Concrete Slab Calculator
Estimate cubic yards of ready-mix or the number of concrete bags for a slab, plus reinforcement.
Enter your slab dimensions to get the concrete volume in cubic yards and see both ways to buy it: a ready-mix order rounded up to the quarter yard, and the equivalent bag count for your bag size. Add reinforcement, a thickened edge, or a vapor barrier and the material list updates — and if you enter prices for both concrete paths, the calculator compares them for your exact volume. The ready-mix quantity is always rounded up, never down — running short mid-pour costs far more than a little extra concrete.
This is a quantity estimate, not a structural design: thickness, reinforcement, and subbase here are inputs for volume math. For driveways, footings, or any slab carrying heavy loads, confirm those choices against local code or an engineer's spec before ordering.
You need about 1.36 cu yd of concrete (36.7 cu ft) including 10% waste.
Order 1.5 cu yd of ready-mix, or buy 62 bags of 80 lb mix.
Project measurements
- Slab area
- 100 sq ft
- Concrete volume (no waste)
- 33.33 cu ft (1.23 cu yd)
- With 10% waste
- 1.36 cu yd
- Estimated wet weight
- ≈ 5,000 lb (2.5 tons)Plan truck access or wheelbarrow trips accordingly.
- Bagged-path dry weight
- ≈ 4,960 lb (62 × 80 lb)
Material breakdown
| Material | Calculated | Buy |
|---|---|---|
| Ready-mix concretedelivered; ordered in 0.25 cu yd stepsAlternative to bagged mix — buy one path, not both. | 1.36 cu yd | 1.5 cu yd |
| Bagged concrete mix80 lb bags @ 0.6 cu ft eachAlternative to ready-mix — 36.7 cu ft with waste. | 61.1 bags | 62 bags |
Shopping list
| Item | Qty | Unit price | Subtotal |
|---|---|---|---|
| Ready-mix concretedelivered; ordered in 0.25 cu yd stepsAlternative to bagged mix — buy one path, not both. | 1.5 cu yd | — | — |
| Bagged concrete mix80 lb bags @ 0.6 cu ft eachAlternative to ready-mix — 36.7 cu ft with waste. | 62 bags | — | — |
Assumptions used in this estimate
- Bag yield
- 0.6 cu ft per 80 lb bag
- Waste
- 10% (applied to volume, never rounded down)
- Ready-mix order
- rounded up to the next 0.25 cu yd
- Wet density
- ≈ 150 lb per cu ft
How this calculator works
Concrete is sold by volume, so the whole estimate flows from one calculation — the slab's volume in cubic feet, converted to cubic yards, with waste added on top:
volume (cu ft) = length × width × (thickness ÷ 12) × slabs
+ thickened edge: (perimeter × edge width − 4 × edge width²) × extra depth
cubic yards = volume ÷ 27
with waste = cubic yards × (1 + waste %)
ready-mix = with waste, rounded UP to the next 0.25 cu yd
bags = (volume with waste, cu ft) ÷ bag yield → rounded upThe calculator then shows both purchase paths side by side: a ready-mix truck order (rounded up to the quarter yard — suppliers won't deliver 1.36 yards, and rounding down risks running short mid-pour) and a bagged-mix count based on the cured yield printed on the bag (0.60 cu ft for an 80 lb bag). Enter a price for either path, or both to get a direct cost comparison in the results.
Wire mesh is estimated from slab area using 5 × 10 ft sheets with 6-inch overlaps; rebar is estimated as a grid at your chosen spacing built from 20 ft bars with a 5% splice allowance; a vapor barrier adds the slab area plus 10% overlap in 10 × 100 ft rolls.
Worked example: a 10 × 10 ft patio slab
A 10 × 10 ft slab poured 4 inches thick, with the default 10% waste:
- Volume: 10 × 10 × (4 ÷ 12) = 33.33 cu ft, which is 33.33 ÷ 27 = 1.23 cu yd.
- With 10% waste: 36.67 cu ft = 1.36 cu yd.
- Ready-mix path: 1.36 rounds up to the next quarter yard → order 1.5 cu yd.
- Bagged path: 36.67 ÷ 0.60 cu ft per 80 lb bag = 61.1 → 62 bags — nearly 5,000 lb of dry mix to haul and mix.
- Wet weight: 33.33 cu ft × 150 lb ≈ 5,000 lb of concrete in place.
That bag count is exactly why the calculator warns when the bagged path gets large: 62 bags is a full day of continuous mixing for two people, and any long pause creates a cold joint. For this slab, most people are happier with a short-load ready-mix delivery.
How to measure for a slab
- Measure the formed size, not the rough hole.Length and width are the inside dimensions of your forms. If you haven't built forms yet, measure the finished slab you want.
- Check thickness in several spots. The subgrade is never perfectly flat. If your excavation varies between 4 and 5 inches deep, estimate at the deeper number or level the base first — thickness errors scale the entire volume.
- Account for the thickened edge.Shed and garage slabs often pour a deeper perimeter footing monolithically. Enable the option and enter its width and extra depth; it can add 50% or more to a small slab's volume.
