Calculate concrete volume
For rectangular areas with a uniform thickness.
How to calculate concrete for a slab or patio
Measure the length and width of the rectangular area and decide the concrete thickness required for your project. The calculator multiplies these three measurements to find volume. It supports multiple identical slabs and adds an optional allowance for waste or site variation.
Step 1: Measure the slab
For metric measurements, enter length and width in metres and thickness in centimetres. For US measurements, enter length and width in feet and thickness in inches. Keep the thickness consistent across the area. For irregular shapes or changing depths, split the project into smaller sections and calculate each separately.
Step 2: Calculate the volume
Metric formula: volume (m³) = length (m) × width (m) × thickness (cm) ÷ 100 × number of slabs.
US formula: volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12 × number of slabs. Divide cubic feet by 27 to obtain cubic yards.
Step 3: Include extra material
Multiply the calculated volume by 1 plus your allowance divided by 100. The default 10% is an editable planning assumption, not a universal requirement. Actual needs depend on excavation accuracy, ground conditions, formwork and how the concrete is placed.
Step 4: Estimate ready-mix or bags
For ready-mix, the result shows the estimated volume in cubic metres or cubic yards. Confirm supplier minimum loads, rounding increments, delivery fees and ordering requirements before placing an order. For bagged concrete, enter the mixed concrete yield printed on the product packaging, in litres per bag or cubic feet per bag. The calculator rounds up to a whole number of bags and multiplies by your price per bag.
Worked example: UK patio slab
A 4 m × 3 m slab at 10 cm thickness has a volume of 4 × 3 × 0.10 = 1.20 m³. Adding 10% gives 1.32 m³ of concrete. If each bag produces 12.5 litres of mixed concrete, 1,320 ÷ 12.5 = 105.6, so you would need 106 full bags. Check the yield on your chosen product before relying on the bag count.
Worked example: US concrete pad
A 12 ft × 10 ft pad with 4 in thickness requires 12 × 10 × (4 ÷ 12) = 40 ft³, or approximately 1.48 yd³ before extra allowance. With 10% added, plan for about 1.63 yd³.
What this calculator does not determine
This is a quantity and cost planning tool, not a structural design tool. It does not decide a safe slab thickness, concrete strength, reinforcement, foundations, ground preparation, drainage or building regulation requirements. For load-bearing structures, driveways or uncertain ground conditions, obtain project-specific guidance from a qualified professional.
Material costs exclude delivery, equipment, labour, reinforcement, formwork, excavation, taxes and any supplier minimum charges.
Frequently asked questions
How many bags of concrete do I need? Divide the total concrete volume, including your chosen allowance, by the mixed yield per bag. Round up to a whole bag. Use the manufacturer's yield rather than assuming every bag has the same output.
How many cubic feet are in a cubic yard? One cubic yard equals 27 cubic feet. One cubic metre is approximately 1.308 cubic yards.
Does the calculator work for footings or steps? You can estimate rectangular sections individually and add their volumes. The tool does not assess structural suitability or required dimensions.
Does changing GBP to USD convert prices? No. Enter the actual price in your chosen currency. The selector changes the symbol only.
Working on the rest of your project? Use our free Tile Calculator or Paint Calculator. Send feedback if you notice an issue.