Concrete Slab Calculator
Work out exactly how much concrete a floor, pad or slab needs. Enter length, width and thickness — get cubic yards, cubic metres, bags, weight and an equipment recommendation for your volume.
How to calculate concrete for a slab
The formula
A slab is a rectangular pour, so its volume is length × width × thickness. The trick is keeping units consistent — thickness is usually in inches or millimetres while length and width are in feet or metres. The calculator handles the conversion for you.
- Volume = length × width × thickness.
- A 4" slab = 0.333 ft thick; a 100 mm slab = 0.1 m thick.
- 1 m³ of concrete covers about 10 m² at 100 mm thick.
Worked example
A 30 ft × 12 ft patio slab at 4 inches:
- Area = 30 × 12 = 360 ft².
- Volume = 360 ft² × 0.333 ft = 120 ft³ = 4.44 yd³.
- Add 10% waste → about 4.9 yd³ (3.7 m³).
Order roughly 5 cubic yards (3.8 m³) for this slab.
Sizing, thickness & waste
Recommended slab thickness
- Patios, garden & shed bases: 4" (100 mm).
- Driveways & garage floors: 4–6" (100–150 mm) with reinforcement.
- Workshop / light industrial: 6" (150 mm) minimum.
Don't forget
- Compact and level the sub-base before pouring.
- Add steel mesh or fibre for slabs that carry vehicles.
- Order 5–10% extra so you never run short mid-pour.
What produces this much concrete?
Slabs above a few cubic metres are where on-site batching starts to pay off. Based on your calculated volume:
Small pour — mix on site
Under ~0.5 m³ (≈0.65 yd³) you can mix on site from bags. A portable mixer saves time over hand-mixing once you pass a few bags.
Medium pour — small mixer or ready-mix
From ~0.5–2 m³, a small or self-loading mixer keeps you independent of delivery schedules; otherwise order ready-mix.
Large pour — self-loading mixer or delivery
At 2–10 m³, compare ready-mix delivery against a self-loading mixer or mixer truck if you pour regularly.
On-site batching becomes economical
Above ~10 m³, repeated or remote pours usually cost less with on-site batching than per-load delivery. A mobile or compact plant sets up fast.
Continuous production — dedicated plant
Beyond ~50 m³ per job, a stationary or ready-mix plant delivers the throughput and consistency large projects need.
Frequently asked questions
How do you calculate concrete for a slab?
Multiply the slab's length × width × thickness, keeping units consistent (convert thickness in inches to feet by dividing by 12), then change to cubic yards or cubic metres. Enter your dimensions above and the calculator returns volume, bags, weight and cost automatically.
How do you calculate concrete slab volume?
Slab volume is length × width × thickness. For a 20 ft × 10 ft slab at 4 inches: 20 × 10 × 0.333 = 66.7 ft³, which is 2.47 cubic yards (1.89 m³) before waste.
How thick should a concrete slab be?
Most residential slabs are 4 inches (100 mm). Driveways and garage floors that carry vehicles should be 4–6 inches (100–150 mm) with reinforcement, and industrial floors 6 inches (150 mm) or more.
How much gravel is needed under a concrete slab?
A compacted granular sub-base of about 4 inches (100 mm) is typical under a residential slab, increased to 6 inches on soft ground. Gravel volume uses the same length × width × depth formula — set thickness to your sub-base depth.
How do you calculate square feet for concrete?
Square footage is simply length × width of the slab. Multiply that area by the thickness in feet to get cubic feet, then divide by 27 for cubic yards.
How do you calculate materials for a concrete slab?
Start with the slab volume, then derive what you need: bags of premix (volume ÷ bag yield), or cement, sand and aggregate from a mix ratio. The bag and mix-ratio calculators handle each case.
How much concrete do I need for a concrete pad?
A pad is calculated the same as a slab: length × width × thickness. A 10 ft × 10 ft pad at 4 inches needs about 1.23 cubic yards (0.94 m³) before waste.
How much extra concrete should I order for waste?
Add 5–10% to your calculated volume to cover spillage, uneven subgrade, and over-excavation. For small or irregular pours, 10% is safer because running short mid-pour creates a cold joint.