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
3.26
80-lb bags (DIY mix)
147
Estimated concrete cost
$538
Total cubic feet
80.0
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter slab length (ft).
- 2Enter slab width (ft).
- 3Enter thickness (in).
- 4Enter waste factor (%).
- 5Enter ready-mix price per cubic yard ($).
- 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
12 by 20 feet is the standard single-car parking pad and a common workshop or carport slab. At 240 square feet it sits right at the point where hand-mixing bags stops making sense and a ready-mix truck becomes the obvious call. This calculator defaults to 12x20 at 4 inches thick, but bump the thickness to 5 or 6 inches if a vehicle heavier than a passenger car will park on it, since load capacity scales with thickness and with the compacted base underneath. It returns cubic yards to order, the equivalent 80-lb bag count, total cubic footage, and the estimated ready-mix cost at your local price.
Worked example
Using the values the calculator loads with:
Inputs
- Slab length: 20 ft
- Slab width: 12 ft
- Thickness: 4 in
- Waste factor: 10 %
- Ready-mix price per cubic yard: 165 $
Results
- Ready-mix needed: 3.26
- 80-lb bags (DIY mix): 147
- Estimated concrete cost: $538
- Total cubic feet: 80
What each field means
Inputs
- Slab length (ft)
- The slab length used in the calculation, measured in ft. Starts at 20 ft so you have a working example on load.
- Slab width (ft)
- The slab width used in the calculation, measured in ft. Starts at 12 ft so you have a working example on load.
- Thickness (in)
- The thickness used in the calculation, measured in in. Starts at 4 in so you have a working example on load. Accepted range: 2–12 in.
- Waste factor (%)
- The waste factor used in the calculation, measured in %. Starts at 10 % so you have a working example on load. Accepted range: 0–30 %.
- Ready-mix price per cubic yard ($)
- The ready-mix price per cubic yard used in the calculation, measured in $. Starts at 165 $ 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
How thick should a 12x20 parking pad be?
Four inches works for passenger cars over a compacted base. Go to five or six inches with rebar if a truck, RV, or trailer will sit on it.
How many yards is a 12x20 slab?
At 4 inches thick it is about 3 cubic yards before waste, roughly 3.3 with a 10% factor — well within a single ready-mix delivery.
Do I need rebar?
Wire mesh or a grid of #3-#4 rebar is cheap insurance on a pad this size, especially in freeze-thaw climates or anywhere vehicles park.
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). 12x20 Concrete Slab Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/12x20-concrete-slab-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/12x20-concrete-slab-calculator" target="_blank" rel="noopener">12x20 Concrete Slab Calculator — RevenueLab</a> (2026).</p>
Source: [12x20 Concrete Slab Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/12x20-concrete-slab-calculator) (2026).