Mulch / Gravel Calculator
Mulch, soil or gravel for a bed: in bags and cubic yards, with the weight to move.
The bed
The material
2–3 inches suppresses weeds without smothering roots. More than 4 holds too much water against stems.
Depth
2 ft³ is standard
This works out how much mulch to buy for a garden bed. Enter the area and the depth you want, and it returns the volume in cubic yards and the number of bags.
It also compares the cost of bags against bulk delivery, which is usually where the real decision lies.
How much mulch a bed needs
Mulch is sold two ways: in bags measured in cubic feet, and in bulk by the cubic yard. One cubic yard is 27 cubic feet, so a yard equals about 13 or 14 of the standard 2-cubic-foot bags.
The calculation is the same as any volume: area times depth. The only conversion needed is turning the depth in inches into feet by dividing by 12.
Depth is the decision that matters most. Two to three inches is right for most beds. Less than that and weeds come through; more than four and you risk suffocating roots and holding damp against plant stems.
One cubic yard covers about 108 square feet at 3 inches deep, or 162 square feet at 2 inches. Those two numbers answer most mulch questions on their own.
What to enter
- What are you spreading?
- Mulch, soil, compost or gravel. They are all measured the same way, but the sensible depths differ.
- Area
- The size of the bed. For an odd shape, split it into rectangles and add them up.
- Depth
- How thick a layer you want, in inches. Two to three is standard for wood mulch.
- Bag size and price
- Used to compare bagged against bulk. Bags are usually 2 cubic feet.
How deep to spread it
- Wood mulch
- 2 to 3 inches. Enough to suppress weeds and hold moisture without smothering roots.
- Around trees
- 2 to 3 inches, but keep it clear of the trunk. Piling it against bark traps damp and invites rot and pests.
- Compost as a topdressing
- 1 to 2 inches, since it is meant to break down into the soil rather than sit on it.
- Gravel
- 2 to 3 inches for paths and decorative beds.
- New beds
- 3 inches, because it settles and thins over the first season.
What this assumes
The area is roughly level and the depth is even. A sloped bed will drift thin at the top and deep at the bottom.
Bags are assumed to be 2 cubic feet unless you change the size.
How to calculate how much mulch you need
Area times depth, then convert. The same three steps as concrete, with gentler consequences for getting it wrong.
- ÷ 12
- Inches of depth into feet
- ÷ 27
- Cubic feet into cubic yards
Measure the bed. Length times width. Break irregular shapes into rectangles and add them.
Choose a depth. Three inches for most beds, two if you are topping up existing mulch.
Convert and divide. Multiply area by depth in feet, then divide by 27.
Decide bags or bulk. Above about 2 cubic yards, bulk delivery is usually cheaper and saves a lot of lifting.
See a worked example: a 500 square foot bed at 3 inches
- Area
- 500 sq ft
- Depth
- 3 inches
Convert the depth: 3 ÷ 12 = 0.25 ft.
Multiply: 500 × 0.25 = 125 cubic feet.
Divide by 27: 125 ÷ 27 = 4.63 cubic yards.
In bags that is about 63 standard 2-cubic-foot bags, which is why bulk delivery makes sense at this size.
About 4.6 cubic yards, or 63 bags
Frequently asked questions
About 108 square feet at 3 inches deep, or 162 square feet at 2 inches.
Those two figures cover most situations. Halve the depth and you double the coverage, since it is a straight trade.
About 13 and a half of the standard 2-cubic-foot bags, since a cubic yard is 27 cubic feet.
For 3-cubic-foot bags it is 9. Always check the bag, because sizes vary between suppliers and the difference adds up over a large order.
Two to three inches for most beds. That is enough to block light from weed seeds and slow evaporation.
Going deeper is not better. Past about four inches you risk starving roots of air and holding moisture against stems, which causes rot.
Bulk is almost always cheaper per cubic yard, often by a wide margin, but it comes with a delivery fee and arrives as a pile on your driveway.
The rough crossover is around 2 cubic yards. Below that, bags are easier to handle and you can buy exactly what you need. Above it, the savings outweigh the shovelling.
Usually not. Old mulch breaks down into the soil, which is part of the point. Just rake it level and top it up to the depth you want.
Do remove it if it has become matted and water is running off rather than soaking through, or if it is showing mould in a way that concerns you.
Problems people actually run into
Piling mulch against tree trunks
The cone of mulch heaped around a trunk, sometimes called a mulch volcano, is one of the most common and most damaging garden habits.
Bark is not built to stay wet. Held against it, mulch traps moisture, encourages rot and gives pests a covered route in. Keep a few inches clear all the way round so the trunk flare stays visible.
Measuring the bed but forgetting it settles
Fresh wood mulch compacts noticeably over the first few weeks, so a bed spread at exactly 2 inches often looks thin by mid-season.
Spread new beds at 3 inches knowing it will settle towards 2, rather than laying it thin and buying more later.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 3, 2026