HardscapeMath

How much mulch do I need?

By Jake Huseman. Published . The figures in the charts are computed by the same engine as the mulch calculator when this page is built, so they always agree with it.

Short answer: measure the bed's area in square feet, multiply by the depth in feet, and divide by 27 to get cubic yards. At the usual 3 in depth, a cubic yard covers 108 sq ft and a 2 cu ft bag covers 8 sq ft, so a 250 sq ft bed needs about 2.31 cubic yards or 32 bags before any allowance for waste. Everything else on this page is about getting the area and the depth right, because those two numbers are where estimates go wrong.

Step 1: measure the beds

Mulch goes to the edge of the bed, so measure the bed, not the planting, and measure to where the lawn or the edging begins. Most beds are one of four shapes or a combination of them:

  • Rectangle: length x width. A 20 x 4 ft foundation bed is 80 sq ft.
  • Circle: 3.1416 x radius x radius, where the radius is half the diameter. An 8 ft round bed is 3.1416 x 4 x 4 = 50.3 sq ft.
  • Ring around a tree: the circle's area minus the trunk's. An 8 ft ring with a 1 ft trunk is 49.5 sq ft; the trunk hardly matters, but the calculator subtracts it anyway.
  • Triangle: half the base x the height. A corner bed 12 ft along one edge and 6 ft along the other is 36 sq ft.
  • Anything else: break it into the shapes above, or pace off the length and measure the width at three or four points, average the widths, and multiply. A kidney-shaped bed is a rectangle plus a half circle at each end, within a few percent.

Write every bed down before you add them up; a project with six beds is where the calculator earns its keep, because it keeps the list and draws each shape so a mistyped dimension is obvious.

Step 2: choose the depth

Depth is the number people guess, and it is the one that scales the whole order. Two things decide it: the kind of mulch and what is already on the bed.

  • Fine, shredded mulch (shredded hardwood, cedar, cypress, dyed wood mulch): 2 to 3 in finished. It packs down and mats, so deeper layers shed water and go sour underneath.
  • Coarse mulch (bark nuggets, wood chips): 3 to 4 in. Bigger pieces leave air gaps, so the soil can still breathe under a thicker layer, and it lasts longer before topping up.
  • Compost, leaf mould, fine bark: 1 to 2 in. These are as much soil food as weed control.
  • Around annuals, vegetables and shallow perennials: 1 to 2 in of anything, so the crowns stay clear.
  • Against trunks and stems: none. Pull it back a few inches. A mound of mulch against a trunk, the "mulch volcano", keeps the bark wet and invites rot, borers and rodents.

Then look at what is there. If an inch of last year's mulch remains, a 3 in finished depth needs a 2 in top-up, and that is the depth to use. Measuring the existing layer with a ruler in three or four places takes a minute and routinely saves a third of the order.

Typical depths and densities for the mulches in the calculator come from:

Coverage chart by depth

A cubic yard is 27 cubic feet. Spread 1 in deep it covers 324 sq ft, and each extra inch of depth divides that figure. The chart gives the coverage of a yard and of each bag size, and the yards needed for four common bed sizes, with no waste allowance:

Mulch coverage per cubic yard and per bag, by depth
DepthSq ft per cu ydSq ft per 2 cu ft bagSq ft per 3 cu ft bagCu yd for 100 sq ftCu yd for 250 sq ftCu yd for 500 sq ftCu yd for 1,000 sq ft
1 in32424.036.00.310.771.543.09
2 in16212.018.00.621.543.096.17
3 in1088.012.00.932.314.639.26
4 in816.09.01.233.096.1712.3
6 in544.06.01.854.639.2618.5

Bulk is sold in half-yard steps, so round up: 4.63 yards for 500 sq ft at 3 in is an order for 5. Bag counts are 27 divided by the bag size per yard (13.5 of the 2 cu ft bags, 9 of the 3 cu ft bags), rounded up.

To read it the other way, from bags to depth: a 2 cu ft bag spread over 12 sq ft is 2 in deep; over 8 sq ft, 3 in; over 6 sq ft, 4 in. If a bed "took eight bags last year", that is 16 cubic feet, and dividing by the bed's area tells you how deep it was actually mulched.

Step 3: add an allowance

Beds are not flat, edges get tucked under, a wheelbarrow spills, and a 3 in layer spread by eye lands between 2 and 4 in. Five percent extra is the usual allowance for beds you know; ten percent for new beds, slopes and rough ground. On a bulk order the allowance mostly disappears into the rounding to the next half yard; on a bag order it is one or two extra bags, which are easy to return or to keep for the first bare patch.

Worked example: a front yard

A 30 x 4 ft foundation bed, an 18 x 2 ft border along the walk and an 8 ft ring around a tree with a 1 ft trunk, all at 3 in with 5 percent extra:

Result

BedSq ftCu ft at 3 in
Foundation bed, 30 x 4 ft120.030.0
Walk border, 18 x 2 ft36.09.0
Tree ring, 8 ft across with a 1 ft trunk49.512.4
  • Total area205.5 sq ft
  • With 5% extra53.9 cu ft
  • Bulk2.00 cu yd
  • 2 cu ft bags27
  • 3 cu ft bags18

205.5 sq ft at 3 in is 51.4 cubic feet, 53.9 with the allowance, or 2.00 cubic yards: an order for 2 yards, or 27 of the 2 cu ft bags. This is right at the size where bulk and bags cost about the same and the choice comes down to whether you want a pile on the driveway or a car full of bags.

Bags or bulk?

Bags cost more per cubic foot but have no delivery fee, can be carried through a gate or a house, and go back to the store if unopened. Bulk is cheaper per yard, is usually the same material landscapers buy, and arrives in one pile. The break-even is where the bag total equals the per-yard price plus delivery, which for most suppliers is somewhere around one to two yards. Below a yard, bags; above two, bulk; in between, price it both ways. The calculator shows both totals when you enter a bag price and a per-yard price.

One more consideration is weight. In the calculator's data, hardwood bark mulch is about 15.5 lb per cubic foot, so a yard is roughly 419 lb (0.21 tons) dry and considerably more wet. A pickup can carry a yard or two of dry mulch, which is the one bulk landscape material most people can collect themselves.

How often to top up

Organic mulch rots from the bottom, which is the point. A 3 in layer typically loses about an inch a year, faster in hot, wet climates and for fine mulch. Each spring, rake the surface to break the crust, measure the depth, and add what is missing, usually 1 to 2 in. Every few years, when the bottom layer has turned to black compost, work some of it into the soil before mulching so the bed does not rise against the siding and the trunks.

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