Concrete Calculator
Calculate how much concrete you need for any project — slabs, footings, post holes, walls, and steps. Get volume in cubic yards, cubic feet, and bag count with waste factor included.
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Enter your values above to see the results.
Tips & Notes
- ✓Always round up to the nearest 0.25 yd³ when ordering ready-mix — concrete cannot be returned, and a second truck delivery costs as much as the first. Order slightly more than you calculate.
- ✓Add the waste factor after calculating the exact volume, not before. Calculate clean volume first, then multiply by 1.05–1.15 depending on application type.
- ✓For post holes, dig 6 inches deeper than the frost line for your region. A post hole that is too shallow will heave during freeze-thaw cycles and fail within 3–5 years regardless of concrete quality.
- ✓Use fiber-reinforced concrete (not plain mix) for driveways and exterior slabs — the fibers reduce cracking from temperature expansion and shrinkage without requiring wire mesh.
- ✓Let freshly poured concrete cure for at least 7 days before loading it (foot traffic after 24–48 hours, vehicle traffic after 7 days). Full structural strength requires 28 days of cure time.
Common Mistakes
- ✗Calculating volume in cubic feet but ordering in cubic yards without dividing by 27 — this is the most common concrete ordering error and results in receiving 27 times too much material.
- ✗Forgetting to account for the waste factor entirely — even a flat patio needs 5% extra. Ordering the exact calculated volume almost guarantees running short.
- ✗Using the wrong thickness — specifying 4-inch thickness in feet (entering 4 instead of 0.333) results in an order 12 times larger than needed.
- ✗Not accounting for the subgrade displacement — if you pour concrete into a trench or form, the volume of the form itself is what you fill, not the exterior dimensions of the structure.
- ✗Ordering ready-mix before forms are fully built and inspected — once the truck arrives, the clock starts. Most batch plants allow 60–90 minutes from arrival before the concrete becomes unworkable, and overtime charges apply.
Concrete Calculator Overview
Concrete volume calculation is one of the most consequential measurements in construction — too little stops a pour mid-project creating a cold joint that permanently weakens the structure, while too much wastes expensive ready-mix that cannot be returned. Getting the number right before you call the batch plant is the difference between a clean project and an expensive mistake.
Slab, patio, and footing volume formula:
Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft) | Convert inches to feet: divide by 12
EX: Patio 12 ft × 16 ft × 4 inches thick → Volume = 12 × 16 × (4÷12) = 12 × 16 × 0.333 = 64 ft³ ÷ 27 = 2.37 yd³ → Order 2.5 yd³ (includes 5% waste)Cylinder formula — fence posts, piers, sonotubes:
Volume (ft³) = π × radius² × depth | Radius = diameter ÷ 2
EX: 12-inch diameter post hole, 3 ft deep → r = 0.5 ft → Volume = 3.14159 × 0.25 × 3 = 2.36 ft³ per hole → 6 holes = 14.14 ft³ = 0.52 yd³Concrete coverage and bag yield — quick reference:
| Bag Size | Yield (ft³) | Yield (yd³) | Best For | Mix Setting Time |
|---|---|---|---|---|
| 40 lb bag | 0.30 ft³ | 0.011 yd³ | Small repairs, anchoring | 24–48 hours |
| 60 lb bag | 0.45 ft³ | 0.017 yd³ | Posts, footings, small slabs | 24–48 hours |
| 80 lb bag | 0.60 ft³ | 0.022 yd³ | Large footings, walls | 24–48 hours |
| Ready-mix (1 yd³) | 27 ft³ | 1.0 yd³ | Slabs, driveways, foundations | 28 days full cure |
| Application | Waste Factor | Reason | Minimum Order (yd³) |
|---|---|---|---|
| Flat slab / patio | +5% | Minor spillage, uneven subgrade | 0.5 yd³ |
| Driveway | +8% | Grade variations, edge forms | 1.0 yd³ |
| Foundation walls | +10% | Form blowouts, irregular forms | 1.0 yd³ |
| Stairs / steps | +15% | Complex forms, significant spillage | 0.5 yd³ |
| Fence posts (holes) | +10% | Irregular hole shape | — |
| Columns / piers | +12% | Form imperfections | — |
Frequently Asked Questions
Volume = 10 × 10 × (4÷12) = 33.33 ft³. Add 5% waste: 35 ft³. Using 80 lb bags (0.60 ft³ each): 35 ÷ 0.60 = 58.3, so 59 bags. Using 60 lb bags (0.45 ft³): 35 ÷ 0.45 = 77.8, so 78 bags. At this volume (1.3 yd³), ready-mix concrete from a batch plant is likely cheaper per unit and far less labor-intensive than mixing 59–78 bags by hand.
One cubic yard = 27 cubic feet = 3 ft × 3 ft × 3 ft. A standard concrete mix weighs approximately 4,050 lbs per cubic yard (150 lbs per cubic foot). A full ready-mix truck carries 8–10 cubic yards — 16–20 tons of concrete. A typical residential driveway (2 cars, 600 sq ft at 4 inches) requires approximately 7.5 cubic yards, or about 30,000 lbs of concrete.
Use bagged concrete for volumes under 0.5 cubic yards (about 23 80-lb bags or less). Use ready-mix for volumes above 1 cubic yard. The crossover zone of 0.5–1.0 cubic yards is a judgment call: ready-mix requires a minimum order charge and site access for the truck, while bagged concrete requires significant mixing labor. Most contractors choose ready-mix for any project requiring more than 30 bags.
Walkways and patios (foot traffic only): 3–4 inches minimum. Residential driveways (passenger vehicles): 4 inches minimum, 5–6 inches recommended. Driveways for heavy vehicles or RVs: 6 inches. Garage floors: 4 inches minimum with wire mesh or fiber reinforcement. Structural foundations: 6–8 inches or more depending on load and local codes. Always check local building codes — many jurisdictions specify minimum thickness for permitted work.
Concrete gains strength over time, not all at once. The standard cure schedule: 24–48 hours for foot traffic, 7 days for vehicle traffic and light loads, 28 days for full design strength. Avoid loading concrete heavily before 7 days. Cold weather slows curing significantly — below 50°F, cover the pour with insulating blankets. Hot, dry weather accelerates surface drying, which causes cracking; mist the surface or use curing compound.
Cement is an ingredient in concrete — not a synonym. Concrete is a mixture of cement (typically Portland cement), aggregates (sand and gravel or crushed stone), and water. Cement is the binding agent that hardens when mixed with water through a chemical process called hydration. A typical concrete mix is approximately 10–15% cement by weight, 60–75% aggregate, and 15–20% water. When people say cement driveway or cement slab, they technically mean concrete.