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
Result
Ready-mix needed
5.43
80-lb bags (DIY mix)
244
Estimated concrete cost
$815
Total cubic feet
133.3
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter slab length (ft).
- 2Enter slab width (ft).
- 3Enter thickness (in).
- 4Enter waste factor (%).
- 5Enter ready-mix price per cubic yard ($).
- 6Read your ready-mix needed on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
20x20 is the classic single-car garage or large-patio footprint — 400 square feet of slab. Because that's enough concrete to matter financially, this calculator defaults to 4 inches thick but lets you bump it to 5 or 6 inches if the slab needs to carry a vehicle, then converts the volume into ready-mix cubic yards and a total cost so you can get an accurate number before calling a supplier.
Worked example
Using the values the calculator loads with:
Inputs
- Slab length: 20 ft
- Slab width: 20 ft
- Thickness: 4 in
- Waste factor: 10 %
- Ready-mix price per cubic yard: 150 $
Results
- Ready-mix needed: 5.43
- 80-lb bags (DIY mix): 244
- Estimated concrete cost: $815
- Total cubic feet: 133.3
What each field means
Inputs
- Slab length (ft)
- The slab length used in the calculation, measured in ft. Starts at 20 ft so you have a working example on load.
- Slab width (ft)
- The slab width used in the calculation, measured in ft. Starts at 20 ft so you have a working example on load.
- Thickness (in)
- The thickness used in the calculation, measured in in. Starts at 4 in so you have a working example on load. Accepted range: 2–12 in.
- Waste factor (%)
- The waste factor used in the calculation, measured in %. Starts at 10 % so you have a working example on load. Accepted range: 0–30 %.
- Ready-mix price per cubic yard ($)
- The ready-mix price per cubic yard used in the calculation, measured in $. Starts at 150 $ so you have a working example on load.
Results
- Ready-mix 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.
- 80-lb bags (DIY mix)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Estimated concrete cost
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Total cubic feet
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Is 20x20 big enough for a two-car garage?
It's tight — most two-car garages run closer to 20x24 or 24x24. 20x20 comfortably fits one large vehicle plus storage, or works well as a detached garage pad.
How thick should a 20x20 garage slab be?
5-6 inches is standard for a vehicle-bearing slab, versus 4 inches for foot-traffic-only patios. Thicker slabs use proportionally more concrete, so adjust the thickness field before ordering.
How much does a 20x20 slab typically cost installed?
Materials alone (concrete plus mesh/rebar and forms) commonly run several thousand dollars; labor for excavation, forming, and finishing often equals or exceeds material cost, so use this tool's material number as a starting point, not a full turnkey quote.
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). 20x20 Concrete Slab Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/20x20-concrete-slab-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/20x20-concrete-slab-calculator" target="_blank" rel="noopener">20x20 Concrete Slab Calculator — RevenueLab</a> (2026).</p>
Source: [20x20 Concrete Slab Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/20x20-concrete-slab-calculator) (2026).