Free Tool — Instant Results
Concrete Calculator: Estimate Slab, Footing, Post Hole, and Bag Count Fast
Enter your measurements below and get an instant estimate in cubic yards, cubic feet, and number of bags — no sign-up required.
🏗️ Concrete Calculator
Calculate concrete volume for slabs, footings, walls, and more
📊 Results
Bag Calculator
| Bag Size | Concrete per Bag | Bags Needed |
|---|---|---|
| 80 lb | 0.60 cu ft | 0 |
| 60 lb | 0.45 cu ft | 0 |
| 40 lb | 0.30 cu ft | 0 |
How the Calculator Works
This tool estimates how much concrete you need based on the shape and dimensions of your project. It works for the most common project types — slabs, footings, walls, post holes, and round columns — and instantly converts your volume into both metric and imperial units, plus bag counts.
Enter Your Measurements
Use a tape measure to get the length, width (or diameter), and depth of your project. For slabs, measure the finished surface. For footings and post holes, measure the actual hole you plan to dig. Always measure twice.
Choose the Project Shape
Rectangular shapes like slabs, walls, and footings use the formula: Length × Width × Depth. Round shapes like columns and post holes use the cylinder formula: π × radius² × depth. The calculator handles the math automatically once you pick a shape from the tabs.
Get Volume and Bag Estimates
Your result appears in cubic yards, cubic feet, and liters. The bag count is based on standard yields: an 80 lb bag covers roughly 0.60 cubic feet; a 60 lb bag covers about 0.45 cubic feet; and a 40 lb bag covers around 0.30 cubic feet. These are approximate — always round up when buying.
The waste factor (10% by default) accounts for uneven subgrades, spillage, and slight over-excavation. Most pros add at least 5–10% to any estimate.
🧱 Slab Calculator
Used for floors, shed bases, garage slabs, and general flatwork. Typical residential slabs are 4 inches thick.
🚗 Driveway Calculator
Driveways are usually 4–6 inches thick. Measure full length and width and add extra for waste.
🏡 Patio Calculator
Patios behave like slabs — use Length × Width × Thickness. Most patios are 3–4 inches thick.
🏗️ Footing Calculator
Footings support structures like decks and fences. Multiply total length by cross-section area.
🕳️ Post Hole Calculator
Post holes are cylindrical. Volume depends on diameter and depth of each hole.
🏢 Wall or Column Calculator
Walls are rectangular. Columns are round. Enter dimensions to calculate required volume.
Concrete Volume Formulas
All calculators do the same thing — apply geometry. Here are the three formulas used behind the scenes:
Slab, Footing, and Wall Formula
Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft)
Convert to yards: Volume (ft³) ÷ 27
Example: 10 ft × 10 ft × 0.33 ft = 33 ft³ ÷ 27 = 1.22 cubic yards
Column and Round Post Hole Formula
Volume (ft³) = π × (diameter ÷ 2)² × depth
Convert to yards: Volume (ft³) ÷ 27
Example: 1 ft diameter, 3 ft deep = 3.14 × 0.25 × 3 = 2.36 ft³ = 0.087 cubic yards
How Many Bags of Concrete Do You Need?
Once you have your volume in cubic feet, the conversion to bags is straightforward. Here’s a quick-reference table based on standard yields from major manufacturers:
| Bag Size | Yield per Bag | Bags per Cubic Yard | Bags per Cubic Foot |
|---|---|---|---|
| 80 lb | ~0.60 ft³ | 45 bags | 1.67 bags |
| 60 lb | ~0.45 ft³ | 60 bags | 2.22 bags |
| 40 lb | ~0.30 ft³ | 90 bags | 3.33 bags |
For projects over 1 cubic yard, most contractors use ready-mix concrete delivered by truck. It’s more economical and consistent for large pours. Bagged mix is best for small jobs — footings, post holes, repairs, and patches under roughly 1–2 yards total.
Always buy a little more than your estimate. Concrete bags don’t go to waste if you store them in a dry place. Running short mid-pour is far more costly than one extra bag.
Concrete Cost Estimate
Material Cost
Bagged concrete mix typically costs between $5 and $10 per 80 lb bag, depending on the brand and retailer. For a 10×10 slab at 4 inches thick, you’d need roughly 56 bags at 80 lbs — that’s around $280–$560 in materials alone, plus tax.
Ready-mix concrete (delivered) usually runs $140–$200 per cubic yard, with minimum delivery charges often starting at 1 yard. Always get a local quote — prices vary significantly by region and fuel costs.
Delivery Cost
Most ready-mix plants charge a short-load fee for orders under 3–5 cubic yards. Expect $50–$100 extra if you’re ordering less than a full truck. Waiting time (short load fees) can also add up if your pour takes longer than expected.
Waste and Overage
Budget for 5–15% extra depending on the complexity of the job. Simple flat slabs need closer to 5–8%. Irregular shapes, steep slopes, and projects with lots of forms may need 10–15%. It’s always cheaper to have leftover concrete than to call for a second delivery.
Best Concrete Mix for Your Project
Standard Slab Mix (General Purpose)
A standard 4,000 PSI mix works well for most residential slabs, driveways, patios, and footings. Look for bags labeled “general purpose” or “structural” concrete. These are typically a sand, gravel, and cement blend — just add water.
Fast-Setting Concrete
For fence posts and mailbox bases, fast-setting concrete is a good choice. You pour it dry into the hole, add water, and it sets in 20–40 minutes. You don’t need to mix it first — which saves time and effort on smaller installs.
Mortar and Specialty Mixes
Mortar is used for setting pavers, bricks, and stone — not structural concrete work. If you’re laying a patio with individual pavers, you want a mortar or polymeric sand, not poured concrete. Crack-resistant mixes are available for high-traffic areas and driveways that experience freeze-thaw cycles.
Common Mistakes to Avoid
Concrete Calculator FAQs
Recommended Tools and Materials
Having the right tools before you pour makes the job safer and produces a better finish. Here’s what you’ll typically need:
- Concrete mix bags — 80 lb bags for efficiency, 60 lb bags if you need lighter handling
- Wheelbarrow — essential for moving mixed concrete or dry bags
- Mixing tub or electric mixer — hand mixing works for small jobs; a rented mixer is worth it for anything over 5–10 bags
- Measuring tape — measure twice, order once
- Wood forms and stakes — contain the pour and set the edge lines
- Level — check your forms before pouring
- Screed board — drag across the top to flatten the surface
- Bull float or trowel — smooth the surface after screeding
- Edger — rounds the edges to prevent chipping
- Concrete control joint tool — cuts relief lines to control cracking
Final Planning Checklist
- Double-check all measurements before ordering
- Convert depth to decimal feet (e.g., 4 in. = 0.33 ft)
- Add at least 10% waste to your total volume
- Decide: bagged mix or ready-mix delivery?
- For ready-mix: schedule delivery and have crew and tools ready
- Prepare your subgrade or gravel base before the concrete arrives
- Have all forms, stakes, and tools in place before you start mixing
- Plan your curing: keep the surface moist for at least 7 days
- Check local permits — some projects require a building permit