Mulch is light, bulky and sold in two systems at once: bags labelled in cubic feet, and bulk piles measured in cubic yards. The calculator accepts rectangles, circles and rings, then reports yards, bags and a rough weight so you can compare the options.
Measuring beds and tree rings
For straight-edged beds, multiply length by width. For a round bed or tree ring, measure the diameter, halve it for the radius, square it and multiply by 3.14. For a ring around a trunk where you leave a bare gap, work out the big circle and subtract the circle of the gap.
Measure depth after you decide how much old mulch you are keeping. University advice is to count total depth, old layer included. If last year's mulch is still 1 inch deep and you want 3 inches, you only need to add 2.
The formula
Square feet times depth in inches, divided by 324, equals cubic yards. For bags, convert to cubic feet (square feet times depth in inches, divided by 12) and divide by the bag size on the label.
Coverage tables are the same arithmetic run backwards. One cubic yard covers:
- 324 sq ft at 1 inch
- 162 sq ft at 2 inches
- 108 sq ft at 3 inches
- 81 sq ft at 4 inches
Worked example: a border and two tree rings
A planting border 5 ft by 20 ft, plus two tree rings, each a 6 ft circle. Everything gets 3 inches.
- Border: 5 x 20 = 100 sq ft.
- Each ring: 3.14 x 3 x 3 = 28.3 sq ft. Two rings: 56.5 sq ft.
- Total area: 156.5 sq ft.
- Cubic feet at 3 inches (0.25 ft): 156.5 x 0.25 = 39.1 cubic feet.
- Cubic yards: 39.1 / 27 = 1.45.
- Bags of 2 cubic feet: 39.1 / 2 = 19.6, so 20 bags.
- Weight of bulk at roughly 600 lb per yard: about 870 lb.
One and a half yards would cover it with a little to spare. If bags have been sitting out in the rain, expect them to be heavier than the label suggests.
Metric: 14.5 square metres at 7.5 cm deep, about 1.1 cubic metres.
Choosing a depth
Extension services are unusually consistent on this, with one outlier:
- Beds with established shrubs and perennials: 2 to 3 inches. University of Florida recommends 3.
- Seedlings, small vegetables, herbs: 1 inch only, so young stems are not smothered.
- Fine or double-shredded mulch: 1 to 2 inches, because it packs and lets less air through (Penn State).
- Trees: 2 to 4 inches out to the drip line, kept off the trunk. Rutgers advises that the total including old mulch should not pass 3 inches, and 2 inches on heavy or poorly drained soil. Mississippi State Extension suggests 4 to 6 inches, which is the highest figure we found.
When a layer has thinned to under 2 inches, put a thin new one on top. You rarely need to strip the old layer out.
Types of mulch compared
Shredded hardwood is the standard bag and bulk mulch, and it decomposes slowly into the soil, which is why it needs topping up. One cubic yard weighs somewhere around 400 to 700 lb dry.
Pine bark nuggets or chips are lighter, about 350 to 500 lb a yard dry. Coarse pieces let more air reach the root zone than fine ones, which is why the extension advice allows a slightly thicker layer of coarse mulch.
Cedar chips sit between the two on weight (one supplier lists 600 lb a yard).
Rubber mulch is far heavier, roughly 800 to 1,250 lb a yard, and does not break down. It does not feed the soil, and it is more of a play-area surface than a garden mulch. The weight figures come from two sites that agree with each other, but we could not find a manufacturer's own spec.
Compost counts as an organic mulch too, and a yard of it weighs more than bark (supplier figures run 1,000 to 1,400 lb).
Volume, not type, drives how many bags you need. Weight changes only how hard the load is to carry.
Bags or bulk
Fourteen 2 cubic foot bags make a cubic yard (13.5 exactly). A job that needs more than that is worth pricing as a delivery. Our bags vs bulk guide has a formula you can run with your local prices.
Common mistakes
- Mulching to 5 or 6 inches "to be safe". The extension sources above all warn that thick mulch holds water against stems and suffocates roots.
- Piling it against the trunk. The "volcano" is the single most-cited mistake. Pull mulch back a few inches from young stems and 8 to 12 inches from mature trunks.
- Forgetting that old mulch counts. Add to what is there, not on top of a full layer.
- Ordering by area alone. A 100 sq ft bed at 2 inches is 0.6 yards, at 4 inches it is 1.2. Depth doubles the answer.
- Skipping the settling. Organic mulch decomposes and sinks. Plan a top-up about once a year.
Values this calculator uses
Weights are loose, dry, typical figures in US tons (2,000 lb). Your supplier's numbers win if they differ.
| Item | Value | Source |
|---|---|---|
| Shredded hardwood mulch | 0.3 tons / cu yd (range 0.2 to 0.5) | genlawn.com, bestbarkmulch.com, blog.harborsoils.com; confidence: medium |
| Pine bark mulch | 0.225 tons / cu yd (range 0.175 to 0.4) | bestbarkmulch.com, genlawn.com, skynursery.com; confidence: medium |
| Rubber mulch | 0.5 tons / cu yd (range 0.4 to 0.625) | bestbarkmulch.com, genlawn.com; confidence: low |
| 2 cu ft bag | 2 cu ft | sakrete.com, skynursery.com, travis-tx.tamu.edu |
| 1.5 cu ft bag | 1.5 cu ft | sakrete.com, skynursery.com, travis-tx.tamu.edu |
| 3 cu ft bag | 3 cu ft | sakrete.com, skynursery.com, travis-tx.tamu.edu |
Sources
- gardeningsolutions.ifas.ufl.edu: mulch
- extension.msstate.edu: mulch how for the landscape
- yardandgarden.extension.iastate.edu: using mulch garden
- extension.psu.edu: mulching landscape trees
- njaes.rutgers.edu: fs099
- extension.oregonstate.edu: fs 270 raised bed gardening
- genlawn.com: how much does a yard of mulch weigh
- bestbarkmulch.com: mulch weight guide
- skynursery.com: bulk calculator