Concrete Calculator

Calculate concrete volume by multiplying length × width × thickness.

Convert to cubic meters or yards, then determine cement, sand, and gravel quantities based on your mix ratio.

• Standard 1:2:4 mix is suitable for most projects.
• Use 1:1.5:3 for structural elements requiring extra strength.
• Lean mix 1:3:6 is for non-structural applications.

To calculate concrete volume for a rectangular slab, multiply length × width × thickness with all dimensions in the same unit: Volume = L × W × T.

For a 10 ft × 12 ft slab that is 4 inches (0.333 ft) thick, V = 10 × 12 × 0.333 = 40 cubic feet.

Because ready-mix concrete is ordered by the cubic yard in the US, divide by 27 (there are 27 cubic feet in one cubic yard): 40 ÷ 27 = 1.48 cubic yards. In metric units the same relationship gives cubic meters directly.

Per the National Institute of Standards and Technology (NIST), the yard and meter are precisely defined length units, so 1 yd³ equals 0.764555 m³.

The SI unit for volume is the cubic meter (m³), defined by the International Bureau of Weights and Measures (BIPM) from the meter, the base unit of length.

In the US construction industry, concrete is customarily measured in cubic yards (yd³), where 1 yd³ = 27 ft³ = 0.764555 m³. Small quantities from bagged pre-mix are often stated in cubic feet (ft³), with 1 m³ = 35.3147 ft³.

Material weight uses kilograms (kg) or pounds (lb); one 50 kg bag of cement equals about 110.2 lb.

Following NIST and BIPM conventions, always keep every dimension in one unit system before multiplying to avoid conversion errors.

For a standard 1:2:4 concrete mix (1 part cement, 2 parts sand, 4 parts gravel by volume), you need roughly 7 bags of 50 kg cement per cubic meter of finished concrete.

This works because cement is about one-seventh of the total volume in a 1:2:4 mix, and a 50 kg bag of cement occupies roughly 0.035 m³ — so a cubic meter of concrete takes about seven bags of cement.

A richer 1:1.5:3 structural mix needs about 9 bags/m³, while a lean 1:3:6 mix needs around 5 bags/m³.

For a 7.5 m³ patio slab in a 1:2:4 mix, that is 7 × 7.5 ≈ 53 bags. These are volumetric estimates; the Portland Cement Association notes actual yield varies with aggregate moisture and compaction.

A round (cylindrical) column uses the volume of a cylinder: V = π × r² × h, where r is the radius (half the diameter) and h is the height.

For a column 0.3 m in diameter (r = 0.15 m) standing 3 m tall, V = 3.14159 × 0.15² × 3 = 3.14159 × 0.0225 × 3 = 0.212 m³. For a square column, use V = side × side × height instead.

Always compute the radius from the diameter first, since squaring the full diameter overstates the volume fourfold. HyperPhysics (Georgia State University) provides the standard cylinder and prism volume relationships used here.

Stair concrete is estimated by summing the volume of each step as a stacked series of rectangular prisms, then adding the underlying landing or throat.

A simple approximation treats the flight as a triangular prism: Volume ≈ ½ × total run × total rise × width. For a 10-step flight with 0.18 m rise and 0.28 m tread over a 1 m width, total rise = 1.8 m and total run = 2.8 m, giving V ≈ 0.5 × 2.8 × 1.8 × 1 = 2.52 m³.

Add extra for the supporting slab beneath the steps. Because forms for stairs are irregular, order 10-15% more than the calculated figure.

The water-cement ratio (w/c) is the weight of mixing water divided by the weight of cement, and it is the single biggest factor controlling concrete strength.

A typical structural w/c is 0.40 to 0.50 by weight: for one 50 kg bag of cement at w/c = 0.45, add 50 × 0.45 = 22.5 liters of water (1 liter of water weighs 1 kg).

Lower ratios produce stronger, more durable concrete but reduce workability, while excess water weakens the cured product and causes shrinkage cracking.

The American Concrete Institute (ACI) publishes w/c limits by exposure class in ACI 318, and Encyclopaedia Britannica describes hydration as the chemical reaction that hardens cement.

Accurate concrete estimating supports many projects:

• residential patios, driveways, garage floors, and sidewalks
• structural footings and foundation walls
• fence and deck post footings
• retaining walls
• cylindrical columns or pier caps

Contractors use volume figures to order ready-mix by the truck (a standard mixer carries about 8-10 cubic yards) and to avoid short loads that create cold joints.

DIY builders use them to count 60 lb or 80 lb pre-mix bags, where an 80 lb bag yields roughly 0.6 ft³. Precise estimates also feed cost budgets, since ready-mix and rebar are priced per cubic yard and per ton respectively.

The most frequent errors are:

• mixing units (multiplying feet by inches without converting)
• forgetting to divide cubic feet by 27 to reach cubic yards
• using the column diameter instead of the radius in V = π × r² × h

Others include ignoring a waste factor, so any spillage or over-excavation causes a short pour, and confusing cement with concrete when ordering bags. Rounding thickness too coarsely also skews thin slabs.

To avoid these, keep every dimension in one unit system, add 5-10% for waste (10-15% for irregular forms), and double-check that radius, not diameter, feeds the cylinder formula. Cross-check final yardage against your ready-mix supplier's quote.

Concrete conversions follow fixed factors defined by NIST and BIPM:

• To convert cubic feet to cubic yards, divide by 27 (since 1 yard = 3 feet and 3³ = 27).
• To convert cubic yards to cubic meters, multiply by 0.764555; to go from cubic meters to cubic yards, multiply by 1.30795.
• For cubic meters to cubic feet, multiply by 35.3147.

Example: a 12 ft³ footing equals 12 ÷ 27 = 0.444 yd³, which equals 0.444 × 0.764555 = 0.340 m³. Keeping these exact factors on hand prevents the ordering errors that come from approximate mental math.