Concrete Slab Calculator
Flat work in one pass: the yards to order, the bags if you must, what the pour weighs — and what each square foot of finished slab cost you.
Concrete Slab Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Rectangular slabs only — no thickenings or openings.
- Grade assumed flat; subgrade corrections ride in the waste %.
- No forming, finishing or labour in the cost cards.
In short: A 20 × 24 ft slab at 4 in: 160 ft³ net, 176 ft³ with 10% waste = 6.518519 yd³ — 294 bags or $1,140.74 of truck. The pour weighs 12.00 tons and each finished square foot carries $2.38 of concrete.
Formula
net ft³ = L × W × thickness ÷ 12
order yd³ = net × (1 + waste) ÷ 27
bag count = ⌈order ft³ ÷ 0.60⌉
weight = net × density ÷ 2,000
[('thickness', 'the squared lever: inches are cheap, yards are not'), ('waste', "subgrade dips, spillage, the mixer's appetite"), ('weight', 'dead load the base and soil carry forever'), ('per ft²', 'the only comparable slab number')]
Worked Example
- Multiply length, width and thickness for the net volume.
- Apply the waste allowance and convert to yards.
- Show both buys: sacks at 0.60 ft³, truck at the yard rate.
- Weigh the pour against the base and soil beneath it.
- State the per-square-foot cost for honest quote comparison.
20 × 24 ft at 4 in: 160 ft³ net, 176 ft³ ordered = 6.518519 yd³. Bagged, 294 sacks; trucked, $1,140.74 at $175. The slab weighs 12.00 tons, and each of the 480 square feet carries $2.38 of concrete.
Strengths & Limits Of This Model
Where this engine is strong
- Weight and per-ft² cards, not just yards
- Defers the buy and the steel to the pages that own them
- Free-entry thickness with code-common chips
Where it stops
- Rectangles only
- No integral footing thickened edge
Practical Use Cases
Patio & shed pads
The classic 4-inch residential pour.
Driveway planning
5-6 inch slabs priced per square foot.
Garage floors
Yards, weight and the steel decision.
Quote comparison
One number that survives between bids.
Delivery sizing
Yards vs the short-load fee boundary.
Methodology & Editorial Standards
Net volume is the rectangular prism L × W × t/12; the order applies the waste allowance multiplicatively and divides by 27. Bag counts ceil at the exact 0.60 ft³ yield, truck cost is order yards times the entered rate, and the dead load multiplies net volume by the entered placed density. The per-square-foot card divides truck cost by finished area so quotes compare on the only slab number that scales.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Concrete Slab Calculator — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How do I calculate yards of concrete for a slab?
Length × width × thickness in feet, divided by 27. For the 20 × 24 ft slab at 4 inches: 160 ft³ = 5.925926 yd³ net, 6.518519 with a 10% allowance — the order number.
How thick should a concrete slab be?
4 inches covers patios, walks and shed pads; residential driveways and garage floors run 5-6 with a compacted base under. Thickness is the cheapest strength you can buy at design time and the most expensive to add afterwards.
How much waste should I add?
Ten percent is the working default for formed flat work: subgrade irregularities and spillage eat it. The waste page's doctrine explains why two small allowances compound — and why the order is always rounded up, never down.
How much does a 20x24 concrete slab weigh?
12.00 tons at normal-weight 150 lb/ft³: 160 ft³ × 150 ÷ 2,000. The placed weight is why the base under it is engineered, not decorative.
How much does a concrete slab cost per square foot?
At $175/yd³ and 4 inches, $2.38 per finished square foot for material. Labour, forming and finishing typically double or triple the all-in number; material is the floor you can quote from.
Does a 4 inch slab need rebar?
It needs reinforcement of some kind — rebar grid or welded wire — because concrete's tensile strength is a tenth of its compressive. The grid decision is the rebar page's: 18-inch centres on #4 is the residential default.
How many bags for a 10x10 slab?
At 4 inches: 33.3 ft³ net, 36.67 with 10% — 62 sacks of 80 lb. That is the concrete page's crossover zone: price the short-load truck before committing to 62 mixes.
What is a cubic yard of concrete?
27 cubic feet — concrete that covers 81 ft² at 4 inches, or 45 of the 80-lb sacks. Every order ticket, supplier quote and truck on the road is priced in these yards.
Should I order extra concrete for the slump test?
The 10% allowance already carries test concrete and the pump's priming charge if your supplier expects it — confirm both when you book. Ordering twice because the first truck came up short costs a second fee, not just a second yard.
Slab-on-grade or a footing first?
Independent slabs on prepared base are the DIY norm; structures, frost lines and local codes want a footing under the perimeter first — that page is next in the family. This page pours what sits on grade.