Concrete Calculator

Getting the right amount of concrete saves time, money, and trips to the hardware store. This calculator gives you the volume in both metric and imperial units, plus the number of 60lb premix bags you need, with a 10% waste buffer built in.

• Garden patio: Calculate concrete needed for a backyard patio slab.
• Driveway: Estimate volume for a driveway at a standard 10–15 cm depth.
• Fence post footings: Calculate how many bags fill a cylindrical post hole (use volume = π × r² × depth).
• Shed foundation: Size the concrete pour for a small outbuilding base.

1. 1Enter the length, width, and depth of the area in meters.
2. 2For thickness, enter in meters. A 10 cm slab is 0.10 m.
3. 3The calculator multiplies the three dimensions for volume in m³.
4. 4It converts to cubic yards and estimates 60lb premix bags.
5. 5A 10% overage recommendation accounts for waste and spillage.

• Not including a waste factor: Concrete pours always involve spills, uneven subgrades, and slightly irregular forms. Order at least 5-10% more than the calculated volume. The calculator adds 10% by default.
• Entering depth in centimeters instead of meters: A 10 cm slab is 0.10 m thick, not 10 m. Entering depth as centimeters (without converting) overstates the volume by 100x. Always convert to meters or use consistent units throughout.
• Mixing premix bags with ready-mix volume: Premix bags are for small pours. Ready-mix trucks are for large volumes. If your slab needs more than 1 cubic meter, ready-mix is usually cheaper and more practical than buying dozens of bags.
• Ignoring subgrade preparation: Concrete placed on soft or uneven ground settles and cracks. The calculator tells you how much concrete to buy, not whether the subgrade is suitable. Compact the base before pouring.

• Order by the cubic meter, not the bag: For any pour above 0.5 cubic meters (about 18 bags of 27 kg premix), get ready-mix quotes. The material cost is similar but you save hours of mixing labor.
• Add expansion joints for large slabs: Concrete expands and contracts with temperature. Slabs over 3 m in any dimension need expansion joints to prevent cracking. The joint spacing does not change the volume calculation, but it affects the pour plan.
• Account for formwork beyond the edge: Your slab dimensions may be 3 m × 4 m, but the formwork sits outside those edges. The concrete fill area is the interior dimension, not the formwork outer edge. Double-check before ordering.
• Use the bag count for stockpile planning: The bag-count output helps you plan transport and storage. One 27 kg bag fills about 0.028 cubic meters. Confirm the bag size matches what your supplier stocks before ordering.