Concrete Footing Calculator
Estimate concrete for a strip or trench footing from its cross-section and run length, plus a yd³-per-100-ft rate.
Content updated September 14, 2026
Rectangular strip/trench-volume estimate only. Footing width, depth, and reinforcement are structural decisions — verify plans, soil conditions, and local code with a qualified professional before excavating or ordering.
Estimate the concrete volume for one or more equal continuous strip or trench footings from a rectangular cross-section — width and depth — run along a linear length, rather than a flat slab area. This matches how footings are actually formed and poured.
Because footings are commonly priced and ordered by run length, the tool also reports a normalized cubic-yards-per-100-linear-feet rate alongside the total order volume, cubic feet, cubic metres, and a whole-bag estimate.
The Concrete Footing formula
Cross-section = width × depth · net volume = cross-section × total run length · order volume = net volume × (1 + allowance% ÷ 100) · rate per 100 ft = cross-section × 100 × (1 + allowance% ÷ 100) ÷ 27Width, depth, and run length are converted to a common unit before multiplication · 1 yd³ = 27 ft³ exactly · bag yield must come from the applicable product.
Worked example
A 16 in × 8 in footing cross-section run 120 ft with a 10% allowance has a net volume of about 106.667 ft³ (3.951 yd³) and an order volume of about 117.333 ft³ (4.346 yd³) — about 3.621 yd³ per 100 linear feet. At 0.6 ft³ per bag that is 195.556 bags, rounding up to 196.
Assumptions, rounding, and limitations
Assumptions
- Each run is a uniform rectangular cross-section (width × depth) extruded along a straight run length.
- All runs entered are equal in cross-section and length; unequal runs must be calculated separately and added together.
- The entered contingency percentage is applied directly to net plan volume.
- International-foot and metric conversions use NIST exact relationships.
Rounding: Volume math retains floating-point precision. Displayed volumes show up to three decimal places; the purchase-oriented bag result always rounds the exact quotient upward to a whole bag.
Limitations
- Does not model corners, T-intersections, stepped footings, keyways, rebar displacement, formwork thickness, uneven subgrade, over-excavation, or supplier batch increments.
- Does not determine footing width, depth, frost depth, or reinforcement — those are structural and local-code decisions.
- The result is a planning estimate and should be checked against approved drawings, field measurements, and applicable project requirements.
Sources
- CIP 31 — Ordering Ready Mixed Concrete — National Ready Mixed Concrete Association
- NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology
A cross-section run along a length, not a slab area
A footing's volume is width × depth × total run length, where width and depth may use one convenient unit (such as inches) and run length another (such as feet). Multiple equal runs — for example, opposite sides of a foundation — multiply the same cross-section by their combined length.
NRMCA states that ready-mix concrete is sold by volume and that an order should generally be 4%–10% above a plan-dimension estimate for contingencies; the default here is 10%, but the input is exposed so the decision stays yours.
Why a per-100-foot rate is included
Footing work is frequently quoted and compared by linear foot rather than by total job volume, so the calculator reports cubic yards per 100 linear feet of the entered cross-section — a figure that stays constant regardless of how long your specific runs are.
The whole-bag estimate is included only for cases where a footing is bagged rather than ordered as ready-mix; for most footing pours the ready-mix cubic-yard figures are what you will actually order.
Frequently asked questions
▶How do I calculate concrete for a footing?
Multiply the footing's cross-section area (width × depth) by its total run length, then apply your chosen contingency allowance to get the order volume.
▶Why does this use a cross-section and run length instead of length × width like a slab?
A footing is a trench-shaped run, not a flat rectangular area, so its volume is naturally modeled as a fixed cross-section extruded along a length — which is also how it is typically formed and inspected on site.
▶What is the yd³-per-100-ft figure for?
It is a normalized rate — the concrete needed per 100 linear feet of your exact width and depth — useful for comparing footing jobs or estimating a longer perimeter run without recalculating from scratch.
▶Does this tell me what footing depth or width I need?
No. Footing size is a structural and local-code decision that depends on soil bearing capacity, frost depth, and the load above it. Always use the depth and width from approved plans or a qualified professional.
▶Will the estimate match what I actually use?
Not necessarily. Trench over-excavation, uneven subgrade, formwork loss, spillage, and supplier batching can all change the final amount from a plan-dimension estimate.
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Open calculator →Disclaimer: Concrete Footing Calculator results are estimates for general information and education only, and are not financial, tax, legal or medical advice. Verify important decisions with a qualified professional.