Concrete Slab Calculator
Concrete slabs show up on patios, garages, driveways, and building foundations. The amount you need depends on surface area and slab thickness. Multiply length × width × thickness, then add roughly 5–10% extra for uneven subgrade, form variation, and pour spillage.
Related: concrete calculator (general box volume), screed calculator for thin topping layers.
Concrete slab
Calculation is based on average density of 2400 kg/m³.
Calculation Results
Slab volume
0.00 m³
Concrete weight
0.00 t
How much concrete is needed for a slab?
Total concrete depends on how large the footprint is and how thick the slab will be after compaction and finishing. The calculator below uses length × width × thickness with your unit selectors, converts internally to meters, and applies 2400 kg/m³ by default for weight planning.
How this calculator works
- Volume = length × width × thickness
- Weight = volume × concrete density
A standard density of 2400 kg/m³ fits many reinforced and non-reinforced slab mixes. All inputs are normalized inside the tool, so you can mix feet, inches, meters, or yards without doing manual conversions first.
Typical slab thickness guide
Different uses call for different thickness:
- 4 inches (≈10 cm) — patios, walkways, light-duty use
- 5–6 inches (≈12–15 cm) — driveways, moderate loads
- 6+ inches (≈15+ cm) — heavy vehicles, structural slabs
Getting thickness right matters as much as getting area right—a thin slab in the wrong spot will not perform like a thicker one.
Where this calculator is used
It is built for flat, rectangular or square layouts such as:
- Patio slabs
- Garage floors
- Shed bases
- Sidewalks and walkways
- Small foundation slabs
Break non-rectangular pours into boxes and add the volumes if your layout is more complex.
Example calculation
For a slab that is:
- 20 ft long
- 12 ft wide
- 4 inches thick
You get roughly 2.27 m³ of concrete and about 5.4 metric tons at 2400 kg/m³. Add a 5–10% buffer so the pour does not stop short if the subgrade or forms vary.
Common mistakes to avoid
- Using the wrong thickness. Even one inch of error across a large area changes volume sharply—measure forms and finished grade carefully.
- Forgetting slope on outdoor slabs. Drainage fall can slightly increase average depth compared to a perfectly level sketch.
- Ignoring space for reinforcement. Rebar and mesh affect how concrete fills the section; keep clearance and cover per your details.
- Skipping sub-base prep. Soft or uneven bases lead to variable thickness and more concrete than a paper estimate.
Practical tips for slab projects
- Compact the sub-base before you pour.
- Use straight, braced form boards to hold thickness.
- Plan control joints to guide cracking.
- On exterior slabs, plan slope and drainage away from structures.
Frequently asked questions
Do I need reinforcement for a slab?
Most slabs benefit from steel reinforcement such as rebar or mesh, especially for load-bearing applications. Follow structural notes on your plans.
Can I pour a slab in sections?
Yes, but joints must be planned so loads and cracking behave as intended—poor sequencing can create weak planes.
Should I order ready-mix concrete?
For larger slabs, ready-mix is usually more consistent and efficient than batching by hand on site.
What happens if I pour too thin?
Thin slabs are more likely to crack and fail under load. Meet minimum thickness for the job, not just the smallest number that clears a budget.