- Convert carefully. Feet for length and width, inches for thickness is the convention this tool uses. Mixing units is the classic slab-ordering error — see Cubic Feet vs. Cubic Yards for the conversions.
Buying concrete: ready-mix vs. bags
- Bags win for small jobs. Under about half a cubic yard (roughly 25 bags of 80 lb mix), bagged concrete is cheaper once you count delivery fees, and you can mix at your own pace.
- Ready-mix wins for slabs. Around a cubic yard and up, a truck delivers consistent, properly mixed concrete all at once — which matters, because a slab should be poured continuously. Ask suppliers about minimum orders and short-load fees; many have a 1 cu yd minimum.
- Order the rounded-up amount. Running out with the finish half-troweled is the most expensive way to save a quarter yard. Excess concrete is cheap insurance; have a small form or post holes ready for the leftover.
- Don't forget the supporting cast: form lumber and stakes, gravel base, reinforcement, vapor barrier where required, and a bull float or screed. Enter prices for the items this tool counts to build the core of your budget.
Common estimating mistakes
- Dividing thickness by 10 instead of 12.Four inches is 0.333 ft, not 0.4. The error inflates an order by 20% — or worse, people "fix" it downward and run short.
- Skipping waste to save money.Uneven subgrade routinely absorbs 5–10% of a pour. Waste on concrete isn't offcuts, it's the ground eating your volume.
- Rounding the truck order down.If the math says 1.36 yards, order 1.5. A second truck for a quarter yard costs more than the whole first order's overage.
- Underestimating the bagged path's labor. Each 80 lb bag yields only 0.6 cu ft. People are often shocked that a modest patio needs 60+ bags — check the dry weight stat before committing your back and your mixer.
- Forgetting the thickened edge.A monolithic shed slab's perimeter footing can hold almost as much concrete as the slab field itself.
Limitations
This tool estimates volume and materials for rectangular slabs only. It does not size the slab structurally: thickness, reinforcement choice and placement, subbase depth and compaction, control-joint layout, and mix strength (PSI) all depend on loads, soil, and climate, and should follow local code or an engineer's guidance for anything beyond a simple patio or walkway. Bag yields are the manufacturer's cured-yield figures and vary slightly by product — check your bag and use the custom yield override if it differs. Delivery fees, pumping, and finishing labor are not included in the cost estimate.
Frequently asked questions
How many bags of concrete equal one cubic yard?
One cubic yard is 27 cu ft. At the standard cured yields, that's 45 bags of 80 lb mix (0.60 cu ft each), 60 bags of 60 lb mix (0.45 cu ft), or 90 bags of 40 lb mix (0.30 cu ft). That's also why the calculator suggests considering ready-mix once the bag count climbs into that range — a yard of bags weighs about 3,600 lb before you add water.
How thick should a concrete slab be?
Common practice is 4 inches for patios, walkways, and shed slabs, and 5 to 6 inches for driveways or anything that carries vehicles. Heavier loads, poor soils, and cold climates push thickness and reinforcement up. Treat these as starting points, not a design — local code or an engineer has the final word.
Do I need rebar or wire mesh in my slab?
For light-duty slabs on well-compacted base, many builders rely on control joints and skip reinforcement or use wire mesh to hold shrinkage cracks tight. Rebar grids add real bending strength for driveways and slabs on questionable soil. Reinforcement placed on the ground does nothing — it belongs in the middle third of the slab thickness, supported on chairs.
Should I order ready-mix or mix bags myself?
A practical dividing line is around half to one cubic yard. Below that, bags are economical and manageable; above it, mixing bags fast enough to avoid cold joints becomes a race. If both prices are entered, the calculator shows a direct cost comparison for your exact volume — but remember bags also cost hours of hard labor that ready-mix doesn't.
What is a short-load fee?
Ready-mix trucks are most efficient delivering full loads (typically 8 to 10 cu yd), so most suppliers add a fee for small orders — often applied below 3 or 4 yards — or set a 1 cu yd minimum. Get the delivered, all-in price for your specific quantity when comparing against bagged mix.
How long does concrete take to cure?
Concrete typically supports foot traffic after 24 to 48 hours, reaches most of its working strength in 7 days, and its rated strength at 28 days. Keeping the slab damp for the first several days (curing) significantly improves final strength and surface durability. Avoid pouring when temperatures will drop near freezing or in heavy rain.
Sources & methodology
Quantity formulas and default assumptions are documented transparently and summarized on our methodology page. Assumptions that reflect common practice rather than a specific published standard are labeled as assumptions in the results. Referenced organizations:
- 811 Before You Dig — Common Ground Alliance. Free national utility-locating service. Referenced for all digging safety guidance.
- Ready Mixed Concrete Resources — National Ready Mixed Concrete Association. Referenced for general ready-mix ordering practices (never round an order down).
Last reviewed: 2026-07-25 · Reviewed by: Site owner review required
This calculator provides planning estimates for educational purposes. It is not engineering, electrical, or construction advice. Product dimensions, coverage rates, site conditions, installation methods, and local code requirements vary — verify them before purchasing or building. See our full disclaimer.