Concrete Calculator
Free concrete calculator. Estimate cubic yards, bag counts (40lb/60lb/80lb), and cost for slabs, footings, and patios. Supports multiple pours and metric.
Slabs
Slab Dimensions
Cost Estimation (Optional)
About this calculator
Concrete is unforgiving to estimate by eye. Order too little and your pour stops short, leaving a cold joint where the fresh batch meets hardened concrete — a built-in crack and a structural weak point. Order too much and you have paid for material that goes to waste, often with a short-load fee on top. This calculator removes the guesswork: enter your slab dimensions, pick a bag size, and get cubic yards, cubic feet, bag counts, and cost in one place. It handles rectangular and circular pours, multiple slabs at once, metric units, and a waste factor — everything you need to walk into a supply yard knowing exactly what to order.
How to Calculate Concrete Needed
Every concrete estimate is a volume calculation. Concrete is sold by the cubic yard (ready-mix) or by the bag (which you measure in cubic feet), so the whole job is converting your slab's dimensions into those units. The core formula is simple:
Cubic Yards = Cubic Feet ÷ 27
The one trap is units. Thickness is almost always given in inches, but length and width are in feet — so you must convert thickness to feet by dividing by 12 before multiplying. A 4-inch slab is 4 ÷ 12 = 0.333 feet thick, not 4.
Worked example — a patio. Say you are pouring a 12 × 16 foot patio at 4 inches thick:
Thickness = 4 ÷ 12 = 0.333 ft
Volume = 192 × 0.333 = 64 cu ft
Cubic yards = 64 ÷ 27 = 2.37 cu yd
With 10% waste = 2.61 cu yd → order 2.75 cu yd
For a circular pad — a pier, a round patio, a hot-tub base — swap the area formula for the area of a circle. Measure the diameter, halve it to get the radius, and use:
A 10-foot-diameter pad at 4 inches is π × 5² × 0.333 = 26.2 cubic feet, or about 0.97 cubic yards. The calculator above does all of this automatically, including adding up several slabs of different shapes into one combined order.
Concrete Slab Thickness Guide
Thickness drives both your volume and your slab's load capacity. Going thicker than needed wastes concrete; going thinner invites cracking and failure. These are the standard residential thicknesses:
| Application | Thickness | Notes |
|---|---|---|
| Walkways & paths | 4 in | Light foot traffic |
| Patios & shed floors | 4 in | Standard residential slab |
| Garage floors | 4–6 in | 5 in if storing heavy equipment |
| Driveways (cars) | 4–5 in | Reinforce with wire mesh or rebar |
| Driveways (trucks/RVs) | 6 in | Heavier loads need more mass |
| Footings | 8–12 in | Depth set by frost line & load |
Two rules apply to nearly every pour. First, always place concrete over a compacted gravel base — typically 4 inches of crushed stone — for drainage and a stable subgrade. Second, slabs that carry vehicles or span any distance should be reinforced with welded wire mesh or rebar. Thickness alone does not prevent cracking; reinforcement holds the slab together when it does crack.
Bagged vs. Ready-Mix: Cost Comparison
The single biggest cost decision is bags versus a ready-mix truck. Bags win on small pours because there is no delivery minimum; ready-mix wins on volume because the per-yard price is far lower and you skip the brutal labor of mixing. Here is roughly how the two compare:
| Method | Unit Price | Per Cubic Yard | Best For |
|---|---|---|---|
| 40 lb bags | ~$4 / bag | ~$360 | Tiny repairs, post setting |
| 60 lb bags | ~$5 / bag | ~$300 | Small pads, steps |
| 80 lb bags | ~$6 / bag | ~$270 | Pads up to ~½ yard |
| Ready-mix | ~$120–175 / yd | ~$120–175 | Anything over 1 yard |
The math is stark: filling a cubic yard with 80 lb bags takes about 45 bags and costs roughly $270, while the same yard of ready-mix runs $120–175 delivered. But ready-mix plants charge a short-load fee — often $50–$150 — when you order under a full truck (about 10 yards), which erases the savings on very small jobs. The practical crossover is around 1 cubic yard : below it, bags are simpler and competitive; above it, ready-mix is both cheaper and far less work. Enter your prices in the calculator to compare the two for your exact slab.
