Concrete Calculator
Enter the slab dimensions to find how much concrete you need in cubic yards and cubic feet, plus how many 60 lb or 80 lb bags it would take.
| If buying bags | Bags needed |
|---|---|
| 80 lb bags (≈0.60 ft³ each) | 56 |
| 60 lb bags (≈0.45 ft³ each) | 75 |
Order a little extra — ready‑mix is usually bought by the cubic yard, and most pros add ~10% for spillage and uneven subgrade. Bag counts already round up.
How to use this calculator
Enter the length and width of your slab in feet, then the thickness in inches (you don't need to convert — the calculator handles it). If you are pouring multiple identical slabs — say, four footings of the same size — set the number of slabs accordingly. The results show the exact volume in cubic feet and cubic yards, a waste-adjusted order quantity, and the number of 60 lb and 80 lb pre-mix bags you would need if mixing by hand.
How concrete volume is calculated
Concrete is sold and ordered by volume. The fundamental relationship is the same as any rectangular prism: multiply the three dimensions together. The only wrinkle is that thickness is usually measured and thought about in inches while length and width are in feet, so you must convert the thickness to feet before multiplying.
Once you have cubic feet, the next step is converting to cubic yards — the unit that ready-mix concrete suppliers use when quoting prices and scheduling deliveries. One cubic yard equals 27 cubic feet (because 3 ft × 3 ft × 3 ft = 27 ft³). Divide your cubic foot total by 27 to get yards.
Worked example — step by step
You are pouring a 12 ft × 20 ft driveway apron at 6 inches thick.
- Convert thickness: 6 inches ÷ 12 = 0.5 feet
- Cubic feet: 12 × 20 × 0.5 = 120 ft³
- Cubic yards: 120 ÷ 27 ≈ 4.44 yd³
- With 10% waste: 4.44 × 1.10 ≈ 4.89 yd³ — order 5 cubic yards
- 80 lb bags equivalent: 120 ft³ ÷ 0.60 ≈ 200 bags — clearly a job for a ready-mix truck
In practice, your concrete supplier will quote by the yard and can usually deliver in 0.5 or 1 yd³ increments. Rounding up to a whole number of yards is standard, hence ordering 5 yards for a 4.89-yard job.
Why you should always order extra
The 10% waste factor the calculator shows is not just a buffer for carelessness — there are real sources of volume loss in every pour. Subgrade (the ground under the slab) is rarely perfectly level, meaning some spots sit a little deeper than your planned thickness. Slab edges require extra concrete that a flat area calculation doesn't fully capture. Spillage during chute discharge and finishing is also unavoidable. Running short of concrete mid-pour is a serious problem — a cold joint between two pours is structurally weak and cosmetically obvious. Ordering slightly more than you think you need is almost always the right call.
Bags vs. ready‑mix: which to choose
Pre-mix bags (60 lb and 80 lb) are convenient for small projects: a fence post base, a small repair patch, or a stepping stone. Once your project reaches roughly half a cubic yard or more, the economics and labor calculus shift. A single cubic yard requires 45 bags at 80 lbs each — that is 3,600 lbs of material to lug, open, mix, and pour, not counting water. A ready-mix truck delivers wet concrete exactly when you need it, and the per-yard cost is typically lower than buying bags for projects over about 1 yard.
Consider bags for: fence posts, mailbox bases, small steps, and repair patches. Consider ready-mix for: patios, driveways, sidewalks, foundations, and any pour requiring 1 cubic yard or more.
Common mistakes to avoid
- Forgetting to convert thickness to feet: Entering thickness in inches directly as feet produces a wildly wrong volume. 4 inches is 0.333 feet, not 4 feet. This calculator handles the conversion for you when you enter thickness in inches.
- Skipping the waste factor: Ordering exactly the calculated volume almost guarantees running short. Always round up and add at least 5–10%.
- Ignoring subgrade preparation: If the ground under your slab has soft spots or the surface is uneven, your actual concrete use will exceed the calculation. Compact and level the base first.
- Confusing cubic feet and cubic yards: A concrete quote in yards and a bag count in cubic feet are not directly comparable until you convert (27 ft³ = 1 yd³). Mixing up units leads to ordering far too little or far too much.
The formula
Cubic feet = Length (ft) × Width (ft) × (Thickness (in) ÷ 12) · Cubic yards = Cubic feet ÷ 27
Waste-adjusted order: Cubic yards × 1.10 · 80 lb bags: Cubic feet ÷ 0.60 · 60 lb bags: Cubic feet ÷ 0.45
How we calculate this
Volume is calculated as Length × Width × (Thickness in inches ÷ 12) to produce cubic feet. Cubic yards = cubic feet ÷ 27. The waste-adjusted order quantity adds 10% to the exact volume. Bag counts are derived from standard yields: 0.60 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. This calculator handles the conversion from inches to feet for thickness and shows the result in both cubic feet and cubic yards.
How many bags of concrete does it take to make a cubic yard?
A cubic yard equals 27 cubic feet. An 80 lb bag of pre‑mix concrete yields about 0.60 cubic feet, so you need roughly 45 bags per yard. A 60 lb bag yields about 0.45 cubic feet, requiring roughly 60 bags per yard. For jobs over 1 cubic yard, ready‑mix delivery is almost always more practical.
Should I order extra concrete?
Yes — always add a waste factor. Most contractors order 5–10% extra to account for spillage during the pour, slightly uneven subgrade that can require more depth, and slab edges that use more material than a flat calculation suggests. The calculator includes a 10% waste estimate alongside the exact volume.
When should I use bags instead of ready‑mix?
Bags are practical for small repairs and projects under about 0.5 cubic yards (roughly half a yard). Above that threshold, the labor of mixing dozens of heavy bags typically outweighs the convenience, and a ready‑mix truck becomes more cost-effective. One cubic yard requires about 45 bags at 80 lbs each — that is significant mixing and hauling effort.
What thickness should a concrete slab be?
For residential applications: sidewalks and patio slabs are typically 4 inches thick, driveways 4–6 inches (6 inches if heavy vehicles will park on them), and structural slabs may be thicker per engineering specs. Check local building codes for your specific application.
How do I calculate concrete for a circular slab?
The area of a circle is π × radius². For example, a 10-foot-diameter circle has a 5-foot radius, giving an area of 3.14159 × 25 ≈ 78.54 square feet. Multiply by the thickness in feet (e.g., 4 inches = 0.333 ft) to get cubic feet, then divide by 27 for cubic yards. This calculator handles rectangular slabs; use the area formula above and enter it as a length × width equivalent if needed.
What is the difference between concrete and cement?
Cement is an ingredient in concrete, not the finished material. Concrete is a mixture of cement (the binding powder), water, sand (fine aggregate), and gravel (coarse aggregate). When people say "cement" in everyday speech they usually mean concrete. Pre-mix bags contain cement, sand, and gravel — just add water.
How long does concrete take to cure?
Concrete reaches about 70% of its design strength in 7 days and full strength around 28 days. You can usually walk on a slab after 24–48 hours and drive on it after 7 days, but avoid heavy loads for the full cure period. Keep the surface moist during the first few days to improve strength and prevent cracking.