Concrete Calculator
Compute concrete volume for slabs, footings, columns, and stairs. Outputs cubic yards and cubic metres, with bag counts at QUIKRETE's published yields and an adjustable waste allowance.
Concrete Calculator
How to use the Concrete Calculator
Pick the pour shape
Four geometries cover almost every residential / light-commercial pour: a flat slab (patio, driveway, garage floor, sidewalk), a footing (wall foundations, deck post footings, strip foundations), a column (round piers for decks, fence posts, structural columns), or stairs (porch steps, stoop, exterior staircase). For irregular pours (L-shaped patios, stepped foundations), break the geometry into rectangles and calculate each separately.
Choose your unit system
Imperial uses feet for length/width/height and inches for thickness — the convention in US / Canada residential construction. Metric uses meters for the large dimensions and centimeters for thickness / depth — the convention in UK / Europe / Australia / Asia and on most engineering drawings worldwide. Convert dimensions before entering; the calculator doesn't mix units within one calculation.
Enter dimensions + set the waste allowance
Type the dimensions for your shape — the fields change with the shape, and counts and steps take whole numbers. The waste field defaults to 10%: a planning allowance for spillage, uneven sub-base and forms that are slightly out, not a published standard. Raise it for awkward pours and lower it for simple, well-formed ones.
Read the bag count, then price it
The result gives the volume in cubic yards, cubic metres and cubic feet, and the number of 60 lb, 80 lb and 25 kg bags at their yields. Ready-mix is priced by local suppliers, often with a minimum order, so for a large pour get a delivered quote as well and compare.
Concrete quantities — volume, bags and the allowance for waste
Every result here starts as a volume: length × width × thickness for a slab, the same times the count for footings, π × radius² × height for a round column, and for steps the sum of solid blocks, each one riser higher than the last. On the sample slab — 20 × 10 ft at 4 in — that is 66.7 ft³, or 2.47 cubic yards, before any allowance. At 10% it becomes 2.72 yd³.
How many bags
QUIKRETE's data sheet for its Concrete Mix (No. 1101) gives the yields this page uses: a 60 lb bag makes about 0.45 ft³ (12.7 L) and an 80 lb bag about 0.60 ft³ (17 L). The sample slab with its allowance therefore takes 163 bags of 60 lb or 123 of 80 lb. Every bag size on that sheet works out to the same 0.0075 ft³ per pound of dry mix, so a 25 kg bag of the same mix makes about 11.7 litres, and the sample slab takes 178 of them. Bagged mixes sold in kilograms are other products with their own yields, so check the figure printed on the bag you buy.
This page has been wrong about the 25 kg bag twice. It first assumed 12.5 litres, which leaves a pour short. It then divided 25 kg by the data sheet's unit weight of 2,242.5 kg/m³, which gives 11.1 litres, but that weight is for fresh concrete with its mixing water already in, while the 25 kg on the bag is dry. That version counted about one bag in twenty too many.
Order to the volume, then add an allowance — running short mid-pour costs more than a spare bag.
Why an allowance at all
Forms are rarely exactly to size, sub-bases are uneven, and some concrete is always lost in mixing and placing. The 10% default is a planning figure, not a rule from a standard; what matters is that a second batch placed after the first has begun to set will not bond like a single pour, so being short is the expensive mistake. The mix, strength and thickness for structural work are set by your local code and engineer, not by a quantity calculator.
Bag yields, the 25 kg correction, and why concrete is specified at 28 days
A 60 lb bag of QUIKRETE Concrete Mix yields about 0.45 ft³; an 80 lb bag about 0.60 ft³ (data sheet No. 1101).
Concrete doesn't "dry" — it cures by hydration, the reaction between cement and water, which is why it is kept moist while it gains strength.
Concrete is specified by its strength at 28 days: in IS 456 the number in a grade such as M20 is the compressive strength, in N/mm², of a 150 mm cube at that age. How fast a pour gets there depends on the cement, the temperature and the curing, so when a slab can take foot or vehicle traffic is for the engineer or the supplier's data sheet to say, not a fixed number of days.
QUIKRETE's yields are one rate in five sizes: 0.0075 ft³ per pound of dry mix, from the 40 lb bag (0.30 ft³) to the 90 lb (0.675 ft³).
The data sheet's 140 lb/ft³ (2,242.5 kg/m³) is the weight of fresh concrete, water included. Dividing a dry 25 kg bag by it gives 11.1 litres; the sheet's own yield per pound gives 11.7.
A cubic yard is 27 ft³, exactly 0.764554857984 m³, so it takes 60 of the 60 lb bags or 45 of the 80 lb bags.
The sample 20 × 10 ft slab at 4 in is 2.47 yd³ before the allowance and 2.72 yd³ with 10%.
Solid steps are counted as blocks one riser higher each step: eight 11 × 7 in steps 3 ft wide are 57.8 ft³.
A round column uses π × radius² × height: four 12 in columns 10 ft high are 31.4 ft³.
Reinforcing bars do not change the volume you order in any meaningful way; their size and spacing are an engineer's design.
Frequently Asked Questions
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Measure as exactly as you can, then allow for what measurement cannot see: forms slightly out of square, a sub-base that is lower in places, and concrete lost in mixing and placing. The 10% default is a planning allowance, not a figure from a standard. Being short is the costly mistake, because a second batch placed after the first has begun to set does not bond like a single pour.
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That is a price comparison for your area. The page tells you how many bags the job takes; ready-mix suppliers quote by the cubic yard or metre and often set a minimum order or charge for small loads. For anything more than a few dozen bags, ask for a delivered price and compare it with the bag count and your own mixing time.
