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Mulch Calculator

How much mulch a bed needs, in bags and in cubic yards — and whether bagged or bulk is actually cheaper.

Enter the size of the bed and how deep you want the mulch.

Two to three inches suits most beds.

Compare bagged against bulk

Enter both and it works out which is cheaper for this job. Bulk usually wins above about two cubic yards, unless the delivery charge eats the difference.

Estimate. A real-world figure based on the assumptions listed below, which you can change.

How this calculator works

The volume is straightforward: area times depth. Multiply the square footage by the depth in inches, divide by twelve for cubic feet, and by twenty-seven again for cubic yards. Bags are the cubic feet divided by the bag size, rounded up — you cannot buy most of a bag.

What actually decides whether an order is right is the depth, and that is a gardening question rather than a geometric one. Under two inches and weeds come straight through, so you are back doing it again in a season. Over about four inches and you start stopping water and air reaching the roots. Two to three is the usual recommendation, and this warns outside that range rather than quietly costing you a lorry-load of bark you did not need.

Bagged mulch costs several times more per cubic yard than bulk, and the crossover is somewhere around two cubic yards — about thirteen or fourteen bags. Enter both prices and the comparison is done for you, because for a small bed bags genuinely are the right answer and for a large one they are not.

Unlike gravel, mulch is bought by volume rather than weight and does not compact in any way worth modelling. Density is irrelevant to the order, which is why there is no density field here.

The formula

Exactly what happens to your numbers, step by step.

  1. Volume

    cubic feet  = area (ft²) × depth (in) ÷ 12
cubic yards = cubic feet ÷ 27

  2. Bags

    bags = ⌈cubic feet ÷ bag size⌉

    Rounded up. The standard bag is 2 ft³.

A worked example

A 20 ft by 10 ft border at 3 inches deep, with mulch at $4 a bag or $35 a cubic yard.

What you enter

Bed
20 ft × 10 ft
Depth
3 inches
Bag size
2 ft³

The working

Area
200 ft²
Volume
200 × 3 ÷ 12 = 50 ft³
In cubic yards
50 ÷ 27 = 1.85
In bags
50 ÷ 2 = 25 bags
Cost in bags
25 × $4 = $100
Cost in bulk
1.85 × $35 = $65

1.85 cubic yards — 25 bags, or $35 cheaper in bulk

At this size bulk wins by about a third, and that is before the twenty-five empty bags. Below roughly two cubic yards the maths flips, because bulk suppliers usually have a minimum order and a delivery charge that a single bed cannot absorb.

Assumptions

Every result here rests on these. Change your inputs and the result changes with them.

  • The bed is a simple rectangle or circle, or you have entered the area yourself.
  • Mulch is laid at an even depth across the whole area.
  • The bag size you enter is the volume printed on the bag, in cubic feet.
  • Bulk is costed on the exact volume, though most suppliers sell in whole or half yards.

What this cannot tell you

  • It does not know your supplier’s minimum order or delivery charge, and on a small job those often decide the answer more than the price per yard does.
  • Existing mulch is not subtracted. If there is an inch of old bark still down, you need less than a full depth on top — measure what is actually there.
  • Beds are rarely simple shapes. For an irregular border, split it into rectangles and add them, or measure the area another way and enter it directly.
  • It does not account for settling or decomposition. Organic mulch breaks down and thins by roughly an inch a year, which is why beds need topping up rather than redoing.
  • Rubber, stone and other inorganic mulches behave differently — they do not decompose, and stone is sold by weight, so use the Gravel Calculator for those.

Questions people ask

How much mulch do I need?

Multiply the bed area in square feet by the depth in inches, then divide by twelve for cubic feet. A 200 ft² bed at 3 inches needs 50 cubic feet, which is 1.85 cubic yards or 25 standard bags.

How deep should mulch be?

Two to three inches for most beds. Thinner than two and weeds push through; deeper than about four and you risk starving the roots of water and air. Never pile it against trunks or stems — that traps moisture against the bark and causes rot.

How many bags are in a cubic yard?

Thirteen and a half standard 2 ft³ bags, since a cubic yard is 27 cubic feet. In practice you would buy fourteen. That is also roughly where bulk starts being cheaper.

Is bulk mulch cheaper than bags?

Per cubic yard, almost always — often by half or more. But bulk usually carries a minimum order and a delivery fee, so for one small bed bags can still work out cheaper overall. Enter both prices above and it compares them properly.

How often does mulch need replacing?

Organic mulch thins by roughly an inch a year as it breaks down, so most beds need an inch or two topping up annually rather than a full replacement. Measure what is left before ordering — this calculator assumes you are starting from bare soil.

Related calculators

Sources and review

Sources

  • Typical bagged mulch sizes and coverage printed on the packaging; 2 ft³ is the common bag

Methodology version 1.0.0 · Last reviewed