Concrete Coverage Reference
Once you know your area, this table tells you how far one cubic yard stretches at common thicknesses — handy for a quick sanity check before you trust any number.
| Thickness | Coverage per Cubic Yard | 80 lb Bags per 100 sq ft |
|---|---|---|
| 2 in | 162 sq ft | ~28 bags |
| 3 in | 108 sq ft | ~42 bags |
| 4 in | 81 sq ft | ~56 bags |
| 5 in | 65 sq ft | ~70 bags |
| 6 in | 54 sq ft | ~84 bags |
Read it like this: one cubic yard of concrete covers 81 square feet at 4 inches thick. So a 9 × 9 foot patio (81 sq ft) at 4 inches is almost exactly one yard. The bag column shows why bagging large slabs gets impractical fast — a 100 square foot slab at 4 inches needs about 56 eighty-pound bags, nearly a ton of material to mix by hand.
Common Mistakes When Ordering Concrete
- Forgetting to convert thickness to feet. Multiplying by 4 instead of 0.333 inflates your order twelvefold. Always divide inches by 12 first.
- Skipping the waste factor. Subgrade is never perfectly flat and forms are never perfectly placed. Without 5–10% extra, you risk running short.
- Ordering exactly to the cubic yard. A short pour means a cold joint. Round up to the next quarter yard, not down.
- Ignoring the short-load fee. Pricing a half-yard pour at the per-yard rate misses the delivery minimum that makes small orders expensive.
- Measuring an over-dug footing as if it were the planned size. Trenches always cave and widen — estimate the actual hole, not the drawing.
- Not accounting for slab slope or thickened edges. Patios slope for drainage and many slabs have a thickened perimeter, both of which add volume.
Pro Tips for Concrete Pours
- Have help and a plan before the truck arrives. Ready-mix sets fast — you have roughly 30–60 minutes to place and screed before it stiffens. Line up enough hands.
- Order a little long, not short. Returning a few extra yards or finding a spot for a small overage is trivial; a cold joint is permanent.
- Pour over compacted gravel. Four inches of crushed stone gives drainage and a stable base that prevents settling and cracking.
- Cure slowly and keep it wet. Concrete gains strength through hydration, not drying. Keep the surface damp for the first week, especially in heat.
- Add reinforcement for anything load-bearing. Wire mesh or rebar on chairs (raised so it sits mid-slab) controls cracking on driveways and large pads.
- Place control joints. Tool or saw joints every 8–12 feet so the slab cracks where you want it to, not randomly across the surface.
Putting It All Together
A good concrete estimate is just careful measurement plus three conversions: inches to feet for thickness, cubic feet to cubic yards for volume, and yards to bags if you are mixing it yourself. Add a sensible waste factor, decide between bags and ready-mix based on whether your pour clears about one cubic yard, and you will order with confidence. Use the calculator at the top of this page to run the numbers for your slab — including multiple pours, circular pads, and a side-by-side cost comparison — and you will know exactly what to buy before you ever pick up a shovel.
Questions
How do I calculate how much concrete I need?+
How many bags of concrete are in a cubic yard?+
How thick should a concrete slab be?+
How much does a yard of concrete cost?+
Should I use bagged concrete or ready-mix delivery?+
How much waste should I add to my concrete order?+
How many 80 lb bags of concrete do I need for a 10x10 slab?+
How long does concrete take to cure?+
Related Calculators
This calculator provides rough estimates for planning purposes only. Default values are based on general industry reference data. Actual quantities may vary. Always verify with a licensed professional before purchasing materials or beginning construction. Full disclaimer.