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How Much Mulch for a 20×4 ft Bed at 3 Inches?

A 20 by 4 foot bed at 3 inches deep needs this many cubic yards and bags of mulch. Here is the answer, plus a calculator you can adjust if your measurements are slightly different.

Answer: 0.74 cubic yards

How it changes with depth (in)

Same area, different depth (in):

Depth (in)	2	3	4
Mulch needed	0.49 cubic yards	0.74 cubic yards	0.99 cubic yards
Depth	2 in	3 in	4 in

What a 20×4 ft bed at 3 inches deep needs

A 20×4 ft bed at this depth is a bag job. At 0.74 yd³ there is no point ordering bulk: most yards have a one-yard minimum and charge delivery on top, so 10 × 2 ft³ bags comes out cheaper and leaves no pile sitting on the driveway. It is also light enough to carry in a normal car in two trips.

The bed is 80 ft². At 3 in deep that is 20.0 ft³ of material, which is 0.74 yd³ — 10 × 2 ft³ bags or 7 × 3 ft³ bags. Every number below is worked out from that volume.

How far it goes

Unit	Covers at 3 in deep	Notes
1 cubic yard	108 ft²	the number your supplier quotes in
1 × 2 ft³ bag	8 ft²	about a 8-ft² patch
This bed	80 ft²	0.74 yd³, 10 bags
Edging for the perimeter	51 lin ft	20 ft + 4 ft doubled, plus 5% for corners

The bed measures 20 ft × 4 ft, so the 20 ft sides are the long runs to rake and the 4 ft sides are the ends. Edging the perimeter takes about 51 linear ft of timber or steel strip.

3 in is the depth most landscape advice lands on — deep enough to hold moisture and stop most weed seeds germinating, shallow enough that rain still reaches the soil. It is the right choice for a mixed bed of shrubs and perennials.

If you are buying bags, remember that a 2 ft³ bag looks generous in the store and covers surprisingly little ground. At 3 in deep, one bag does 8 ft² — so a bed of this size needs 10 of them. Buying a couple of spare bags is cheaper than a second trip once you are half-covered.

Bags or bulk?

Option	Quantity	Estimated cost
2 ft³ bags	10 bags	$40
3 ft³ bags	7 bags	$39
Bulk, loose	0.74 yd³	$28 + $65 delivery

At this volume, bags win. You would pay $40 in bags, and a bulk order would still carry a minimum quantity plus delivery — more money for material you have to shovel. Stick with bags.

Costs assume $4 per 2 ft³ bag, $6 per 3 ft³ bag and $38 per cubic yard loose, with a $65 delivery or small-load fee under one cubic yard. Mulch is bulky and local, so freight moves the price more than the material does — always compare a delivered quote.

Getting it from pile to bed

Edge the bed first. Mulch spills into the lawn, and once the grass is growing through it you cannot fix the edge without raking it all back.
Clear weeds and water the ground the day before. Mulch holds moisture in; it does not create it, and it will not kill established perennial weeds.
You are moving 20.0 ft³, which is about 4 heaped wheelbarrows. Pace it properly and it is an afternoon.
Spread with a rake or fork to an even 3 in, working backwards so you never kneel in what you have just spread.
Keep it clear of trunks and stems by a couple of inches — mulch piled against bark rots it — and stop a few inches short of the house so termites have no bridge.
Water it in once to settle it and knock the dust off. Organic mulch fades over a season; that is normal, not a defect.

Common questions

How many bags of mulch for a 20×4 ft bed at 3 inches?

10 bags if you buy 2 ft³ bags, or 7 if you buy the larger 3 ft³ bags. The bed is 80 ft² and needs 20.0 ft³ (0.74 yd³) at 3 in deep. One 2 ft³ bag covers about 8 ft² at this depth.

How many cubic yards is that?

0.74 cubic yards. For reference, one cubic yard spread at 3 in deep covers 108 ft², and a full-size pickup bed holds roughly 2 cubic yards level.

Is it cheaper to buy mulch in bags or by the yard?

At 0.74 yd³ it depends on delivery. Bags come to about $40 in material; bulk is about $28 before delivery and any minimum order. Below about one cubic yard bags usually win; above two, bulk almost always does once you count the handling.

How deep should mulch be?

Two to three inches is the standard for beds, and three to four inches for a fresh bed on bare soil. Do not go beyond four inches — deeper than that and rain cannot reach the soil, roots suffer, and mulch piled against stems causes rot. This page is worked at 3 in.