Home Improvement
Concrete Calculator
Slab length, width and depth in — cubic metres and bags out.
How much concrete do I need?
Order a little extra concrete; short loads are miserable and cold joints look permanent. Enter length, width and depth to estimate volume, then compare premix bag counts with a volumetric truck quote. Bag yields differ by brand — use the litres or m³ printed on your bag rather than a generic internet chart. Uneven ground and deep footings need more than a flat slab formula; measure the deepest points and consider stepped pours. Related DIY volume tools include garden soil and flooring calculator for covering area rather than cubic volume. Reinforcement, damp-proof membranes and shuttering are separate purchases. This is a quantity estimate, not a structural design or Building Regulations approval. Cold weather slows curing and can tempt people to under-order because the truck feels expensive on the day — the shortfall still hurts more. Check access for a chute or barrow run before you commit to ready-mix; bags remain sensible only when the volume is small. Ask the supplier which mix suits paths versus structural pads, then stick to the volume maths rather than rounding down to save a few pounds.
Worked example
A path 8 m long, 1.2 m wide and 100 mm deep: volume = 8 × 1.2 × 0.10 = 0.96 m³. Adding 10% waste suggests ordering about 1.06 m³. If bags yield 0.012 m³ each after mixing, you need roughly 88 bags — usually enough reason to prefer a small truck for anything beyond a tiny pad.
Limits and assumptions
Assumes a rectangular prism. Slopes, trenches and pier holes need segmented volumes. Does not specify mix strength (C20, C25, etc.) — ask your supplier for the job. Not engineering advice.
Frequently asked questions
How much waste should I add?
Many DIYers add 5–10% for spillage and uneven bases. Complex forms may need more.
Should depth be in millimetres or metres?
Keep units consistent. Convert everything to metres for m³, or use the unit mode the form expects.
Bags or ready-mix?
Small pads suit bags; larger slabs usually suit a truck. Compare cost using the volume this tool estimates.
How do I convert feet and inches?
Convert to metres first (1 inch = 0.0254 m) so volume stays in m³ for UK suppliers.
Does this include hardcore sub-base?
No — calculate sub-base volume separately if you are ordering Type 1 or similar.