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How Many Concrete Bags for a 10×8 Slab?
The formula
Length × width × (thickness ÷ 12) = cubic feet, × 1.1 for waste, ÷ bag yield. A 60-lb bag yields 0.45 cu ft; an 80-lb bag yields 0.6 cu ft. A 10×8 ft slab at 4 inch = 26.7 cu ft × 1.1 = 29.3 cu ft → 66 sixty-lb or 49 eighty-lb bags. Check with our Concrete Bags Calculator.
Check the yield printed on your bag rather than trusting a rule of thumb — yields drift between products, and a slab that comes up three bags short mid-pour means an ugly cold joint where the two batches meet. The 10% waste factor covers spillage, an uneven subgrade, and the low corner you will inevitably find once the mix is wet.
Bags by slab size (4 inch thick, with waste)
| Slab | 60-lb bags | 80-lb bags |
| 6×6 ft | 30 | 22 |
| 8×8 ft | 53 | 40 |
| 10×8 ft | 66 | 49 |
| 10×10 ft | 82 | 62 |
| 12×12 ft | 118 | 88 |
The 80-lb bag always looks like the better deal per pound, and it usually is — but 80 lbs is the practical limit for lifting into a mixer all day, so pick the size your back, not just your budget, can handle.
Worked example: 10×10 ft patio at 4 inch
Volume = 33.3 cu ft, plus 10% = 36.7 cu ft. That is 82 sixty-lb bags or 62 eighty-lb bags. Two adults can mix and place about 30–40 sixty-lb bags per day — plan two pours with a cold joint, or order a short-load ready-mix truck past 60 bags.
If you do split the pour, place a control joint exactly where the two days meet. A straight groove there turns a guaranteed crack into a designed one, and the joint reads as intentional to every future eye.
When bags beat ready-mix
- Under ~40 bags: mixing yourself is cheaper
- 40–80 bags: compare short-load fees vs your weekend
- Over ~2 cu yd: ready-mix almost always wins on price and quality
- Footings and posts: bags are ideal, no truck access needed
The real deciding factor is usually access, not cost. If a truck cannot reach the pour without a chute that overhangs a fence or a flowerbed, bags win by default — and a wheelbarrow relay from the street can turn a one-hour pour into a whole day.
Steps that prevent cracks
Compact 4 inch of gravel, set forms level with 1/8 inch per foot drainage slope, and lay rebar or mesh chairs — not lying on the dirt. Pour, screed, float, edge, and broom for grip. Mist-cure under plastic for 7 days; most cracks come from drying too fast, not bad mix.
Add control joints every 8–10 ft in both directions before the surface hardens, and keep the slab away from rigid posts and steps with an isolation joint. Concrete always shrinks; the only choice is whether it cracks where you plan or wherever it likes.
FAQ
- How thick for cars? 4 inch minimum for passenger vehicles on compacted base; 5–6 inch for trucks.
- Quikrete vs Sakrete? Equivalent at equal bag weight — compare yield (cu ft), not brand.
- Cold weather? Below 40°F use hot water mix and insulating blankets, or wait.
- How much water? Only the label amount. Extra water makes the mix easier to move and permanently weaker once cured.
- Can I add bags later? No — a second batch poured onto a set slab is a cold joint, not a repair.
Limits
Bag yields assume exact water ratio — soupy mixes shrink and weaken. Fiber mesh helps surface cracks but never replaces rebar structurally. For slabs over 200 sq ft, price ready-mix before committing. See home improvement.
Cost reality check
At $5 per 60-lb bag, our 10×10 example costs $410 in bags alone plus gravel, forms, and rental. A short-load ready-mix truck often quotes $400–$600 for the same volume placed in an hour. Price both before your back votes — and remember bag concrete has no return policy once the pallet is opened.
Pro tip: stage bags uphill from the forms, cut all bags open before adding water, and mix no more than two bags per wheelbarrow batch. Consistent water ratio batch to batch is what separates a slab that lasts decades from one that dusts and cracks in two summers.