CalculatorMulchHow Much Mulch Do I Need? The 27 Rule Explained
Why does dividing by 27 turn feet into yards?
Because a cubic yard is a cube three feet on every side, and 3 × 3 × 3 = 27 cubic feet. That is the entire origin of the number. Bulk landscape material is priced by the cubic yard, bagged material is labelled in cubic feet, and every mulch question is a conversion between those two units.
Written out, the rule is:
length (ft) × width (ft) × depth (ft) ÷ 27 = cubic yards
The step almost everyone gets wrong is depth. Depth gets quoted in inches while everything else is in feet, so it has to be converted first — inches divided by 12.
| Depth | Depth in feet | Multiply your area by |
|---|---|---|
| 1 inch | 0.0833 | 0.0833 |
| 2 inches | 0.1667 | 0.1667 |
| 3 inches | 0.2500 | 0.2500 |
| 4 inches | 0.3333 | 0.3333 |
| 5 inches | 0.4167 | 0.4167 |
| 6 inches | 0.5000 | 0.5000 |
Miss that division and the answer comes out twelve times too big, which is how people talk themselves into ordering forty yards for a front bed. The mulch calculator handles the conversion for you and shows cubic feet and cubic yards side by side, so the mistake is visible immediately.
How many square feet does one cubic yard cover?
One cubic yard covers 324 square feet at 1 inch deep, and that single number generates all the others — just divide it by the depth in inches. It is the fastest mental shortcut in the whole subject, because most people know their area long before they know their volume.
| Depth | One cubic yard covers | Cubic feet per 100 sq ft | Cubic yards per 100 sq ft |
|---|---|---|---|
| 1 inch | 324 sq ft | 8.33 | 0.31 |
| 2 inches | 162 sq ft | 16.67 | 0.62 |
| 3 inches | 108 sq ft | 25.00 | 0.93 |
| 4 inches | 81 sq ft | 33.33 | 1.23 |
| 5 inches | 64.8 sq ft | 41.67 | 1.54 |
| 6 inches | 54 sq ft | 50.00 | 1.85 |
The 3-inch row is the one worth memorising. 108 square feet per yard covers the standard landscape mulch depth, and it means a yard of mulch is roughly a 10 × 10 patch — smaller than most people picture when they hear “a whole yard of mulch”.
How much mulch do common beds need?
Here is the same formula applied to the bed sizes that actually show up in a suburban yard, all at 3 inches, with bag counts rounded up the way a supplier would ring them up.
| Bed | Area | Cubic feet | Cubic yards | 2 cu ft bags | 3 cu ft bags |
|---|---|---|---|---|---|
| 4 × 25 ft strip | 100 sq ft | 25 | 0.93 | 13 | 9 |
| 10 × 20 ft bed | 200 sq ft | 50 | 1.85 | 25 | 17 |
| 15 × 20 ft bed | 300 sq ft | 75 | 2.78 | 38 | 25 |
| 20 × 20 ft bed | 400 sq ft | 100 | 3.70 | 50 | 34 |
| Front-of-house total | 500 sq ft | 125 | 4.63 | 63 | 42 |
| Whole-property refresh | 1,000 sq ft | 250 | 9.26 | 125 | 84 |
Two things jump out of that table. First, bag counts climb fast — a perfectly ordinary 400 sq ft frontage is fifty bags, which is more than a single trip in most vehicles. Second, the 2 cu ft and 3 cu ft columns are not interchangeable: the same job takes a third fewer bags in the larger size, which matters when you are carrying them. The mulch bags calculator shows all five US bag sizes at once for your own area.
How do you measure a bed that is not a rectangle?
Break it into shapes you can measure, calculate each one, then add the volumes together. Curves are the reason people give up on measuring, and they should not be — a curved bed is almost always a rectangle plus a couple of half-circles, and being 5 percent out on a curve costs one bag.
- Circle (tree rings, round beds): area = 3.1416 × radius². A ring 6 feet across has a radius of 3 ft, so 3.1416 × 9 = 28.3 sq ft. At 3 inches that is 7.1 cubic feet — four 2 cu ft bags.
- Triangle (corner beds): area = ½ × base × height.
- Long curved border: measure the length along the curve and the average width. A border 40 ft long that swells from 3 ft to 5 ft averages 4 ft, so 160 sq ft.
Add the areas first, then apply the depth once at the end. That is fewer operations and fewer chances to fumble a decimal.
Should you add extra for settling and waste?
Add about 5 percent for mulch, and 10 to 15 percent for compost or soil. Mulch itself barely compresses, but it vanishes into the gaps at bed edges, around plant crowns and into any dip in the ground, and you will always spread the last portion thinner than you meant to.
The materials that genuinely settle are the organic soil products. Compost and screened topsoil lose noticeable volume once they are watered in and the air is driven out of them, which is why a raised bed filled level on Saturday is dished by the following weekend. That is covered in how to fill a raised bed.
What if you only know the square footage?
Divide the area by the coverage number for your depth. For 3 inches that is area ÷ 108; for 2 inches, area ÷ 162; for 4 inches, area ÷ 81. A 750 sq ft yard at 3 inches is 750 ÷ 108 = 6.94 cubic yards, so you order 7.
This is also how to check a quote. If a supplier says two yards will do a 400 sq ft bed at 3 inches, the coverage table says 400 ÷ 108 = 3.7 yards, and the quote is short by nearly half. Getting that number right before you pick up the phone is the entire point of the cubic yards calculator, which works the same way for any material.
Do you order bags or bulk?
Compare the per-yard cost, not the sticker price. A cubic yard is 13.5 bags at 2 cu ft, so bags at $4.29 work out to $57.92 per yard, against roughly $38 per yard for bulk plus a one-time delivery fee. The delivery fee is what decides it, and the crossover for those numbers lands near 3.3 yards — meaning the 500 sq ft example above is a bulk job, and the 200 sq ft bed is not.
That comparison is worth doing with your own local prices rather than trusting a rule of thumb; bagged vs bulk mulch works through the break-even properly, and the mulch cost calculator does it with your numbers.
Frequently asked questions
How many cubic yards of mulch do I need for 500 square feet?
4.63 cubic yards at 3 inches deep, which is the depth most beds want. At 2 inches the same area needs 3.09 yards, and at 4 inches it needs 6.17. The arithmetic is 500 × depth-in-feet ÷ 27, so the depth you pick changes the order by a factor of two across the normal 2–4 inch range. That is why the depth question is worth answering before the area question.
Is it better to order in cubic yards or in bags?
Bags win on small jobs and bulk wins on large ones, with the crossover usually between two and four cubic yards once a delivery fee is included. Below about a yard and a half, bags are simpler, cleaner and you can return the unopened ones. Above three yards you are carrying forty-plus bags, and the plastic alone becomes a chore worth paying to avoid.
Should I round up or down when the calculator gives a decimal?
Round up, and to a sensible ordering unit. Bulk yards are typically sold in half-yard increments, so 4.63 yards becomes an order of 5. Bags round up to the next whole bag every time. Running 5 to 10 percent short is the single most common ordering mistake, because beds are never quite the shape you measured and mulch settles into edges and low spots as you spread it.
Does the 27 rule work for soil and gravel too?
Yes — volume is volume, and the formula is identical for topsoil, compost, sand and gravel. What changes is weight, not the calculation. Heavy materials are frequently sold by the ton rather than the yard, so gravel and topsoil orders need a second conversion using density. That step is covered in the density chart guide and built into the cubic yards to tons calculator.
How we calculate this
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