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Estimate continuous strip or isolated pad footings with unit conversion, concrete allowance, premix bags, ready-mix cost, granular base and approximate reinforcement length.
Footing calculation symbols
Concrete footing formulas
For a strip footing, length is the total continuous run. For a pad footing, enter the plan length and width of one pad, then enter the pad quantity.
Footing measurement diagram
Measure concrete dimensions, not trench dimensions, unless the trench sides form the concrete directly. Include stepped levels as separate sections when their width or depth changes.
Worked strip-footing example
A 40 ft long footing, 18 in wide and 8 in deep contains:
Depth = 8 ÷ 12 = 0.6667 ft
Net volume = 40 × 1.5 × 0.6667 = 40 ft³
Net cubic yards = 40 ÷ 27 = 1.481 yd³
With 10% allowance = 1.630 yd³
At 0.60 ft³ per bag, the adjusted quantity requires 74 bags.
Worked pad-footing example
Six pads measuring 3 ft × 3 ft × 12 in contain 54 ft³, or 2.00 yd³, before allowance.
Footing types and planning use
| Footing type | Typical planning use |
|---|---|
| Continuous strip | Supports a continuous wall or closely spaced loads. |
| Isolated pad | Supports a single post, pier or column load. |
| Combined footing | Supports two or more columns; use engineered dimensions. |
| Stepped footing | Follows sloping grade in level steps; calculate each section separately. |
These descriptions identify geometry only. A qualified professional must determine bearing area and reinforcement where required.
Excavation, base and reinforcement checklist
- Confirm soil-bearing assumptions and required undisturbed bearing surface.
- Locate utilities before excavation.
- Keep loose soil, water, ice and organic material out of the footing base.
- Check width, depth, elevation and wall or column centerline before placement.
- Support reinforcing steel for required concrete cover.
- Brace forms against concrete pressure.
- Coordinate dowels, anchor bolts and sleeves before the pour.
The reinforcement result is an approximate two-direction grid takeoff with 10% added for cutting and laps. It does not specify bar diameter, development length, cover or structural capacity.
Frequently asked questions
What is the difference between a strip and pad footing?
A strip footing runs continuously under a wall or line of loads. A pad footing is an individual rectangular base under a post or column.
Should trench over-excavation be included?
Include only concrete-filled over-excavation in the concrete dimensions. Granular backfill or working space should be estimated separately.
How much extra concrete should I order?
Five to ten percent is common for planning, but irregular trenches, unstable sides or difficult access may require more.
Does the tool size a footing for soil bearing?
No. It uses dimensions you provide and does not calculate structural loads or allowable soil pressure.
Does ready-mix cost include all charges?
The estimate includes the entered delivery amount but may exclude tax, pump, waiting, environmental and short-load fees.
Common mistakes
- Using trench width when forms reduce the concrete width.
- Mixing inches, feet and metric units.
- Forgetting repeated pads or footing segments.
- Ignoring steps, thickened sections and column pedestals.
- Placing reinforcement directly on soil.