Concrete Calculator

A concrete calculator estimates the volume of concrete needed for a slab, footing, column, or wall, and converts that volume into the number of pre-mixed bags required. It is used by builders, contractors, and DIY homeowners to order the right amount of material and avoid costly over-ordering or running short mid-pour.

How to Use the Concrete Calculator

1. Select the shape of your project: rectangular slab, round column, footing, or staircase.
2. Enter the dimensions: length, width, and thickness (or depth and diameter for round shapes).
3. Select your units: metres or feet.
4. Click "Calculate" to see the total volume in cubic metres or cubic yards.
5. If using pre-mixed bags, select the bag size (20 kg, 25 kg, 40 kg, or 60 lb/80 lb bags) to see the number of bags required. Add 5-10% for wastage.

The Formula

For a rectangular slab (the most common shape):

Volume (m³) = Length (m) x Width (m) x Thickness (m)

For a circular column or footing:

Volume (m³) = Pi x Radius² x Height (m)

Where Radius = Diameter / 2 and Pi = 3.14159.

For a cylindrical hole (e.g., post footing):

Volume (m³) = Pi x (Diameter / 2)² x Depth (m)

Converting cubic metres to bags:

• A standard 20 kg bag of pre-mixed concrete yields approximately 0.009 m³.
• A 25 kg bag yields approximately 0.011 m³.

Number of bags = Volume (m³) / Yield per bag

Standard ready-mix concrete is ordered by the cubic metre. One cubic metre requires approximately 2,300 kg of wet concrete mix, which is the weight typically used by ready-mix suppliers for pricing.

Real-World Example

You want to pour a concrete patio slab measuring 4 metres long, 3 metres wide, and 100 mm (0.1 metres) thick.

Volume = 4 x 3 x 0.1 = 1.2 m³

Adding 10% for wastage and uneven subbase: 1.2 x 1.10 = 1.32 m³

If ordering ready-mix concrete: order 1.4 m³ (rounding up to the nearest supplier increment).

If using 25 kg bags: Number of bags = 1.2 / 0.011 = 109 bags

At 10% wastage allowance: 120 bags

This is a case where ready-mix is more practical and cost-effective than mixing 120 bags by hand. For projects under 0.5 m³, pre-mixed bags are generally more economical.

Tips for Planning Your Concrete Pour

Always add at least 5% to your calculated volume as a wastage allowance; 10% is safer for irregularly shaped pours or those with complex formwork. Ensure the subbase is compacted and level before pouring; an uneven base increases the actual volume required. For slabs thicker than 100 mm or subject to vehicle loads, reinforce with mesh or rebar. Check the minimum and maximum ambient temperatures for pouring; concrete must not freeze within the first 24 hours. In hot weather, schedule the pour for early morning to slow the hydration rate and reduce cracking risk. For large pours, order ready-mix and plan the pour sequence in advance, as concrete begins setting within 90 minutes of mixing.

Frequently Asked Questions

How thick should a concrete slab be? For a standard pedestrian patio or path, 75-100 mm (3-4 inches) is sufficient. For a driveway subject to car traffic, use 100-125 mm (4-5 inches). Garage floors that will carry heavy vehicles typically require 125-150 mm (5-6 inches). For structural or load-bearing slabs, consult a structural engineer for the correct specification.

What is the difference between concrete and cement? Cement is an ingredient in concrete, not the same thing. Concrete is a mixture of cement (typically Portland cement), sand (fine aggregate), gravel or crushed stone (coarse aggregate), and water. Cement is the binding agent; concrete is the finished construction material. When people say "cement slab" they typically mean a concrete slab.

How many bags of concrete do I need per cubic metre? A 25 kg bag of pre-mixed dry concrete typically yields approximately 0.011 m³ of cured concrete. This means you need approximately 91 bags per cubic metre. A 20 kg bag yields approximately 0.009 m³, requiring about 111 bags per cubic metre.

How long does concrete take to cure? Concrete reaches approximately 70% of its design strength after 7 days and full strength (typically 25-30 MPa for standard mixes) after 28 days. It is safe to walk on after 24-48 hours and to drive on after 7 days in most conditions, but avoid heavy loads and sustained stress until the 28-day cure is complete.

