Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Cuts, breakage, and mistakes. 10% is standard.
Result
Concrete needed
3.26
Estimated ready-mix cost
$538
60 lb bags (if mixing)
196
80 lb bags (if mixing)
147
Slab area (sq ft)
240
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter length (ft).
- 2Enter width (ft).
- 3Enter thickness (in).
- 4Enter ready-mix price ($/yd³).
- 5Enter waste allowance (%) — Cuts, breakage, and mistakes. 10% is standard..
- 6Read your concrete needed on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Ready-mix is sold by the cubic yard and short-load fees are brutal, so it pays to order slightly over rather than run out mid-pour. This gives you volume, bag counts if you are mixing on site, and an estimated delivered cost.
Worked example
Using the values the calculator loads with:
Inputs
- Length: 20 ft
- Width: 12 ft
- Thickness: 4 in
- Ready-mix price: 165 $/yd³
- Waste allowance: 10 %
Results
- Concrete needed: 3.26
- Estimated ready-mix cost: $537.78
- 60 lb bags (if mixing): 196
- 80 lb bags (if mixing): 147
- Slab area (sq ft): 240
What each field means
Inputs
- Length (ft)
- The length used in the calculation, measured in ft. Starts at 20 ft so you have a working example on load.
- Width (ft)
- The width used in the calculation, measured in ft. Starts at 12 ft so you have a working example on load.
- Thickness (in)
- The thickness used in the calculation, measured in in. Starts at 4 in so you have a working example on load. Accepted range: 1–24 in.
- Ready-mix price ($/yd³)
- The ready-mix price used in the calculation, measured in $/yd³. Starts at 165 $/yd³ so you have a working example on load.
- Waste allowance (%)
- Cuts, breakage, and mistakes. 10% is standard.
Results
- Concrete needed
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Estimated ready-mix cost
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 60 lb bags (if mixing)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 80 lb bags (if mixing)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Slab area (sq ft)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the concrete slab calculator work?
Ready-mix is sold by the cubic yard and short-load fees are brutal, so it pays to order slightly over rather than run out mid-pour. This gives you volume, bag counts if you are mixing on site, and an estimated delivered cost. The underlying maths is: Volume (yd³) = Length × Width × Thickness (ft) ÷ 27, plus a waste allowance.
What do I need to enter?
5 values: length, width, thickness, ready-mix price, and waste allowance. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the concrete needed result mean?
It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Concrete Slab Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/concrete-slab
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/concrete-slab" target="_blank" rel="noopener">Concrete Slab Calculator — RevenueLab</a> (2026).</p>
Source: [Concrete Slab Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/concrete-slab) (2026).