🧱 Concrete Calculator

The volume formula depends on the shape you pick. Slab/footing: volume = length × width × thickness. For example, a 4m × 3m slab at 0.1m thick = 1.2 m³. Cylinder (column/footing): volume = π × (diameter ÷ 2)² × height. A 0.3m-diameter, 3m-tall column comes out to 3.14159 × 0.15² × 3 ≈ 0.212 m³. Stairs use a triangular-prism approximation: volume = width × number of steps × (rise × run) ÷ 2. The result is then multiplied by your chosen waste/overage % to get the volume to actually order or pour. For the material estimate, we apply the widely used construction-estimating rule of a nominal 1:2:4 mix (cement:sand:coarse aggregate by volume) with a dry-volume factor of 1.54 — dry, loose materials need roughly 1.54× more volume than the finished wet concrete because the particles pack tighter once mixed and hydrated. Example: 1.2 m³ total pour → dry volume 1.2 × 1.54 = 1.848 m³ → cement 1.848/7 = 0.264 m³ → at 1,440 kg/m³ that is about 380 kg → roughly 10 bags of 40kg cement. Sand comes out to 1.848 × 2/7 = 0.528 m³ (about 845 kg), and gravel to 1.848 × 4/7 = 1.056 m³ (about 1,605 kg).

Shape: pick Slab for floors, footings, and pavements; Cylinder for columns or isolated/pile footings; Stairs for staircases and outdoor steps. Typical waste allowances: small DIY pours (planters, stepping stones) can often use 0-5%; standard residential slabs and footings typically use 5%; uneven subgrade, formwork that may leak, or large/complex pours warrant 10% or more. The waste allowance accounts for mixing losses, formwork deflection, subgrade settlement that thickens the pour, and measurement error. Set it too low and you risk running short mid-pour (creating a cold joint); set it too high and you overpay for unused concrete — adjust based on your job-site conditions.

Cement can only be bought in 40kg bag increments, so a result of 9.3 bags should always be rounded up to 10 (this calculator already does that for you). Sand and gravel are usually sold in bulk (by m³ or ton), but for small jobs, bagged product can help avoid leftover waste. Factor in weather and site conditions: rain or high wind on pour day can increase mix-water loss or spillage, so bump up the waste allowance when conditions are unfavorable. If you are ordering ready-mix trucks, deliveries are rounded up to truck-capacity increments (4.5 m³, 5 m³, 6 m³, etc.), so plan ahead for how to use any leftover concrete (stepping stones, reinforcement collars) to minimize waste.