How the Mulch Calculator Works
Mulch is a layer of material, such as wood chips or bark, that you spread over the soil of a garden bed. It keeps the soil moist, stops many weeds, and makes the bed look tidy. Buying the right amount matters. Too little leaves bare patches, and too much is a waste of money and a heavy load to move. The Mulch Calculator works out the volume of mulch for a bed from its size and the depth of the layer.
What you enter
- The bed: the area if you know it, or the length and width of a rectangle, or the diameter of a round bed.
- The depth of the mulch: in inches or in centimeters. A depth of 2 to 3 inches (5 to 8 cm) suits most beds.
- The number of beds of the same size.
- Extra for settling (optional): mulch sinks and breaks down. Many people add 5% to 10%.
- The bag size, and optionally the price of a bag and the price of a cubic yard in bulk.
How the results are worked out
volume in cubic feet = area in square feet x depth in inches / 12 cubic yards = cubic feet / 27 cubic meters = cubic yards x 0.7646 bags = cubic feet / bag size in cubic feet, rounded up
The depth in inches is divided by 12 to change it to feet, so that the area in square feet and the depth are in the same unit. A cubic yard is a cube that measures 3 feet on each side, which is 3 x 3 x 3 = 27 cubic feet. For a circle, the area is pi x (diameter / 2) x (diameter / 2).
A worked example
A garden bed is 400 square feet, and you want the mulch 3 inches deep.
| Step | Result |
|---|---|
| Volume (400 x 3 / 12) | 100 cubic feet |
| Cubic yards (100 / 27) | 3.7 |
| Cubic meters (3.7 x 0.7646) | 2.83 |
| Bags of 2 cubic feet (100 / 2) | 50 bags |
| Bags of 1.5 cubic feet (100 / 1.5, rounded up) | 67 bags |
| Bags of 3 cubic feet (100 / 3, rounded up) | 34 bags |
The number of bags is worked out from the exact number of cubic feet. Rounding the cubic yards first, as some calculators do, can add a whole bag that you do not need. For 60 square feet at 3 inches, the volume is exactly 15 cubic feet, which is exactly 10 bags of 1.5 cubic feet.
How deep should the mulch be?
| Depth | Use |
|---|---|
| 1 to 2 inches (2.5 to 5 cm) | Topping up a bed that already has mulch. Also fine mulch, and small plants. |
| 2 to 3 inches (5 to 8 cm) | The usual depth for flower beds, shrubs and trees. |
| 3 to 4 inches (8 to 10 cm) | To hold back weeds in a bed with many of them, and for coarse chips. |
| More than 4 inches (10 cm) | Not usually needed. A thick layer can keep the soil too wet, and stop water and air from reaching the roots. |
Do not pile mulch against the trunk of a tree or the stems of plants. Keep a gap of a few inches. Mulch that touches the trunk can hold moisture, and cause rot.
Chart: the mulch for the same bed at different depths
For a bed of 400 square feet. Green is the usual range, and amber and red are thick layers.
1 inch ███████ 1.23 cubic yards 2 inches █████████████ 2.47 cubic yards 3 inches ████████████████████ 3.70 cubic yards 4 inches ███████████████████████████ 4.94 cubic yards 6 inches ████████████████████████████████████████ 7.41 cubic yards
The amount is in proportion to the depth. Going from 3 inches to 6 inches doubles the mulch that you need.
How far does a cubic yard go?
| Depth | One cubic yard covers |
|---|---|
| 1 inch | 324 square feet |
| 2 inches | 162 square feet |
| 3 inches | 108 square feet |
| 4 inches | 81 square feet |
| 6 inches | 54 square feet |
The bags and the bulk amount for the same 400 square foot bed:
| Depth | Cubic yards | Bags of 2 cubic feet |
|---|---|---|
| 1 inch | 1.23 | 17 |
| 2 inches | 2.47 | 34 |
| 3 inches | 3.70 | 50 |
| 4 inches | 4.94 | 67 |
| 6 inches | 7.41 | 100 |
Bags or bulk?
Mulch is sold in bags at garden centers, and in bulk by the cubic yard at landscape suppliers, who may deliver it. Bags are easy to carry, and good for small jobs. Bulk is often much cheaper for larger jobs. The calculator compares the two when you enter your prices. Bulk is usually sold in whole or quarter yards, so it is rounded up to the next quarter of a yard. With example prices of $4.50 for a 2 cubic foot bag and $35 for a cubic yard, for the 100 cubic feet:
| Choice | Amount | Cost |
|---|---|---|
| Bags of 2 cubic feet | 50 bags | $225.00 |
| In bulk | 3.75 cubic yards | $131.25 |
Bags ████████████████████████████████████████ $225.00 Bulk ███████████████████████ $131.25
Bulk saves $93.75 in this example. Check for a delivery fee, and for a minimum order.
Metric measurements
If you choose metric units, you enter the area in square meters, or the sides in meters, and the depth in centimeters. The result is in cubic meters and in liters. One cubic meter is 1.308 cubic yards, and it covers 20 square meters at a depth of 5 cm, or about 13.3 square meters at 7.5 cm.
Tips
- Measure the bed carefully, and split an odd shape into a rectangle and a circle. Enter each as a bed.
- Count the old mulch. If there is already 1 inch, you only need to add enough for the total depth you want.
- Prepare the bed first. Pull weeds, and edge the bed. A layer of mulch does not stop weeds that are already growing.
- Check the bag size on the label. Bags of the same price can hold different amounts.
- Buy a little extra, especially for a bed on a slope, where the mulch can wash away.
- Think about the weight. A cubic yard of mulch can weigh several hundred pounds, and more when it is wet.
What this calculator does not include
- The different kinds of mulch, which have different weights and coverage.
- Delivery fees, taxes and the cost of labor.
- The gaps around trees and plants, which you should not cover. They lower the area a little, so the result is a safe upper amount.
This tool gives an estimate for general information only. Check the amounts with your supplier before you buy.