What code does this calculator follow?

Strength checks follow ACI 318-19 for reinforced concrete footings (two-way and one-way shear, flexure). Service sizing uses allowable soil bearing entered by the user.

Do I need both service and factored loads?

Yes. Service load with allowable bearing determines the plan area, while the factored load Pu governs shear and flexural design per ACI 318.

Is footing self-weight included?

The calculator sizes the plan area using net allowable bearing (typical geotechnical convention). If your allowable bearing is gross, increase the service load to include footing self-weight and overburden.

Can I switch US/SI units?

Yes. Toggle units at the top; values are converted intelligently. All formulas are unit-consistent.

Are flexural steel results conservative?

The lever arm is approximated as z ≈ 0.9d, which is standard for preliminary footing design and generally conservative for under-reinforced sections.

Is this tool suitable for final design?

Use it for preliminary sizing and verification. Final design must be performed and sealed by a licensed structural engineer with project-specific loads and soil data.

ACI 318 Concrete Footing Design Calculator

Data Source and Methodology

Authoritative Data Source: ACI Committee 318. 2019. Building Code Requirements for Structural Concrete (ACI 318-19) and Commentary. American Concrete Institute. Direct link to ACI 318-19.

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

Service sizing: Areq = Pservice / qa (net allowable bearing).
Strength checks: two-way (punching) shear, one-way shear, and flexure per ACI 318-19 Chapters on slabs and footings.
Strength reduction factors: φv = 0.75 (shear), φb = 0.9 (flexure).

The Formulas Explained

Required area (service):

$$ A_{req} = \frac{P_{service}}{q_a} $$

Adopted plan dimensions for square footing:

$$ B = L = \sqrt{A_{req}} $$

Factored soil pressure:

$$ q_u = \frac{P_u}{B \cdot L} $$

Two-way (punching) shear around column:

$$ b_o = 2\,(c_1 + c_2) + 4d \quad,\quad A_0 = (c_1 + d)(c_2 + d) $$

$$ V_u = P_u - q_u \, A_0 \quad,\quad \phi V_c = \phi_v \, 4 \sqrt{f'_c}\, b_o\, d $$

One-way shear (worst direction shown):

$$ a_x = \frac{B - c_1}{2} \quad,\quad V_{u,x} = q_u \, L \, (a_x - d) $$

$$ \phi V_{c,x} = \phi_v \, 2 \sqrt{f'_c}\, L\, d $$

Flexure per unit width (cantilever strip, lever arm z ≈ 0.9d):

$$ M_{u,strip} = \frac{q_u\, a^2}{2} \quad,\quad A_s \approx \frac{M_u}{\phi_b \, f_y \, z} $$

Unit consistency is required throughout (US or SI).

Glossary of Variables

Pservice: Service axial load at footing level (kips or kN).
Pu: Factored axial load (kips or kN) per governing load combination.
qa: Allowable soil bearing pressure (ksf or kPa), net recommended.
B, L: Footing plan dimensions along x and y (ft/in or m/mm).
c1, c2: Column side dimensions along B and L (in or mm).
h: Overall footing thickness (in or mm).
d: Effective depth from compression face to centroid of tension steel (in or mm).
f′c: Concrete compressive strength (psi or MPa).
fy: Steel yield strength (ksi or MPa).
bo: Critical perimeter at distance d/2 from column face (in).
Vu: Factored shear demand (kips or kN).
φVc: Design shear capacity after φ (kips or kN).
As: Required steel area per unit width in each direction (in²/ft or mm²/m).

How It Works: A Step‑by‑Step Example

Given: US units, square footing; Pservice = 200 kips, qa = 4 ksf, Pu = 300 kips; column 24 × 24 in; f′c = 4000 psi, fy = 60 ksi; cover = 3 in; bars #5.

Area: Areq = 200 / 4 = 50.0 sf → Square size B = L = √50 ≈ 7.07 ft ≈ 85 in (rounded).
Factored pressure: qu = 300 / 50.17 ≈ 5.98 ksf (using 85 in exact size → 50.17 sf).
Punching shear: assume h ≈ 18 in → d ≈ 18 − 3 − 0.3125 ≈ 14.69 in. bo = 2(24+24)+4·14.69 ≈ 154.8 in. A0 = (24+14.69)²/144 ≈ 10.4 sf. Vu = 300 − 5.98·10.4 ≈ 237.8 kips. φVc = 0.75·4√4000·bo·d / 1000 ≈ 431.8 kips > Vu OK.
One-way shear (x-dir.): ax = (B − c1)/2 = (85 − 24)/2 in = 30.5 in = 2.54 ft. Vux = qu · Lft · (ax − d) ≈ 5.98 · 7.08 · (2.54 − 1.22) ≈ 55.8 kips. φVc1 ≈ 0.75·2√4000·L(in)·d /1000 ≈ 118.5 kips > Vux OK.
Flexure (per ft strip): Mu = qu·a²/2 ≈ 5.98·(2.54²)/2 ≈ 19.3 kip-ft. With z ≈ 0.9d, As ≈ Mu / (φ·fy·z) ≈ 0.32 in²/ft; As,min ≈ 0.0018·12·18 ≈ 0.39 in²/ft → use 0.39 in²/ft. With #5 bars (0.31 in²), spacing s ≈ (0.31/0.39)·12 ≈ 9.5 in.

Result: 85 in × 85 in footing, h ≈ 18 in, #5 @ 9.5 in each way (round to the nearest practical spacing, e.g., 9 in or 10 in, based on detailing and cover rules).

Frequently Asked Questions (FAQ)

Which edition of ACI 318 is used?

ACI 318-19 is referenced for shear and flexure provisions. Always verify with the latest adopted code in your jurisdiction.

What’s the difference between service and factored loads?

Service loads reflect unfactored real-world actions and are used with allowable soil bearing for plan sizing. Factored loads (Pu) incorporate load factors for ultimate strength checks per ACI 318.

Does the calculator include footing self-weight?

Plan sizing assumes a net allowable bearing (typical). If your qa is gross, include the footing and soil overburden in your service load before entering values.

How is punching shear perimeter computed?

The critical perimeter is taken at d/2 from the column face. For a rectangular column, b_o = 2(c₁ + c₂) + 4d.

Are the reinforcement results exact?

Flexural steel is based on a standard z ≈ 0.9d lever-arm approximation and per-foot strip design. This is robust for preliminary design; refine as needed for final detailing.

Can I use metric units?

Yes. Toggle to SI to switch to kN, kPa, and mm. The tool converts values and enforces unit consistency in formulas.

Is this tool a substitute for an engineer?

No. It is intended for preliminary design. Final design must be checked and sealed by a licensed structural engineer with project-specific inputs.

Strumento sviluppato da Ugo Candido,. Contenuti verificati da,.
Last reviewed for accuracy on: September 14, 2025.