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
cu yd
Truck loads (at capacity)
0.54
Total cubic feet
133.3
Slab area
400
sq ft
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter slab thickness (in).
- 2Enter waste factor (%).
- 3Enter truck capacity (cu yd).
- 4Read your ready-mix needed on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
A 20x20 slab, at 400 square feet, is the classic footprint for a two-car detached garage and represents a large enough pour that ordering strategy really matters: this is squarely in ready-mix truck territory, not bagged concrete, and getting the yardage right avoids either an expensive second short-load delivery or paying for unused concrete. This calculator fixes the area at 400 square feet and lets you set thickness (garage slabs bearing vehicle weight are commonly poured at 4-5 inches over a compacted gravel base, sometimes thicker at the perimeter footing) and waste factor, then returns the cubic yards to order. At this size, a single standard ready-mix truck holds about 10 cubic yards, so the tool also shows how many truck loads your pour would need, which matters for scheduling a pour that has to finish before the leading edge sets up.
Worked example
Using the values the calculator loads with:
Inputs
- Slab thickness: 4 in
- Waste factor: 10 %
- Truck capacity: 10 cu yd
Results
- Ready-mix needed: 5.43
- Truck loads (at capacity): 0.54
- Total cubic feet: 133.3
- Slab area: 400
What each field means
Inputs
- Slab thickness (in)
- The slab thickness used in the calculation, measured in in. Starts at 4 in so you have a working example on load. Accepted range: 3–10 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: 0–25 %.
- Truck capacity (cu yd)
- The truck capacity used in the calculation, measured in cu yd. Starts at 10 cu yd so you have a working example on load. Accepted range: 5–12 cu yd.
Results
- Ready-mix needed
- cu yd
- Truck loads (at capacity)
- Returned as a decimal number. 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.
- Slab area
- sq ft
FAQ
How many cubic yards of concrete for a 20x20 garage slab?
At 4 inches thick, about 4.94 cubic yards raw, roughly 5.4-5.5 cubic yards with a 10% waste factor — comfortably a little over half of one standard 10-yard truck.
Should a garage slab be thicker than a patio?
Yes, garage slabs bearing vehicle weight are typically poured 4-5 inches minimum, sometimes with a thickened edge (turned-down footing) around the perimeter for wall support, versus the 3.5-4 inch standard for a walking patio.
Can one truck deliver the whole 20x20 pour?
Usually yes at 4-5 inches thick, since that's under 6 cubic yards and most trucks carry 9-10 yards; thicker pours (6+ inches with reinforcement) can approach or exceed a single truck's capacity.
What happens if I underestimate and the truck runs short?
A second truck has to be dispatched while the first pour is already setting, risking a visible cold joint; ordering slightly over your calculated need (or confirming with the batch plant) is cheaper insurance than a mid-pour shortage.
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/concrete-slab-20x20-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/concrete-slab-20x20-calculator" target="_blank" rel="noopener">20x20 Concrete Slab Calculator — RevenueLab</a> (2026).</p>
Source: [20x20 Concrete Slab Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/concrete-slab-20x20-calculator) (2026).