Concrete Calculator
Cubic yards, bag count, and a sanity check on whether to order ready-mix — for slabs, footings, post holes, and sonotubes. Works for the single 2 cu ft post hole or a full driveway.
How much concrete do I need?
Default is a 10x10 ft slab, 4 in thick — a standard patio.
What you'll need to buy
- Quikrete Concrete Mix — 60 lb bag0.45 cu ft
- #4 Rebar — 20 ft
Product links go to Home Depot search results — pick the best match for your project. Prices shown are rough averages.
How we calculated it
Concrete volume formulas depend on the shape. Every calculation ends with a conversion to cubic yards and bag count:
- Slab: length × width × (thickness in inches ÷ 12)
- Footing (rectangular): length × (width in / 12) × (depth in / 12)
- Post hole / Sonotube (round): π × (diameter in / 24)² × depth ft × count
Once you have cubic feet:
- Cubic yards = cubic feet ÷ 27
- 60 lb bags = cubic feet ÷ 0.45 (each bag yields 0.45 cu ft)
- 80 lb bags = cubic feet ÷ 0.60 (each bag yields 0.60 cu ft)
- Waste factor: +10% standard, +15% for uneven subgrade or complex forms
Ready-mix truck threshold: above 1 cubic yard, a truck almost always wins on price and time. Most suppliers have a 1-yard minimum with a short-load fee for anything under about 3 yards. Call 2-3 local suppliers for same-day pricing — it varies more than you would think.
Concrete bags needed per yard
How many bags to mix yourself to reach various total volumes.
| Volume | 60 lb bags (0.45 cu ft) | 80 lb bags (0.60 cu ft) | Total weight (60 lb) |
|---|---|---|---|
| 1/4 yd (6.75 cu ft) | 15 | 11 | 900 lb |
| 1/2 yd (13.5 cu ft) | 30 | 23 | 1,800 lb |
| 3/4 yd (20.25 cu ft) | 45 | 34 | 2,700 lb |
| 1 yd (27 cu ft) | 60 | 45 | 3,600 lb |
| 1.5 yd (40.5 cu ft) | 90 | 68 | 5,400 lb |
| 2 yd (54 cu ft) | 120 | 90 | 7,200 lb |
| 3 yd (81 cu ft) | 180 | 135 | 10,800 lb |
Above ~1 yd, you are mixing for hours. Most DIYers switch to a ready-mix truck at 1.5-2 yards, accepting a $50-200 short-load fee below a typical 3 yd minimum.
Slab thickness by project type
IRC + industry standards for residential concrete slabs.
| Project | Minimum thickness | Rebar / reinforcement |
|---|---|---|
| Sidewalk | 4" | 6×6 wire mesh mid-depth |
| Patio (pedestrian) | 4" | 6×6 mesh or #3 rebar 16" OC |
| Shed floor (light storage) | 4" | 6×6 mesh or #4 rebar 16-24" OC |
| Shed floor (tractor / heavy) | 5-6" | #4 rebar 16" OC grid |
| Driveway (cars) | 4-5" | #4 rebar 16" OC grid |
| Driveway (RV / truck) | 6" | #4 rebar 12-16" OC grid |
| Garage floor | 4-5" | #4 rebar 16" OC + thickened edge |
| Footings (residential) | 8" min, below frost | 2× #4 rebar longitudinal, #3 ties 12" OC |
Below 4 in any slab cracks under normal residential loads. Rebar does not stop cracking — it holds cracks tight when they inevitably happen.