Concrete Footing Calculator — Volume and Bags for Strip Footings
Strip footings are rectangular trenches filled with concrete to support walls or structures above. This calculator gives you the total volume and number of premix bags needed — allowing a waste margin for spills and uneven trench bottoms.
How we calculate
Volume = length × width × depth (m). Volume with waste = volume × (1 + waste %). Bags = ⌈volume with waste ÷ bag volume⌉.
Worked example
6 m run, 400 mm wide, 300 mm deep: 6 × 0.4 × 0.3 = 0.72 m³. With 10% waste: 0.79 m³. At 0.012 m³/bag: 66 bags.
Common mistakes
- Measuring trench length along one side only — include all sides of a perimeter footing.
- Not compacting the trench base before pouring — soft ground leads to settlement cracks.
- Pouring in freezing temperatures without frost protection — fresh concrete can fail below 5 °C.
- Underestimating depth for shrinkable clay soils — consult a structural engineer for reactive soils.
Frequently asked questions
How wide and deep does a strip footing need to be?+
For a typical single-storey residential wall, a strip footing is usually 300–450 mm wide and 300 mm deep. The exact dimensions depend on the load above and the soil bearing capacity. Always check local building codes or consult a structural engineer for load-bearing work.
How many bags of concrete for a strip footing?+
A 6 m run at 400 mm wide and 300 mm deep needs about 66 bags (25 kg each) with 10% waste. Volume is simply length × width × depth — use this calculator to adjust for your specific dimensions.
Do I need to compact the trench base before pouring?+
Yes. Loose or disturbed soil under a footing can lead to settlement and cracking. Compact the base with a plate compactor or hand tamper, and remove any soft spots and replace with compacted hardcore before pouring.
Can I pour footings in cold weather?+
Avoid pouring when temperatures are below 5 °C. Fresh concrete loses strength rapidly if it freezes before it cures. If you must pour in cold conditions, use frost protection blankets and a winter-grade admixture, and do not strip formwork early.