Measure the poured slab rather than the excavation
A patio, shed pad or flat walkway is a shallow rectangular prism. Measure the inside of the forms for length and width, then enter the intended concrete thickness. The excavation can be deeper because it also contains a prepared subbase. That gravel is not concrete and should not be included in this depth. Metric thickness is entered in centimetres while plan dimensions use metres. In US mode, thickness uses inches and the two plan dimensions use feet. These mixed units are converted before multiplication.
Separate geometric volume from the order
Slab volume = length × width × thickness; order = volume × (1 + allowance ÷ 100)The first result includes the editable order allowance. Another row preserves the exact geometric slab volume so the extra quantity remains visible. Uneven formation, small spills and form variations can increase consumption. Choose an allowance for your own project rather than treating the example percentage as a structural requirement. Thickened edges, footings, steps and isolated pads require additional measured volumes.
A five by four metre pad
A 5 metre by 4 metre pad with 10 centimetres of uniform concrete contains 2 cubic metres. Adding ten percent produces an order of 2.2 cubic metres, approximately 2.878 cubic yards. If one bag yields 20 litres of mixed concrete, that order requires 110 bags. The yield must describe finished mixed material: the bag's dry weight is not a volume. Different bag weights and products have different yields, so read the packaging.
Before arranging a pour
For a nonrectangular slab, split the outline into simple rectangles and calculate each portion. Avoid overlapping pieces, then combine their quantities before applying any supplier minimum or delivery rounding. Batching hundreds of bags may require more labour and equipment than a ready-mix delivery. Ask the supplier about access, unloading time and ordering increments. This worksheet measures material; it does not specify reinforcement, concrete strength, curing practice or the thickness needed to carry a load. Decide those details from the actual ground conditions and design. Keep bag yield and allowance editable when comparing products.
Frequently asked questions
Does the calculator include gravel?
No. Enter only the concrete layer. Measure the compacted gravel base separately and do not use the total excavation depth as slab thickness.
Why show cubic yards?
Ready-mix suppliers in the US commonly quote by the cubic yard. The tool converts the order volume using 27 cubic feet per cubic yard.
Can I enter zero waste?
Yes. A zero allowance gives the geometric volume without extra material. It does not cover irregular formation or spills during placement.
What bag yield should I use?
Use the package's finished mixed-concrete yield in litres or cubic feet. Do not enter kilograms or pounds in this volume field.
Are thickened edges included?
Only if you account for them separately. The equation assumes one uniform depth across the entire rectangular slab.
Last reviewed: 2026-10-05 · How we calculate