Mulch and Soil Calculator
Cubic yards and bag count for garden beds from length, width, and depth.
What it does
Garden beds eat volume faster than intuition suggests. A modest 20 by 10 foot bed at three inches deep already holds nearly two cubic yards, which is a full pickup load or close to thirty bags. Guessing at the store usually means a second trip.
The tool above sizes the job from bed dimensions and depth. It returns bulk cubic yards including a settling allowance, the equal count in your bag size, and the cost at a bulk price.
Mulch and soil play different roles. Mulch sits on top to hold moisture and block weeds, two to four inches deep. Soil fills the bed itself, and new raised beds need six inches at minimum for shallow roots, twelve for tomatoes and carrots.
Delivery changes the economics. Bulk yards often carry a delivery fee with a one or two yard minimum, while bags cost more per yard but need no scheduling. The crossover usually lands near one cubic yard.
How to use
- Switch the unit system first: imperial for feet and cubic yards, metric for meters and cubic meters.
- Measure the bed length and width in feet at the longest points.
- Pick depth: 2 to 3 inches for mulch refresh, 6 plus for new soil.
- Keep the 10% settling allowance unless the bed was filled this season.
- Compare bulk cost against bags. Past one yard, bulk almost always wins.
- For odd shapes, split into rectangles, run each, and add the yards.
Formula
- length x width
- Bed footprint in feet. For circles, use 3.14 x radius x radius instead.
- depth / 12
- Depth converted from inches to feet, since the other measures are feet.
- waste
- Settling and spillage share as a rate. 10% means multiply by 1.10.
- 27
- Cubic feet in one cubic yard (3 x 3 x 3).
One cubic yard covers about 100 square feet at three inches deep, a handy mental check: a 200 ft2 bed at 3 inches needs about two yards. Bags state cubic feet on the front, so divide the same volume by that number and round up.
Worked example
Example: 20 x 10 ft bed, 3 inches of mulch, 10% settling
Length 20, width 10, depth 3 inches, waste 10%, bulk at $45 per yard, 2 ft3 bags.
- Depth in feet: 3 / 12 = 0.25.
- Raw volume: 20 x 10 x 0.25 = 50 ft3.
- With settling: 50 x 1.10 = 55 ft3.
- Cubic yards: 55 / 27 = 2.04 yd3. Order 2 yards.
- Bags: 55 / 2 = 27.5, so 28 bags.
- Cost: 2.04 x $45 = $91.67 before delivery.
Reference table
| Depth | Area one yard covers | Bags of 2 ft3 per yard |
|---|---|---|
| 2 inches | about 162 ft2 | 14 bags |
| 3 inches | about 108 ft2 | 14 bags |
| 4 inches | about 81 ft2 | 14 bags |
| 6 inches | about 54 ft2 | 14 bags |
| 12 inches | about 27 ft2 | 14 bags |
Limits
- Assumes a flat rectangle of even depth. Slopes and mounds need extra.
- Settling varies by material. Fine compost sinks more than coarse bark.
- Bag volumes are nominal. Fluffed bark bags can read generous and packed soil bags tight.
- Does not price edging, fabric, delivery minimums, or labor.
Common mistakes
- Spreading one inch and expecting weed control. Two inches is the working minimum.
- Piling mulch against trunks and stems, which rots bark. Keep a hand width gap.
- Ordering exact yards with no settling share, then running short after rain.
- Mixing soil types by eye. A yard of dense clay weighs nearly twice a yard of bark.
- Filling deep beds with pure topsoil. Layer cheap filler below and good mix on top.
Try it yourself
Quick checks. Answers sit tucked under each question.
A 12 x 8 ft bed needs 2 inches of mulch. About how many yards with no waste?
12 x 8 x (2/12) = 16 ft3, which is 16 / 27 = 0.59 yd3. Order half a yard or about 8 bags of 2 ft3.
One yard covers 108 ft2 at 3 inches. What covers 300 ft2?
300 / 108 = 2.78 yards, so order 3 yards.
Questions
How deep should mulch be?
Two to three inches for beds, up to four for paths. Past four inches water struggles to reach roots.
How deep for a new vegetable bed?
At least six inches of loose mix, twelve for root crops. Shallow soil stunts carrots and parsnips.
Bulk or bags?
Bulk past about one cubic yard: cheaper per yard and less plastic. Bags win for top ups and upper floor balconies.
How much does a yard of mulch weigh?
Roughly 400 to 800 pounds depending on wood and moisture. Wet hardwood sits at the heavy end.
Will mulch steal nitrogen from soil?
Only at direct contact while fresh wood decays. Aged bark on top causes no trouble for established plants.
How often should I top up?
Yearly, one to two inches. Decomposed mulch simply becomes soil organic matter.