This concrete calculator estimates how much concrete you need for a slab, footing or round column — in cubic yards for ready-mix delivery, in 80 lb and 60 lb bags for mixing yourself, and as an approximate cost. It adds a waste allowance automatically.
What Is a Concrete Calculator?
A concrete calculator turns your project dimensions into an order quantity. Concrete is sold by the cubic yard (ready-mix trucks) or by the bag (DIY mixing), and both mistakes are expensive: order too little and a cold joint weakens the pour; order too much and you pay for concrete you haul away. This calculator computes the volume, adds a sensible waste allowance, and converts to bags and cost. It is part of our Construction Calculators collection.
Concrete Calculator Example (Verified)
A 10 ft × 10 ft slab, 4 inches thick:
- Volume: 10 × 10 × (4/12) = 33.33 ft³ = 1.23 yd³
- With 10% waste: order 1.36 yd³ of ready-mix
- Or in bags: 56 × 80 lb bags, or 75 × 60 lb bags
- At $180/yd³ ready-mix: about $245 of concrete
Rule of thumb for bags: above roughly 1 yd³, ready-mix delivery usually beats mixing 50+ bags by hand — in time, consistency and often cost once delivery minimums are met. For the slab area itself, our Square Footage Calculator helps with layout, and the Tonnage Calculator covers the gravel base underneath.
How the Calculation Works
- Slabs and footings: volume = length × width × thickness (all in feet; thickness in inches ÷ 12).
- Round columns/post holes: volume = π × (diameter ÷ 2)² × depth.
- Cubic yards = cubic feet ÷ 27.
- Waste: 5–10% covers uneven ground, spillage and over-excavation. Ground is never as flat as the drawing — 10% is the safe default.
- Bag yields: an 80 lb bag of pre-mix yields about 0.6 ft³; a 60 lb bag about 0.45 ft³.
Thickness Guide
- 4 inches: patios, walkways, shed floors, most residential slabs
- 5–6 inches: driveways and garage floors (especially with vehicle loads)
- Footings: typically 8–12 inches thick and below the frost line — local code decides
Related Tools
- Square Footage Calculator — area of the pour
- Tonnage Calculator — gravel and aggregate base
- Construction Calculators — the full construction collection
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick: about 56 bags of 80 lb mix (or 75 of 60 lb), including a 10% waste allowance. That is over 2 tons of dry mix — most people order ready-mix at this size.
How much does a cubic yard of concrete cover?
At 4 inches thick, 1 yd³ covers about 81 square feet. At 6 inches, about 54 square feet.
How much concrete do I need for a 20×20 slab?
At 4 inches thick: 20 × 20 × 0.333 ft = 133 ft³ ≈ 4.9 yd³ — about 5.4 yd³ with a 10% waste allowance. Enter your exact dimensions above to confirm.
Should I really add 10% extra?
Yes for most ground pours. Running short mid-pour creates a cold joint and a second delivery fee; a small surplus is far cheaper insurance.
What is the difference between cement and concrete?
Cement is the powder binder; concrete is the full mixture of cement, water, sand and aggregate. Bags labeled “concrete mix” already contain everything but water.
Is this concrete calculator free?
Yes — free, instant, no sign-up. Prices are estimates; confirm current ready-mix rates and delivery minimums with your supplier.