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
20.37
80-lb bags (DIY mix)
917
Estimated concrete cost
$3,056
Total cubic feet
500.0
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter slab length (ft).
- 2Enter slab width (ft).
- 3Enter thickness (in).
- 4Enter waste factor (%).
- 5Enter ready-mix price per cubic yard ($).
- 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
30x40 is a popular footprint for a pole barn, workshop, or large detached garage — 1,200 square feet of slab that adds up fast in material cost. This calculator defaults to 5 inches thick for equipment-bearing use and returns the ready-mix cubic yardage, an approximate bag count if hand-mixing any portion, and total cost at your local price per yard.
Worked example
Using the values the calculator loads with:
Inputs
- Slab length: 40 ft
- Slab width: 30 ft
- Thickness: 5 in
- Waste factor: 10 %
- Ready-mix price per cubic yard: 150 $
Results
- Ready-mix needed: 20.37
- 80-lb bags (DIY mix): 917
- Estimated concrete cost: $3,056
- Total cubic feet: 500
What each field means
Inputs
- Slab length (ft)
- The slab length used in the calculation, measured in ft. Starts at 40 ft so you have a working example on load.
- Slab width (ft)
- The slab width used in the calculation, measured in ft. Starts at 30 ft so you have a working example on load.
- Thickness (in)
- The thickness used in the calculation, measured in in. Starts at 5 in so you have a working example on load. Accepted range: 2–12 in.
- Waste factor (%)
- The waste factor used in the calculation, measured in %. Starts at 10 % so you have a working example on load. Accepted range: 0–30 %.
- Ready-mix price per cubic yard ($)
- 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
How much concrete does a 30x40 slab need?
At 5 inches thick with 10% waste, roughly 20.4 cubic yards — enough that most suppliers will schedule a dedicated truck run or two.
Should a 30x40 pole barn slab be thicker than 5 inches?
If it will hold heavy equipment like tractors or lifts, 6 inches with rebar reinforcement is common; 4 inches is enough for light storage only.
Is it cheaper to pour 30x40 in sections?
Pouring in one continuous placement with control joints cut afterward is usually cheaper and stronger than separate pours, since cold joints between separate pours are a common crack point.
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). 30x40 Concrete Slab Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/30x40-concrete-slab-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/30x40-concrete-slab-calculator" target="_blank" rel="noopener">30x40 Concrete Slab Calculator — RevenueLab</a> (2026).</p>
Source: [30x40 Concrete Slab Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/30x40-concrete-slab-calculator) (2026).