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Only the amount: QUIKRETE's data sheet for its Concrete Mix gives about 0.45 ft³ (12.7 L) from a 60 lb bag and 0.60 ft³ (17 L) from an 80 lb bag (40 lb 0.30, 50 lb 0.375, 90 lb 0.675). A cubic yard is 60 of the smaller or 45 of the larger. Lighter bags are easier to carry; heavier ones mean fewer to mix. Other brands and products yield differently, so check the bag.
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Yes, for quantity: bars take up very little of a slab's volume, so the volume you order does not change in any meaningful way. The bar size, spacing and cover are part of the structural design and come from an engineer or your local code. Our rebar weight calculator turns a chosen size and spacing into a weight to order.
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Break the shape into rectangles. An L-shape is two rectangles; a T-shape is three. Calculate each rectangle separately, add the volumes, then apply the waste allowance once to the total. For a curved edge, approximate with rectangles plus a triangular wedge, or use the formula for a circular segment if it is a true arc. The more approximate the geometry, the larger the allowance you will want.
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That is set by your local building code and, for anything structural, an engineer — not by a quantity calculator. Grades are named differently from country to country, some by cube strength and some by cylinder strength, so tell your supplier the grade your design calls for in its own terms.
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It depends on the use, the sub-base and the reinforcement, and it is set by your local code or engineer. This page will compute any thickness you enter; it does not recommend one.
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It models solid steps on the ground: step 1 is one riser high, step 2 two risers, and so on, so the side area is tread × rise × (1 + 2 + … + n) and the volume is that times the width. That is exact for a solid flight with a flat top. For steps built over a sloping slab, or with a landing, work out the landing as a separate slab.
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No. All calculations run entirely in your browser via JavaScript. There's no server roundtrip — open DevTools → Network and confirm zero outbound requests as you change inputs. Pour dimensions stay on your device. Safe for confidential commercial work, multi-property dev calculations, or any pour-volume estimate that shouldn't leave your machine. Close the tab and nothing remains.
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The results always show cubic yards, cubic metres and cubic feet side by side, so there is nothing to convert. The calculation runs in metres from the exact definitions (1 ft = 0.3048 m, 1 yd³ = 0.764554857984 m³), so the three figures agree to the last digit shown. Bag counts are for 60 lb and 80 lb QUIKRETE Concrete Mix and for a 25 kg bag at the same mix's yield per kilogram. The unit switch at the top changes only the units you type the dimensions in.
Method & sources
How it computes
Volume in m³ from exact units (1 ft = 0.3048 m, 1 in = 0.0254 m): slab L×W×T; footing L×W×D×count; column π(D/2)²H×count; solid steps tread×rise×(1+…+n)×width; then × (1 + waste %). Shown in yd³ (0.764554857984 m³), m³ and ft³. Bags at QUIKRETE 1101's 0.0075 ft³ per lb of dry mix: 60 lb 0.45 ft³, 80 lb 0.60 ft³, 25 kg 0.4134 ft³ (11.7 L), rounded up. ConcreteKernel, tested.
What this tool implements
- Removed as unsourced or wrong: ready-mix prices and minimum-load fees, the bag/ready-mix 'crossover', concrete grade and slab-thickness recommendations (now referred to local code and an engineer), '#4 at 12 in' rebar advice, ASEAN grade and brand claims, and trivia including a Hoover Dam volume of '3.3 million m3' and Burj Khalifa pumping records.
- Bag yields: QUIKRETE Concrete Mix (No. 1101) data sheet — 40 lb 0.30 ft³, 50 lb 0.375, 60 lb 0.45 (12.7 L), 80 lb 0.60 (17 L), 90 lb 0.675; one rate, 0.0075 ft³ per pound of dry mix. A 25 kg bag of the same mix: 25 ÷ 0.45359237 × 0.0075 = 0.4134 ft³ = 11.7 L.
- Units are exact: 1 ft = 0.3048 m and 1 lb = 0.45359237 kg (NIST SP 811), 1 yd³ = 0.764554857984 m³ (NIST Handbook 44, Appendix C). Bag counts round up after removing float residue, so an exact cubic yard is 60 × 60 lb and 45 × 80 lb.
- Grade naming: IS 456:2000 Table 2, note 1 — the number in a grade such as M20 is the specified compressive strength of a 150 mm cube at 28 days, in N/mm².
- Corrected 2026-10-16: the 25 kg bag yield of 11.1 L (25 kg ÷ 2,242.5 kg/m³) divided DRY mix by the unit weight of FRESH concrete; it is 11.7 L at the data sheet's yield per pound, so 25 kg bag counts were about 5% high. Also: bag counts could gain a bag on a float residue; volumes used rounded ft³/m³ and ft/m factors; a waste range attributed to ACI without a source was removed; blank step or footing counts no longer default silently to one step.
Sources
- QUIKRETE. Concrete Mix No. 1101 product data sheet (yields by bag size; density approximately 140 lb/ft3, 2,242.5 kg/m3). https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf
- NIST Special Publication 811 (2008), Guide for the Use of the International System of Units, Appendix B (foot = 0.3048 m and pound = 0.45359237 kg, exact). https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublicatio…
- NIST Handbook 44 (2024), Appendix C, General Tables of Units of Measurement (1 cubic yard = 0.764 554 857 984 m³). https://nvlpubs.nist.gov/nistpubs/hb/2024/NIST.HB.44-2024.pdf
- Bureau of Indian Standards. IS 456:2000 Plain and Reinforced Concrete — Code of Practice, Table 2 note 1 (grade = characteristic compressive strength of 150 mm cube at 28 days). https://law.resource.org/pub/in/bis/S03/is.456.2000.pdf
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