Foundation · 320 sq ft shed
16x20 shed foundation: how much gravel, concrete or pavers?
A 16x20 shed needs a 18 x 22 ft gravel pad (1 ft border), which is 7.5 tons of crushed stone at 4 in deep or 11.3 tons at 6 in. A 4 in concrete slab takes 4.35 cubic yards, or 196 bags of 80 lb mix; a paver base takes 189 pavers at 16 in.
At 320 sq ft, a 4 in concrete slab or a 6 in compacted gravel pad inside a timber frame is the usual choice, and a wood kit may want concrete piers or blocks under its runners. Every quantity below is computed for a 16 x 20 ft footprint. Change any assumption in the shed foundation calculator.
How much gravel for a 16x20 shed pad?
Crushed 3/4 in stone at about 1.4 tons per cubic yard, plus 10 percent for compaction. The border is how far the pad extends past the shed on each side.
| Border | Pad size | 4 in deep | 6 in deep |
|---|---|---|---|
| None (flush) | 16 x 20 ft | 4.35 yd³ / 6.1 t | 6.52 yd³ / 9.1 t |
| 1 ft (recommended) | 18 x 22 ft | 5.38 yd³ / 7.5 t | 8.07 yd³ / 11.3 t |
| 2 ft | 20 x 24 ft | 6.52 yd³ / 9.1 t | 9.78 yd³ / 13.7 t |
Frame it with 80 linear ft of treated 4x6 or 6x6 timber, and lay about 436 sq ft of weed fabric (10 percent overlap) under the stone. See how to build a gravel shed pad for the order of work.
How much concrete for a 16x20 shed slab?
| Thickness | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 in | 4.35 | 196 | 261 |
| 6 in | 6.52 | 294 | 392 |
Includes 10 percent waste. At 196 bags, get a ready-mix quote: short-load fees still usually beat hand mixing this much. Slabs also need a compacted gravel sub-base and, in most towns, a permit or inspection once they pass a size threshold.
How many pavers or blocks for a 16x20 shed?
| Base | Quantity | Notes |
|---|---|---|
| 16 x 16 in pavers | 189 | over 4 in gravel and 1 in sand, 5% spare |
| 12 x 12 in pavers | 336 | same base, more joints to level |
| Concrete deck blocks | 30 | 5 runners, a block at each end and every 4 ft or less |
| Anchors | 8 | corners plus one per 6 to 8 ft of long wall |
What does a 16x20 shed foundation cost?
Material only, at $45 per ton of stone, $6.50 per 80 lb bag and $4 per 16 in paver. Local prices vary a lot, and delivery minimums often matter more than the per-ton price.
| Foundation | Materials |
|---|---|
| 4 in gravel pad (1 ft border) | $339 |
| 4 in concrete slab (bagged) | $1,274 |
| 16 in pavers on gravel and sand | $1,090 |
Foundation gear for a 16x20
Grid kits, weed fabric and the tools that make a pad flat.
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Do you need a permit for the foundation?
The shed itself is over every common exemption, so plan on a permit. Concrete slabs sometimes trigger their own review even when the shed does not, and utility lines matter before any digging: call 811 a few days before you excavate or drive long anchors. Check your local code.
Anchor it the day you finish
About 8 anchors for this footprint.
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Frequently asked questions
How many bags of concrete for a 16x20 shed slab?
A 4 in slab for a 16x20 shed is 4.35 cubic yards including 10 percent waste. An 80 lb bag yields about 0.6 cu ft, so that is 196 bags of 80 lb mix, or 261 bags of 60 lb. At 6 in thick it rises to 294 bags. Above about 60 bags, ordering ready-mix is usually cheaper and much less work than mixing by hand.
How big should a gravel pad be for a 16x20 shed?
Make the pad 1 ft larger than the shed on every side, so 18 x 22 ft for a 16x20. The border catches roof drip, keeps splash off the walls and gives you a level surface to stand on while assembling. Some kit manuals ask for 2 ft; at a 2 ft border the pad is 20 x 24 ft and takes 9.1 tons at 4 in.
How deep should gravel be under a 16x20 shed?
Four inches of compacted 3/4 in crushed stone is standard on firm, well-drained soil: 7.5 tons for this size. On clay, soft or wet ground go to 6 in (11.3 tons) and compact in two lifts. Pea gravel rolls underfoot and does not lock together, so avoid it for the structural layer.
How many anchors does a 16x20 shed need?
At minimum, one anchor at each of the four corners. Along the 20 ft walls, add one for every 6 to 8 ft of length, which gives about 8 anchors for a 16x20 in a typical yard, more in open or coastal sites. Use earth auger anchors on gravel or soil and wedge anchors into a concrete slab, and follow the kit's own anchoring instructions.
This page is researched guidance, not professional engineering, legal or building-code advice. Sizes, loads and permit rules vary by product and by town: check your local code and the kit manufacturer's instructions before you build.