Free mulch calculator (garden bed)
Find out exactly how many cubic yards of mulch your garden beds need — plus bag count — by entering length, width, and depth.
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Retail mulch: 2 cu ft and 3 cu ft bags. Bags rounded up — buy a few extra.
Results are estimates. Consult a professional.
How the mulch calculator works
Mulch is sold and applied by volume. To find how much you need, the calculator converts your bed's length and width into square feet, factors in the desired depth in inches, and converts the result to cubic yards — the unit most suppliers use for bulk deliveries. If you prefer bagged mulch, it also tells you how many 2 ft³ bags to buy.
Worked example: a 12 × 4 ft garden bed at 3 inches deep
James has a front-yard border bed that is 12 feet long and 4 feet wide. He wants to lay 3 inches of shredded hardwood mulch to suppress weeds for the season. How much should he order?
Step 1 — convert depth to feet: 3 in ÷ 12 = 0.25 ft.
Step 2 — find cubic feet: 12 × 4 × 0.25 = 12 ft³.
Step 3 — convert to cubic yards: 12 ÷ 27 = 0.44 yd³.
Step 4 — bag count: 12 ft³ ÷ 2 = 6 bags of 2 ft³ mulch.
Mulch needed for common garden bed sizes
Use this table to estimate at a glance. All figures are at a 3-inch depth — the middle of the recommended 2–4 inch range. For a 2-inch layer, multiply the yd³ figure by 0.67; for 4 inches, multiply by 1.33.
| Bed size | Area (ft²) | Volume (ft³) | Cubic yards | 2 ft³ bags |
|---|---|---|---|---|
| 4 × 4 ft | 16 | 4.0 | 0.15 | 2 |
| 4 × 8 ft | 32 | 8.0 | 0.30 | 4 |
| 4 × 12 ft | 48 | 12.0 | 0.44 | 6 |
| 4 × 20 ft | 80 | 20.0 | 0.74 | 10 |
| 6 × 20 ft | 120 | 30.0 | 1.11 | 15 |
| 8 × 20 ft | 160 | 40.0 | 1.48 | 20 |
| 10 × 30 ft | 300 | 75.0 | 2.78 | 38 |
All rows use a 3-inch application depth (depth in ft = 0.25). Bag count is rounded up to the nearest whole bag.
When to use mulch and how to apply it well
The right depth matters as much as the right volume. Too little and weeds break through; too much and roots suffocate. Aim for 2 inches in wet climates or for shallow-rooted plants, and 3–4 inches in hot, dry regions where moisture retention is the priority.
- Keep mulch 2–3 inches away from plant stems and tree trunks — piling it against woody tissue traps moisture and invites rot and pests.
- Buy in bulk for large jobs — a cubic yard (27 ft³) of bulk mulch typically costs less than the equivalent number of bags once you exceed about 10–12 bags.
- Add 10% for waste and settling — mulch compresses as it weathers; ordering a little extra avoids a second trip mid-project.
- Apply in spring or fall — spring mulching conserves soil moisture heading into summer; fall mulching insulates roots before the first frost.
- Refresh, don't double-stack — if last year's layer is still intact, rake it before adding new material so the total depth stays in the 2–4 inch range.
How accurate is this mulch calculator?
The math is exact geometry: length × width × depth (converted to feet) ÷ 27 is the precise volume in cubic yards for a rectangular, flat-bottomed bed. If your measurements are accurate, the calculated volume is accurate. Real-world variation comes from the bed itself, not the formula.
Three factors commonly cause the actual amount needed to differ from the calculated figure. First, irregular or curved beds — the calculator assumes a rectangle; an oval or kidney-shaped bed will need slightly less. Second, existing mulch that has not been cleared — if old mulch remains, you only need enough to top it up to the target depth, not the full depth. Third, mulch density varies by type — fine-ground wood mulch compacts more than chunky bark, so a cubic yard of fine mulch covers slightly less depth than one of coarse bark. Treat the result as a reliable starting estimate, and adjust for these factors when you order.
Mulch calculator terms
About this mulch calculator
This calculator runs entirely in your browser. No measurements or personal data are sent to any server — every calculation happens locally the moment you type. There are no sign-ups, ads, or tracking cookies on this page.
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