Introduction
Strip footing questions fall into a few predictable groups: how wide, how deep, how much concrete, does it need steel, and what the inspector will want to see. The answers are short once the concepts are clear — width comes from load divided by soil capacity, depth comes from the frost line, and volume comes from multiplying the trench geometry. This page collects the questions footing calculators and contractors hear most often.
The Concrete Footing Calculator answers the quantity question directly — enter run length, width, depth, and a waste allowance for the concrete volume in metric and imperial. The Q&As below cover everything around that number: sizing logic, frost depth, reinforcement, stepped footings, and inspections. For the full narrative version, see the complete guide.
Sizing Questions
How wide should a strip footing be?
Width = wall load per unit length ÷ allowable soil bearing capacity, rounded up to standard sizes and never below local minimums (commonly 40–45 cm / 16–18 in for houses). Example: 28 kN/m on 120 kPa soil → 28 ÷ 120 = 0.233 m (9.2 in) by the math, built at 45 cm (18 in). The formula guide derives this with cross-checked metric and imperial arithmetic.
How thick should a footing be?
Typically 20–30 cm (8–12 in) for residential work — 20 cm for light garden walls, 25–30 cm for houses. Thickness is the concrete’s structural depth, not the trench depth; the trench goes deeper to get the footing’s base below the frost line.
How do I know my soil’s bearing capacity?
Use your local building authority’s default values for the area as a starting point. Soft clay runs ~75–100 kPa (1,500–2,000 psf), firm soils ~150 kPa (3,000 psf), dense gravel or rock 200–400+ kPa. For anything beyond a light single-story structure on known ground, a geotechnical report is the reliable answer — soil capacity is the one input you should never guess at.
Depth and Frost Questions
How deep should a footing trench be?
Deep enough that the footing’s base sits below the local frost line, plus the footing’s own thickness. In frost-free areas that may be 0.3–0.5 m (12–20 in); in cold climates 1.0–1.5 m (40–60 in). Your building authority publishes the required depth — it varies by locality, not by nationality. See the code-compliance guide for the rules.
What happens if the footing is above the frost line?
Frost heave: water in the soil freezes, expands, and lifts the footing unevenly each winter. The result is cracked foundation walls and settlement damage that costs far more than the deeper trench would have.
Can the trench be wider than the footing?
Yes — trenches are often dug slightly wider for working room, and the extra space is backfilled after the pour. But order concrete from the footing dimensions, not the trench dimensions, unless you’re pouring directly against the trench walls with no forms.
Reinforcement Questions
Do strip footings need rebar?
Residential strip footings typically include two continuous longitudinal bars (12 mm / #4), one near each face. The steel resists cracking from minor differential settlement and temperature movement. Unreinforced footings are generally limited to very light, non-structural work — check local requirements.
How much concrete cover does footing rebar need?
At least 75 mm (3 in) of concrete between steel and soil on every side. Achieve it with chairs or concrete blocks every 1.0–1.5 m (3–5 ft) — bars laid on the trench bottom have no cover and will corrode.
How long should rebar laps be?
About 40 bar diameters: 0.48 m (19 in) for 12 mm (#4) bars. Stagger the laps between the two bars so joints don’t align at one cross-section, and run bars continuously around corners with L-shaped corner bars.
Pour and Quantity Questions
How much concrete do I need for footings?
Volume = run length × width × depth × (1 + waste%). Example: 40 m × 0.6 m × 0.3 m with 10% waste = 7.92 m³ (10.36 yd³) — about 466 eighty-pound bags, so ready-mix is the practical choice. The imperial cross-check: 131.23 × 1.97 × 0.98 ft = 254.3 ft³ = 9.417 yd³ × 1.10 = 10.36 yd³.
Should I use bags or ready-mix for footings?
Bagged mix only makes sense below roughly 1 m³ (1.3 yd³) — about 65 eighty-pound bags. Above that, ready-mix is cheaper per unit and pours in a fraction of the time; a continuous pour also avoids the cold joints that bag-by-bag placement invites.
What is a stepped footing, and when do I need one?
On sloped ground, the footing is built as level steps dropping downhill — each step no deeper than the footing’s thickness (typically 0.3 m / 12 in), each at least 0.6 m (2 ft) long. Stepping keeps every section bearing on level soil. Estimate each step as its own rectangle and add them.
Can I pour the footing and the slab together?
Yes for light structures — a monolithic pour with a thickened edge around the perimeter is standard for garages and sheds. Houses usually pour the footing separately so it can be inspected before the foundation wall goes on.
Inspection and Code Questions
Do footings need a building permit and inspection?
In most jurisdictions, yes for structural footings: a permit before work starts, and an inspection of the open trench with reinforcement in place before concrete is poured. Garden-wall and other non-structural footings are often exempt — confirm with your local authority rather than assuming.
How soon can I build on a new footing?
Foundation walls or framing typically start a few days after the pour in normal weather, once the concrete has gained enough strength — longer in cold conditions. Keep the footing moist during the first days; strength comes from hydration, not drying.
Related Guides
- Concrete Footing Complete Guide — strip footing sizing, trench dimensions, and the full worked example.
- Concrete Footing Formula Explained — the width, volume, and rebar math derived step by step.
- Concrete Footing Cost Guide — what footings cost per linear metre and where the money goes.
- Concrete Footing Pro Tips — pour-day and ordering advice from the trade.
- Concrete Footing Common Mistakes — the errors that waste concrete and weaken footings.
- Concrete Footing Checklist — everything to verify before the concrete arrives.
Frequently Asked Questions
What is the formula for concrete footing volume?
Volume = run length × footing width × footing depth × (1 + waste%). Keep all three dimensions in consistent units, convert with 1 m³ = 1.308 yd³, and round bag counts up.
How deep must footings be below the frost line?
The footing’s base — not its top — must sit below the local frost depth. Required depths range from ~0.3 m (12 in) in frost-free areas to 1.0–1.5 m (40–60 in) in cold climates; your local building authority sets the number.
How wide is a standard residential footing?
Commonly 45–60 cm (18–24 in) wide and 25–30 cm (10–12 in) thick for houses, but “standard” never replaces the calculation: width = wall load ÷ soil bearing capacity, checked against local minimums.
Do I need rebar in a concrete footing?
Typically yes — two continuous 12 mm (#4) bars on chairs with 75 mm (3 in) cover is the residential norm. Unreinforced footings suit only very light, non-structural work.
How much waste should I add to a footing estimate?
Ten percent for straight trenches in firm ground; 12–15% for hand-dug, irregular, or over-excavated trenches. Trench work is less predictable than slab formwork, so footing waste runs slightly higher.
Can footing concrete be poured in cold weather?
Yes with precautions: protect the concrete from freezing for the first several days (insulated blankets, heated enclosures), never pour on frozen ground, and expect slower strength gain. Below-freezing pours without protection risk permanent strength loss.
Still Deciding? Run the Quantity First
Most footing decisions get easier with the concrete volume in hand — it sets the delivery method, the budget’s concrete line, and the scale of the job. The Concrete Footing Calculator takes your run length, width, depth, and waste allowance and returns the volume in cubic meters, cubic yards, and bags. Start there, then use the guides above for the sizing, code, and pour-day decisions around it.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Actual footing requirements vary with soil conditions, local frost depth, trench accuracy, and mix design. Verify volumes with your supplier and consult a licensed professional or your local building authority for structural or code-related decisions.
