Mulch Calculator

About this mulch calculator

The mulch question is always the same: area × depth. A 20×4 ft bed at 3 inches deep needs 20 cubic feet — about three-quarters of a cubic yard, or 10 two-cubic-foot bags. The calculator handles the unit conversions and tells you both the bulk yardage and the bag count, so you can price both options.

Three inches is the sweet spot for weed suppression and moisture retention. Less than 2 inches and weeds punch through; more than 4 inches and you risk suffocating shallow roots and inviting voles. Top up annually — mulch decomposes, which is the point, but it means last year's 3 inches is this year's 1.5.

The bulk-vs-bag decision is pure economics. Bagged mulch runs 2–3× the per-yard price of bulk, but bulk has delivery minimums (often 2–3 yards) and needs a driveway spot for the pile. Under ~2 yards, bags usually win on total cost and convenience.

Worked example: mulch for a 20×4 flower bed

A 20×4 ft bed at 3 inches deep: 20 × 4 × (3 ÷ 12) = 20 cubic feet, divided by 27 = 0.74 cubic yards. In bags: 20 ÷ 2 = 10 bags of standard 2-cu-ft mulch.

Here is the buying decision that example forces: 10 bags at roughly $4–$5 each is $40–$50, while three-quarters of a bulk yard delivered often costs $25–$40 plus a delivery fee. The crossover is usually around 5–6 bags — above that, bulk wins even after delivery. One cubic yard equals 13.5 two-cu-ft bags, so keep that conversion handy when comparing the bulk price board against the bag stack.

Buying tips

Depth discipline matters more than brand. Two to three inches suppresses weeds and holds moisture; past four inches you start suffocating roots. And keep mulch 2–3 inches away from stems and trunks — the piled-against-the-bark “mulch volcano” traps moisture against bark and rots the plant. It is the single most common mulching mistake, and it kills more shrubs than underwatering does.

Natural hardwood mulch is usually the best value: it is cheaper than dyed, and it feeds the soil as it breaks down. Dyed mulch holds its color longer in the bag but fades in full sun within a season, so you end up re-mulching for looks rather than for the plants.

How to use

1. Measure each bed's length and width in feet (irregular beds: measure the bounding rectangle and shave 10–20%).
2. Enter your target depth in inches — 3 is standard.
3. Press Calculate for cubic yards and bag count.
4. Price bulk vs. bags: bulk wins above ~2 cubic yards.

Good to know

• Assumes rectangular beds; irregular shapes need the bounding-box adjustment noted above.
• Bag size assumed at 2 cu ft — check your product (1.5 and 3 cu ft bags exist).
• Same math works for topsoil and compost — but soil compacts, so add ~10% for soil beds.
• Bulk mulch (by the yard) beats bags on price once you need more than about 5–6 bags — a yard equals 13.5 two-cu-ft bags and often costs less than 8–10 bagged.
• Keep mulch 2–3 inches away from stems and trunks. Piled-against-the-bark “mulch volcanoes” trap moisture and rot the plant — the #1 mulching mistake.