Concrete Calculator Guide: Cubic Yards, Bag Counts & Costs
How to calculate concrete for any pour — slabs, footings, columns, and steps. Includes ready-mix vs. bagged cost comparison, waste factor explanation, and step-by-step examples.
Concrete is sold by the cubic yard when ordering ready-mix and by the bag for small pours. Converting between square footage, thickness, cubic feet, and cubic yards is where mistakes happen — and running short on a pour creates a cold joint, which is a structural defect that costs far more to fix than ordering extra concrete upfront.
How Concrete Is Sold
Ready-mix concrete is ordered by the cubic yard (27 cubic feet). A standard truck carries 9–11 yards. Most suppliers charge a short-load fee for orders under 3–5 yards — often $50–$150 plus a higher per-yard rate. Ready-mix comes in standard PSI mixes:
| Mix Strength | Common Use |
|---|---|
| 2,500 PSI | Sidewalks, patios (light foot traffic) |
| 3,000 PSI | Standard residential slabs, driveways |
| 3,500 PSI | Driveways with heavy vehicle traffic, footings |
| 4,000 PSI | Commercial floors, structural slabs, high-load footings |
Bagged concrete comes in 40lb, 60lb, and 80lb bags. An 80lb bag yields approximately 0.60 cubic feet (roughly 45 bags per cubic yard). Bagged concrete is practical up to about 1–2 cubic yards — beyond that, the labor of mixing becomes prohibitive and ready-mix is almost always cheaper per yard.
Ready-Mix vs. Bagged: When to Use Which
| Volume | Recommendation | Why |
|---|---|---|
| Under 0.5 yd³ (<14 cu ft) | Bags | Short-load fees make ready-mix expensive |
| 0.5 – 1.5 yd³ | Either (compare costs) | Depends on your short-load fee structure |
| Over 1.5 yd³ | Ready-mix | Labor savings and consistent quality |
| Any finish-critical pour | Ready-mix | Consistent water-cement ratio, fiber additive options |
| Fence posts and deck footings | Bags | Small quantities, easy access, fast-setting bags available |
How to Calculate Concrete Volume
The formula is the same for all pour types:
Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft)
Then convert to cubic yards: divide by 27.
The most common mistake: thickness is usually given in inches. Always convert to feet first.
- 4 inches = 4 ÷ 12 = 0.333 ft
- 6 inches = 6 ÷ 12 = 0.500 ft
- 12 inches = 12 ÷ 12 = 1.000 ft
Example — 20×20 garage slab, 4" thick:
20 × 20 × (4÷12) = 20 × 20 × 0.333 = 133.3 cu ft ÷ 27 = 4.94 cubic yards
For round columns and fence posts: Volume = π × radius² × height
A 12" diameter post hole 3 ft deep: π × 0.5² × 3 = 2.36 cu ft = 0.087 yd³
Enter your pour dimensions and the calculator handles all the unit conversions, bag counts, and waste factor automatically.
Why Waste Factor Matters — and What to Use
Concrete calculations assume perfectly flat ground, perfectly square forms, and no spillage. Reality adds waste. Standard waste factors by pour type:
| Pour Type | Recommended Waste Factor | Notes |
|---|---|---|
| Simple slab on flat grade | 5–8% | Well-prepared subgrade |
| Standard slab with some variation | 10% | Default for most residential work |
| Rocky or uneven subgrade | 12–15% | Compaction inconsistencies |
| Footings | 10% | Form movement under vibration |
| Columns (round) | 5% | Tube forms are precise |
Never under-order to save money. Running short forces a cold joint — a construction defect. The inspector may require removal and replacement. Always add 10% waste for typical pours.
Concrete Thickness by Application
| Application | Minimum Thickness | Recommended |
|---|---|---|
| Sidewalk (foot traffic only) | 3.5" | 4" |
| Residential patio | 3.5" | 4" |
| Garage floor (passenger cars) | 4" | 5" |
| Residential driveway | 4" | 5–6" |
| Driveway with truck traffic | 6" | 6–8" |
| Continuous footings | 8" | Per engineered plan |
| Basement floor | 3.5" | 4" |
Always verify with local building codes — frost depth and soil conditions affect requirements significantly by region.
Ready-Mix Concrete Pricing by Region (2025–2026)
| Region | Price Range per Cubic Yard |
|---|---|
| Southeast (GA, SC, FL, AL) | $130–$165 |
| Midwest (OH, IN, IL, MI) | $140–$175 |
| South Central (TX, OK, AR) | $135–$170 |
| Northeast (NY, NJ, CT, MA) | $165–$220 |
| Mountain West (CO, UT, AZ) | $145–$185 |
| Pacific Coast (CA, OR, WA) | $165–$230 |
Prices include standard 3,000 PSI mix delivered. Short-load fees apply to orders under plant minimums. Rural delivery surcharges are common.
Ordering Tips for Contractors
- Schedule the pour for mid-morning. Avoid the hottest part of the day — heat accelerates set time and reduces workability.
- Have your subgrade ready before the truck arrives. Compacted base, vapor barrier, rebar, and wire mesh should all be in place. Pump trucks charge by the hour.
- Know your plant's hold time. Most ready-mix is good for 90 minutes from batch. Don't order more than you can place and finish in that window.
- Get a delivery ticket. The ticket shows exact batch time, mix design, water additions, and load size. Keep it for your job records.
- Order slightly more, not less. If you're between 3.8 and 4.0 yards, order 4. The cost of extra concrete is far less than a cold joint repair.
Step-by-Step Examples
Example 1: 24×24 garage slab, 4" thick
- Volume: 24 × 24 × (4÷12) = 192 cu ft
- Net cubic yards: 192 ÷ 27 = 7.11 yd³
- With 10% waste: 7.11 × 1.10 = 7.82 yd³ (order 8 yards)
- Cost at $155/yd³: $1,240
- 80lb bags equivalent: 353 bags at $6.50 = $2,295 — ready-mix wins by $1,055
Example 2: 6 deck footings, 12" diameter × 42" deep
- Volume per footing: π × (0.5)² × 3.5 = 2.75 cu ft
- 6 footings: 16.5 cu ft
- Net cubic yards: 16.5 ÷ 27 = 0.61 yd³
- With 5% waste: 0.64 yd³ = 17.4 cu ft
- 80lb bags: 17.4 ÷ 0.60 = 29 bags at $6.50 = $189
- Ready-mix with short-load fee: ~$220 minimum — bags win at this volume
Add multiple pours, choose ready-mix or bags, set your waste factor — get volume, bag counts, and material cost instantly.