How Many Bags of Concrete for a 10x10 Slab?

A 10 × 10 slab is 100 square feet. At 4 inches thick:

1. Thickness in feet: 4 ÷ 12 = 0.333 ft
2. Volume: 10 × 10 × 0.333 = 33.3 cubic feet
3. 80 lb bags: 33.3 ÷ 0.6 = 55.6 → 56 bags
4. Cubic yards: 33.3 ÷ 27 = 1.23 yd³

Fifty-six 80 lb bags is 4,480 lb of material — more than a full pallet, and about $280–$390 at typical bag prices. Ready-mix for 1.25 yards runs roughly $175–$250 in concrete plus a short-load fee of $100–$180, so the totals are close.

The real difference is labour. Fifty-six bags is around 28 mixer batches over several hours, and a slab needs to be placed continuously — stop for too long and you get a cold joint that will crack. For a 10 × 10 pad, most people are better off ordering ready-mix or splitting the pour into sections with control joints designed in.

Four inches is standard for a shed floor, hot tub pad, or patio. Go to 5–6 inches if a vehicle or heavy equipment will sit on it. See how thick a concrete slab should be for the full breakdown.

Hot tubs are the common trap — a filled tub with people in it can exceed 5,000 lb, so 6 inches with reinforcement is the usual recommendation.

• 4–6 inches of compacted gravel under the slab
• 6 × 6 welded wire mesh or #3 rebar on 18 in centres
• Thickened edges of 8–12 inches if the slab carries a wall or load
• Vapour barrier under anything that will be enclosed or finished