Also try these free tools:

• Square Footage Calculator
• Volume Calculator
• Area Calculator

A round post footing in metric

A garden post needs a footing 300 mm across and 600 mm deep. Convert to metres before anything else: the diameter is 0.3 m, the depth is 0.6 m, and the radius is half the diameter, so 0.15 m.

The volume of a cylinder is pi times the radius squared times the depth. Radius squared is 0.15 times 0.15, or 0.0225 square metres. Multiply by the depth 0.6 and by pi at 3.14159 and the footing takes 0.042412 cubic metres.

At a nominal yield of 0.009 cubic metres per 20 kg bag that is 0.042412 divided by 0.009, or 4.7124 bags, so buy 5. Carry a wastage allowance of 10 percent and the requirement rises to 0.046653 cubic metres, which is 5.1836 bags, so buy 6. The second bag costs less than a return trip for one more.

The same footing in inches and pounds

The same post in imperial sizes is a hole 12 inches across and 24 inches deep. Half the diameter gives a radius of 6 inches, or 0.5 feet, and the depth is 2 feet.

The volume is pi times 0.5 squared times 2, which is 1.570796 cubic feet, or 0.04448 cubic metres. That figure sits close to the metric answer, and the difference comes from the hole itself: a 300 mm diameter is 11.811 inches, slightly narrower than a 12 inch hole.

Imperial bag yields are quoted in cubic feet and they are larger in volume than the metric yields. A 60 lb bag yields about 0.45 cubic feet, which is 12.7426 litres, and an 80 lb bag yields about 0.60 cubic feet, which is 16.9901 litres. Both exceed the 9 and 11 litres of the 20 kg and 25 kg bags, partly because each imperial bag holds more material and partly because the yield per kilogram differs between products.

Metric and imperial bag counts for one pour

The footing takes 0.042412 cubic metres whichever bag you buy. The table converts that single volume into bags for six bag sizes and carries the wastage column through, because the rounding at the end is where orders go wrong.

Two effects show up in that table. Yield per kilogram is a property of the product rather than a constant: the six yields above run from 0.44 to 0.47 litres per kilogram, so a bag count read off the weight alone will be wrong. And the wastage column changes the answer only when the raw count lands just above a whole number, which is exactly the case where the mistake is easy to make.

Metric and imperial conversions used above

A flat slab converts the same way. A slab 12 feet by 10 feet at 4 inches thick is 40.0 cubic feet, which is 1.132674 cubic metres. At the 2300 kilograms per cubic metre that ready-mix suppliers use for pricing, the wet weight comes to 2605.15 kilograms. Weight rather than volume is what makes delivery the practical choice once a pour passes about half a cubic metre.

Method and assumptions behind the bag count

The volume arithmetic is exact for the shape. The bag count is not, because bag yield is a nominal figure printed by the manufacturer. Three assumptions decide the final order.

• Yields are nominal. A 25 kg bag of dry mix yields about 0.011 cubic metres of cured concrete, and the real figure moves with the water you add and the compaction you achieve.
• The subbase sits outside the calculation. An uneven or uncompacted base takes extra concrete to bring level, and over-excavation adds volume the dimensions in the form never mention.
• Ready-mix carries a minimum order. Suppliers price by the cubic metre and charge for a part load, so a 1.2 cubic metre pour often costs more per metre than a 6 cubic metre pour.
• The wastage allowance is a judgement, not a formula. Five percent covers a flat slab on a level base and ten percent covers formwork, irregular shapes, and a first pour on new ground.

Read the printed yield on the bag you actually buy. Metric and imperial bags carry different yields, and a substitution from a 25 kg bag to a 20 kg bag changes the count without changing the volume.

A note on the concrete standards

Strength and durability requirements come from specification standards rather than from volume arithmetic. ACI CODE-318 sets minimum requirements for structural concrete in the United States, and EN 206, current as EN 206:2013+A2:2021 from the European Committee for Standardization, covers specification, performance, production and conformity in Europe with BS 8500 as its British complement. Neither standard states a bag yield. Yields come from the manufacturer's data sheet for the product in the bag. Quikrete, for example, publishes 0.30, 0.375, 0.45 and 0.60 cubic feet for its 40, 50, 60 and 80 lb bags, and those are the figures used in the tables above.