Concrete Floor Calculator
Concrete Slab Calculator works out Volume and Volume including waste allowance from How many identical elements, Length and Width. The default figures on this page give volume of 4.9383 yd3; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- slab: 20 ft x 20 ft x 0.3333 ft
- Volume = 133.3333 cu ft = 4.9383 cu yd = 3.7756 m3
- Adding 10% waste: 146.6667 cu ft = 5.4321 cu yd (order 5.5 cu yd)
- Bags: 146.6667 cu ft / 0.6 cu ft per 80 lb bag = 244.44 -> 245 bags
- Weight: 133.3333 cu ft x 150 lb/ft3 = 20000 lb
Figures from concrete mix and bag yield tables, checked 31 August 2026. Its 9 worked examples are re-run on every release, last on 15 September 2026. What changed · how reference data is kept current
Sources and references
7 sources
Keep this result
Save it, change one figure, then compare the two
The calculation stays free. Save this scenario, adjust one assumption, save that too, and put them side by side to see exactly what the change costs you. A free account keeps 5 scenarios and 2 comparisons; Personal makes both unlimited and adds exports and private links. Nothing is saved unless you press Save.
What the Concrete Slab Calculator calculates
Computes concrete volume in cubic feet, cubic yards and cubic metres for slabs, footings, walls, columns, stairs and curbs, then the bags, ready-mix loads, weight, cement/sand/aggregate split and cost that volume implies.
How to use this Concrete Slab Calculator
Enter How many identical elements, Length, Width and Thickness / depth. Volume updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Volume, Volume including waste allowance and Order this much ready-mix.
The method follows QUIKRETE Concrete Mix No. 1101 Technical Data Sheet (bag yields, ASTM C138 density), ASCE/SEI 7-22, Table C3.1-1a Minimum Design Dead Loads (stone concrete 150 lb/ft3 reinforced, 144 lb/ft3 plain), ASTM C150/C150M Standard Specification for Portland Cement (94 lb US bag), Portland Cement Association, Design and Control of Concrete Mixtures, 16th ed. (nominal volumetric proportioning, 94 lb = 1 ft3 sack) and ACI 318-19, Building Code Requirements for Structural Concrete (Table 19.3.2.1 maximum water/cementitious ratios).
What you can compare
Change one input at a time to see how it affects Volume, Volume including waste allowance and Order this much ready-mix. The inputs you can vary are How many identical elements, Length, Width, Thickness / depth, Unit for length, height and landing depth and Unit for thickness, rise, run and flag thickness.
Understanding Volume, Volume including waste allowance and Order this much ready-mix
Footing, wall and column dimensions must come from a design by a qualified engineer or from a prescriptive table in the code adopted locally (e.g. IRC Table R403.1). This tool quantifies concrete for dimensions you supply; it does not size structural elements.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
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Frequently asked questions
How to calculate material for concrete slab?
Computes concrete volume in cubic feet, cubic yards and cubic metres for slabs, footings, walls, columns, stairs and curbs, then the bags, ready-mix loads, weight. Open “How this is worked out” beside the result to see each step for your own figures.
What is the 4 2 1 rule for concrete?
On the figures this page opens with — length 10, width 10, thickness / depth 4 and waste / over-order allowance 0% — the volume is 4.9383 yd3. No single number fits everyone: change any figure above and the answer updates as you type, with each step of the arithmetic beside it.
How much concrete for a 20x20 slab costs?
It needs How many identical elements, Length, Width, Thickness / depth, Unit for length, height and landing depth and Unit for thickness, rise, run and flag thickness. Nothing else is required, and the figures stay in your browser.
How many 60lb bags of concrete for 10x10 slab?
The result gives Volume, Volume including waste allowance, Order this much ready-mix, Rounded up to the quarter yard, Bags needed and Total weight of dry bags, each labelled with its unit so the figure can be read on its own.
What information do I need for the Concrete Slab Calculator?
You need How many identical elements, Length, Width, Thickness / depth, Unit for length, height and landing depth, Unit for thickness, rise, run and flag thickness, Unit for width and Unit for the column / Sonotube diameter. The page opens with default figures that give volume of 4.9383 yd3, so you can see the scale before entering your own.
What does this Concrete Slab Calculator show?
It shows Volume, Volume including waste allowance, Order this much ready-mix, Rounded up to the quarter yard, Bags needed, Total weight of dry bags, Weight of the finished concrete and Ready-mix truck loads, with the working and any warnings for your entries.