How to estimate concrete for a slab
Compute volume from area × thickness, then add a realistic ordering buffer.
Concrete is typically ordered by volume. For slabs, volume comes from the slab area multiplied by thickness, then converted into cubic yards or cubic meters.
Ordering too little can stall a pour. Ordering too much is usually cheaper than a second delivery. This guide focuses on safe, practical estimating.
The basic formula
Start with the slab dimensions, convert thickness into the same unit system, then compute volume and convert to the unit your supplier uses.
- Area = length × width (or sum multiple rectangles).
- Volume = area × thickness.
- Convert volume to cubic yards or cubic meters for ordering.
Ordering tips
- Confirm whether thickness is uniform or includes thickened edges.
- Add an ordering buffer (often 5–10%) for uneven subgrade and spillage.
- Ask about minimum load and short-load fees before you order.
- Plan access and timing—late trucks can ruin finishing quality.
Estimate volume quickly
Use the concrete calculator to estimate cubic yards/meters and optional cost.
FAQ
Do I need to add extra for rebar or mesh?
Steel reinforcement does not meaningfully change the concrete volume. The buffer is for site conditions and waste, not rebar.
What thickness should I use?
It depends on load and local requirements. Many sidewalks are around 4 inches, while driveways are often thicker. Check your project specs.
Why does ordering too little cause problems?
You risk cold joints or a second delivery that arrives too late, which can reduce strength and increase finishing issues.
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