Design a square or rectangular spread footing under a single column. Enter the column size and service loads, a trial footing size and depth, and the allowable soil bearing pressure, and the calculator checks bearing pressure (with an optional applied moment and middle-third eccentricity check), one-way shear, two-way (punching) shear, and works out the required flexural steel in both directions plus a straight-bar development length check — all against ACI 318.
Switch between metric and imperial units at the top of the form; every result converts with it.
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This is a preliminary design aid, not a substitute for a full geotechnical and structural analysis. Always have a licensed engineer check the final design.
What the calculator checks
- Service bearing pressure. Column dead and live loads plus footing and soil self-weight, divided by the footing area — with an optional applied moment, giving the eccentricity, a middle-third check, and the resulting maximum/minimum edge pressures — compared against your allowable net soil bearing pressure.
- One-way shear. Factored shear at a section d away from the column face, checked against the concrete shear capacity in each direction.
- Two-way (punching) shear. Factored shear on the critical perimeter at d/2 from the column face, checked against the least of the three governing ACI 318 punching-shear equations (assumes an interior column, αs = 40).
- Flexural design. Required steel area both directions using the Whitney rectangular stress block at the critical section (the column face), with minimum steel and bar spacing.
- Development length. Straight-bar development length required per ACI 318 25.4.2.4, checked against the length actually available past the critical section.
What to have ready
- Column size and unfactored (service) dead and live axial load, plus any applied moment
- Trial footing length, width and thickness, and the depth to the top of the footing (for surcharge)
- Soil unit weight and allowable net bearing pressure
- Concrete strength, steel yield strength, clear cover and bar size
Notes and limitations
The punching-shear check assumes an interior column (αs = 40); for an edge or corner column use the lower αs value from ACI 318 22.6.5.3 and check the governing case by hand. This tool checks a single concentric or uniaxial-moment case — it does not check biaxial moment, combined footings, or footings on piles. Final acceptance of the design rests with the responsible engineer.
Isolated Footing Design Reference Table — ACI 318-19
| Design Parameter | Value / Formula | ACI 318-19 Section | Notes |
|---|---|---|---|
| Required footing area | A = P_service / q_allow | 13.3.1.1 | Use unfactored service load P and allowable bearing q_a |
| Net soil pressure for design (qu) | qu = Pu / A_footing | 13.3.2.1 | Use factored column load Pu (LRFD) |
| Critical section for flexure | Face of column / pedestal | 13.2.7.1 | For concrete columns; at edge of base plate for steel |
| Critical section for one-way shear | d from face of column | 13.2.7.2 | Beam-action shear; φVc = 0.75 × 2λ√f’c × b × d |
| Critical perimeter for two-way shear | b_o = 4(c + d) | 22.6.4.1 | Square column size c; b_o = perimeter at d/2 from column face |
| Two-way shear capacity (Vc) | min of 3 ACI equations | 22.6.5.2 | Governs: typically (2 + 4/βc)λ√f’c × b_o × d / 1000 (kips) |
| Minimum footing depth (concrete cover) | 3 in cover + bar diameter | 20.6.1.3 | 3 in cover for soil contact per ACI 318 Table 20.6.1.3 |
| Development length (ld) — #8 bar, f’c = 4 ksi | ≈ 47 in | 25.4.2.3 | Grade 60; no transverse reinforcement; clear spacing ≥ 2d_b |
| Minimum reinforcement ratio (ρ_min) | 0.0018 (Grade 60 deformed) | 13.3.3.2 | For footings: As,min = 0.0018 × b × h |
| Maximum bar spacing | min(3h, 18 in) | 13.3.3.3 | h = footing thickness; applies in each direction |
Source: ACI 318-19 (Building Code Requirements for Structural Concrete) Chapter 13 (Foundations).
Isolated Footing Design Calculator FAQ
How do I size an isolated footing for a column load?
Step 1 — Size the footing using service loads: Required area A = P_service / q_allowable, where P_service is the unfactored column load plus the estimated footing self-weight, and q_allowable is the allowable bearing capacity from the geotechnical report (psf). For a square footing, B = √A. Step 2 — Check bearing using factored loads: net factored soil pressure qu = Pu / A (kip/ft²), where Pu is the factored column load. Step 3 — Check flexure at the face of the column. Step 4 — Check one-way shear at a distance d from the column face. Step 5 — Check two-way (punching) shear at a perimeter d/2 from all sides of the column. If any check fails, increase footing dimensions or depth and repeat.
What is two-way (punching) shear, and how do I check it?
Two-way shear (also called punching shear) is the tendency of a column to punch through a slab or footing due to concentrated load. The critical perimeter b₀ is located at d/2 from each face of the column (ACI 318-19 Section 22.6.4.1), where d is the effective depth of the footing. For a square column of size c: b₀ = 4(c + d). The factored punching shear force is Vᵤ = Pᵤ − qᵤ × (c + d)². The concrete punching shear capacity is the minimum of three ACI 318-19 equations (22.6.5.2); the governing equation for most square columns is φVc = 0.75 × 4λ√f’c × b₀ × d (in pounds). If Vᵤ > φVc, increase the footing depth d or add shear reinforcement.
How do I design the flexural reinforcement in a footing?
The critical section for bending in an isolated footing is at the face of the column (ACI 318-19 Section 13.2.7.1). The factored moment Mᵤ = qᵤ × L × (B/2 − c/2)² / 2, where L is the footing dimension in the direction of bending and c is the column dimension. The required steel area is found using the standard ACI beam bending equations with φ = 0.90: As = Mᵤ / (φ × fy × j × d), where j ≈ 0.90 for typical shallow beams (iterate as needed). Distribute As uniformly across width L. The minimum reinforcement for footings is As,min = 0.0018 × b × h (ACI 318-19 Section 13.3.3.2 for Grade 60). Bar spacing must not exceed min(3h, 18 in) per Section 13.3.3.3.
What is the development length requirement for footing bars?
The tension development length ld must be available from the critical section (column face) to the end of the bar, accounting for the 3-inch concrete cover at the edge. Per ACI 318-19 Section 25.4.2.3, for a #6 bar (Grade 60, f’c = 4,000 psi, clear spacing ≥ 2d_b, no transverse reinforcement): ld ≈ 38 in; for a #8 bar: ld ≈ 47 in. The available development length = B/2 − c/2 − 3 in (cover). If the available length is less than ld, use smaller diameter bars, increase footing width, or use standard hooks. Standard 90° hooks may also be used at the bar ends to reduce required straight development length (ldh per ACI 318-19 Section 25.4.3).
Do I need to check bearing stress at the column-footing interface?
Yes — ACI 318-19 Section 16.3.3 requires checking bearing stress where the column meets the footing (or pedestal). The design bearing strength is: φBn = φ × 0.85 × f’c × A1 × √(A2/A1), where A1 is the loaded area (column footprint), A2 is the maximum area of the supporting surface that is geometrically similar to A1 (typically the footing area), and the ratio √(A2/A1) may not exceed 2.0. The strength reduction factor φ = 0.65 for bearing. If the column bearing stress exceeds φBn, dowels or extended column bars must be provided to transfer the excess force into the footing. Minimum dowel area is 0.005 × Ag of the column (ACI 318-19 Section 16.3.4.1).