From bed size to bags: how the math works
Mulch is one of the few landscaping materials where the calculation is exact end to end, because it is bought by volume rather than weight. The volume is simply area times depth, with the depth converted into the same unit as the area:
Volume = length × width × depth (US: ft × ft × in ÷ 12 → ft³, then ÷ 27 → yd³)
From there the purchase units fall out by division: a cubic yard is 27 cubic feet, so it equals 13.5 of the standard 2 cu ft bags or 9 of the larger 3 cu ft bags. A typical 10 × 20 ft bed at 3 inches deep is 50 cubic feet — about 1.85 cubic yards, or 25 bags of 2 cu ft mulch before any allowance. The calculator rounds bags up, because a bed that needs 24.2 bags needs 25 bags.
Pick the depth first
Depth is the whole game in mulching — it drives the quantity linearly, and it is also where most jobs go wrong. The working range is 2 to 4 inches. Around perennials and annuals, 2 inches suppresses weeds without smothering crowns. Shrub and tree beds take the classic 3 inches. Coarse bark or wood chips on paths and open ground can go to 4. Thinner than 2 inches, light reaches weed seeds and the layer breaks down before the season ends; thicker than 4, water and air stop reaching the soil. And whatever the depth in the bed, taper to nothing near stems: the infamous "mulch volcano" piled against a trunk keeps bark permanently damp and invites rot, insects, and girdling roots. Leave a 2–3 inch gap around trunks.
Bags or bulk?
The crossover logic is the same as for gravel, but the threshold sits higher because mulch is light enough to handle. Bags are convenient, stackable, and easy to move through a gate — for anything up to roughly one cubic yard (a dozen or so bags), they are usually the pragmatic choice. Beyond that, a bulk delivery by the cubic yard costs a half to a third as much per unit and skips the plastic. The ledger always shows both figures, so you can compare the bag count against the bulk volume and price the two options at your local yard. One practical note for bulk: a cubic yard of mulch is a wheelbarrow marathon — figure 6–7 realistic loads per yard — so have the pile dropped close to the beds.
Why there is no tonnage row
Unlike the gravel calculator, this tool never converts to weight. Mulch weight swings wildly with moisture — the same yard of shredded hardwood can weigh 400 lb dry or double that after a rainy week — which is exactly why the industry sells it by volume. If a supplier quotes you by the ton, ask for the cubic-yard equivalent; volume is the number your beds actually consume.
Frequently asked questions
How many bags of mulch do I need for 100 square feet?
At 3 inches deep: 25 cubic feet, which is 12.5 → 13 bags of 2 cu ft mulch (or 8.3 → 9 bags of 3 cu ft). At 2 inches, 8.4 → 9 bags of 2 cu ft.
Should I remove old mulch before adding new?
Usually no — rake and fluff the old layer, then top up only to the target depth. Remove old mulch only if it is matted into a water-shedding crust or diseased. Counting the existing layer toward your depth is also the easiest way to cut this year's order in half.
Do I need landscape fabric under mulch?
Under organic mulch, generally no — the mulch layer itself suppresses weeds, and fabric blocks the decomposing mulch from feeding the soil. Fabric earns its keep under inorganic cover like stone, where mixing with soil is the problem.
How often does mulch need topping up?
Organic mulch decomposes — expect to lose roughly an inch a year, faster in hot, wet climates. Most gardeners refresh annually in spring with a thin top-up rather than re-laying the full depth.
Filling the bed before you top it? Get the soil quantity from the topsoil calculator. Edging with stone instead? Size it with the gravel calculator — and if a path or pad is part of the project, the concrete calculator turns the pour into bags.