Frequently asked questions
How many bags of mulch make a cubic yard?
A cubic yard is 27 cubic feet. So it takes 13.5 bags of 2 ft³ (call it 14), or exactly 9 bags of 3 ft³. In metric, a cubic yard is about 765 liters, or a little over 15 bags of 50 L.
How deep should mulch be?
Around 2 to 4 inches (5 to 10 cm) is the usual advice for shrub and flower beds. Fine mulch can be on the thinner side, and chunky bark nuggets on the thicker side. Keep mulch pulled back a few inches from tree trunks and plant stems.
How much area does one cubic yard of mulch cover?
At 3 inches deep, one cubic yard covers 108 square feet. That's because 3 inches is 0.25 ft, and 27 ft³ divided by 0.25 ft is 108 ft². At 2 inches it covers 162 ft², and at 4 inches it covers 81 ft².
How many square feet does a bag of mulch cover?
Divide the bag's volume by the depth in feet. A 2 ft³ bag covers 12 ft² at 2 inches, 8 ft² at 3 inches and 6 ft² at 4 inches. A 3 ft³ bag covers half as much again: 18, 12 and 9 ft².
Is bulk mulch cheaper than bags?
Per cubic yard, bulk is usually cheaper. But delivery costs extra, and some yards have a minimum order. Compare the two cost lines using your local prices. For one small bed, bags often win once you add the delivery fee.
Can I use this for wood chips, bark or compost?
Yes. The math only works out a volume, so it works for any loose material spread in a layer. Only the bag size and depth change from one product to another.
How this calculator is checked
Method: Slab volume (area × depth) with exact US–metric volume factors; bag count rounded up after removing floating-point noise. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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