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
Concrete needed (with waste)
2.72
Total cubic feet
66.7
Raw volume (no waste)
2.47
Equivalent in 80lb bags
122
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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 waste factor (%).
- 5Read your concrete needed (with waste) on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
Ready-mix concrete is sold and delivered by the cubic yard, so before you call a batch plant or a pump truck you need your pour's volume converted out of feet and inches. This calculator takes length, width, and thickness in feet (thickness entered in inches is converted automatically) and multiplies them to get cubic feet, then divides by 27 because one cubic yard equals 27 cubic feet. It also applies a waste factor, typically 5-10%, to cover subgrade irregularities, form leakage, and the reality that a slab is rarely a perfectly flat rectangle at exactly the thickness on the plan. Most ready-mix suppliers have a minimum load, often 1 cubic yard or a short-load fee below 3-4 yards, so knowing your real number before you call avoids both under-ordering (a cold joint mid-pour is a real defect) and over-ordering, which wastes money on a product you can't return. This is the same math used behind footings, patios, sidewalks, and driveway pours, just with different waste factors depending on how forgiving the subgrade prep was.
Worked example
Using the values the calculator loads with:
Inputs
- Length: 20 ft
- Width: 10 ft
- Thickness: 4 in
- Waste factor: 10 %
Results
- Concrete needed (with waste): 2.72
- Total cubic feet: 66.7
- Raw volume (no waste): 2.47
- Equivalent in 80lb bags: 122
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 10 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.
- 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 %.
Results
- Concrete needed (with waste)
- 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.
- Total cubic feet
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Raw volume (no waste)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Equivalent in 80lb bags
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why divide by 27?
One cubic yard is 3 feet × 3 feet × 3 feet, which equals 27 cubic feet. Ready-mix trucks and batch plants price and measure in cubic yards, so every slab or footing calculation eventually needs that conversion.
How much waste factor should I use?
5% for a well-formed, flat, prepped slab; 10% is the safe default for most DIY pours; 15%+ for irregular ground, thick footings, or if you're new to finishing concrete and expect some spillage or overpour at the edges.
What if my slab thickness varies across the pour?
Use the thickest section for safety, or better, average several measurement points across the form since ready-mix trucks won't make a second trip for a shortfall without a hefty short-load fee.
Is it cheaper to order ready-mix or mix bags for this volume?
Above roughly 1-1.5 cubic yards, ready-mix delivered is almost always cheaper per yard and far less labor than mixing 45-70 bags by hand or with a rented mixer.
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
80lb Concrete Bags Calculator
Turn a pour's volume into the exact count of 80lb bags to buy.
10x10 Concrete Slab Calculator
Ready-mix yards and bag counts specifically for a 100 sq ft slab like a shed pad or patio.
12x12 Concrete Slab Calculator
Yardage and bag counts for a 144 sq ft slab — a common patio, carport, or workshop pad size.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Concrete Cubic Yards Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/concrete-cubic-yards-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/concrete-cubic-yards-calculator" target="_blank" rel="noopener">Concrete Cubic Yards Calculator — RevenueLab</a> (2026).</p>
Source: [Concrete Cubic Yards Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/concrete-cubic-yards-calculator) (2026).