How to Calculate Concrete Volume
To determine the amount of concrete needed, you calculate the volume using Length × Width × Depth. To convert the wet volume of concrete to dry materials, we multiply by a dry volume factor of ~1.54.
From the total dry volume, we can calculate the individual proportions of cement, sand, and aggregate based on the mix ratio.
Frequently Asked Questions
To determine the amount of concrete needed, you calculate the volume using Length × Width × Depth. To convert the wet volume of concrete to dry materials, multiply by a dry volume factor of ~1.54.
Calculate the volume in cubic metres (m³): Volume = Length × Width × Depth. For a slab 4m × 3m × 0.1m deep, volume = 1.2 m³. Add 5–10% waste allowance. Order ready-mix or calculate bags needed (a 25kg bag of premix typically yields ~0.012 m³ of concrete at standard mix).
Common concrete mix ratios by volume (cement:sand:aggregate): C20 general purpose — 1:2:4; C25 structural — 1:1.5:3; C30 high strength — 1:1:2. The water-cement ratio (w/c) critically affects strength — typically 0.4–0.6 by weight. Lower w/c = stronger, less permeable concrete.
Approximately 83 bags of 25kg premix concrete are needed per cubic metre (m³). For site-batched concrete, you need roughly: 6–7 bags (50kg) of cement, 0.5 m³ of sand, and 0.8 m³ of 20mm aggregate per 1 m³ of finished concrete at a 1:2:4 mix ratio.
Concrete gains strength over time: at 1 day it reaches ~20% of 28-day strength; at 7 days ~65%; at 28 days ~100% (design strength). However, curing continues for years. Keep concrete moist for at least 7 days by covering with wet hessian, plastic sheeting, or curing compound. Avoid foot traffic for 24–48 hours, vehicular traffic for 7–10 days.
Cement is a binding powder (typically Portland cement) made from limestone and clay. Concrete is a composite material made by mixing cement with water, sand (fine aggregate), and gravel or crushed stone (coarse aggregate). Cement is just one ingredient of concrete — roughly 10–15% by volume.