Concrete Calculator — Bags & Cubic Yards
Running short mid-pour ruins a slab. Enter your dimensions for exact cubic yards and 40/60/80-lb bag counts — plus when to skip bags and call a truck. Free, instant, no signup.
Quick answer
Multiply length × width × thickness (all in feet) and divide by 27 for cubic yards. A 10×12 ft patio at 4 inches thick needs about 1.6 cubic yards — order 2. Add 5–10% for uneven ground.
Standard: 4" for patios and sidewalks, 5–6" for driveways. Measure in a few spots — ground is never perfectly level.
Your concrete estimate
Concrete needed
1.23
cubic yards
- Volume
- 33.3 cu ft
- 80-lb bags
- 56
- 60-lb bags
- 75
- 40-lb bags
- 112
Over a cubic yard? Call a ready-mix truck — bags get expensive and exhausting fast.
How the calculation works
- Volume in cubic feet: length × width × (thickness in inches ÷ 12). A 10×10 ft slab at 4" = 33.3 cu ft.
- Cubic yards: cubic feet ÷ 27. Ready-mix is ordered by the yard — that's the industry's unit.
- Bag counts: cubic feet ÷ yield per bag. An 80-lb bag yields ~0.6 cu ft, 60-lb ~0.45 cu ft, 40-lb ~0.3 cu ft.
- Round up every bag count. A cold joint from running short is a permanent weak seam.
Bag yield reference
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 80-lb | 0.60 | 45 |
| 60-lb | 0.45 | 60 |
| 40-lb | 0.30 | 90 |
Rule of thumb: if your calculator result tops 1 cubic yard, price a ready-mix delivery before buying bags.
Tips from the pros
- Over-excavate, then compact. A firm, well-compacted gravel base matters more than extra concrete.
- Measure depth in several spots. Ground dips — the deepest spot decides how much you actually need.
- Have help lined up. Concrete waits for no one; a 1-yard bag mix is a full day for two people.
- Cure it right. Keep the slab damp for at least 3–7 days — fast drying causes surface cracks.
- Cut control joints every 8–10 ft while the concrete is fresh, so cracks happen where you choose.
Common questions
Multiply length × width × thickness (in feet) for cubic feet, then divide by 27 for cubic yards. Example: a 10×10 ft slab at 4" thick = 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cubic yards. Order or buy for 1.25+ yards to be safe.