Foundation Footing Size Calculator
ANA›Life Services Authority›National Calculator Authority›Foundation Footing Size Calculator
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Foundation Footing Size Calculator
Calculate the minimum required footing area and dimensions based on structural load and allowable soil bearing capacity.
Total Column / Wall Load (kips or kN)
Load Unit
Kips (1 kip = 1,000 lbf) Kilonewtons (kN)
Allowable Soil Bearing Capacity (ksf or kPa)
Bearing Capacity Unit
ksf (kips per square foot) kPa (kiloPascals)
Estimated Footing Self-Weight (% of column load)
Footing Shape
Square Rectangular (2:1 ratio) Circular
Calculate
function fouCalc() { var load = parseFloat(document.getElementById('fou-load').value); var loadUnit = document.getElementById('fou-load-unit').value; var bearing = parseFloat(document.getElementById('fou-bearing').value); var bearUnit = document.getElementById('fou-bearing-unit').value; var swPct = parseFloat(document.getElementById('fou-footing-weight').value); var shape = document.getElementById('fou-shape').value; var resultDiv = document.getElementById('fou-result');
// --- Validation --- if (isNaN(load) || load ⚠ Please enter a valid positive column/wall load.'; return; } if (isNaN(bearing) || bearing ⚠ Please enter a valid positive soil bearing capacity.'; return; } if (isNaN(swPct) || swPct 30) { resultDiv.style.display = 'block'; resultDiv.innerHTML = '⚠ Self-weight percentage must be between 0 and 30%.'; return; }
// --- Unit Normalisation to kips & ksf --- var loadKips, bearKsf;
if (loadUnit === 'kips') { loadKips = load; } else { // 1 kN = 0.224809 kips loadKips = load * 0.224809; }
if (bearUnit === 'ksf') { bearKsf = bearing; } else { // 1 kPa = 0.020885 ksf bearKsf = bearing * 0.020885; }
// --- Core Formula --- // Total design load includes self-weight of footing // P_total = P_column * (1 + swPct/100) var totalLoad = loadKips * (1 + swPct / 100);
// Required Area A_req = P_total / q_allow var areaReqSqFt = totalLoad / bearKsf;
// --- Dimension Calculation by Shape --- var dimStr = ''; var areaActual = areaReqSqFt;
if (shape === 'square') { var side = Math.sqrt(areaReqSqFt); // Round up to nearest 3-inch (0.25 ft) increment var sideRounded = Math.ceil(side / 0.25) * 0.25; areaActual = sideRounded * sideRounded; dimStr = 'Side Length' + sideRounded.toFixed(2) + ' ft (' + (sideRounded * 12).toFixed(1) + ' in)' + 'Actual Footing Area' + areaActual.toFixed(2) + ' ft²'; } else if (shape === 'rectangular') { // L = 2B → A = 2B² → B = sqrt(A/2) var B = Math.sqrt(areaReqSqFt / 2); var L = 2 * B; var BRounded = Math.ceil(B / 0.25) * 0.25; var LRounded = Math.ceil(L / 0.25) * 0.25; areaActual = BRounded * LRounded; dimStr = 'Width (B)' + BRounded.toFixed(2) + ' ft (' + (BRounded * 12).toFixed(1) + ' in)' + 'Length (L = 2B)' + LRounded.toFixed(2) + ' ft (' + (LRounded * 12).toFixed(1) + ' in)' + 'Actual Footing Area' + areaActual.toFixed(2) + ' ft²'; } else { // Circular: A = π r² → r = sqrt(A/π) → D = 2r var r = Math.sqrt(areaReqSqFt / Math.PI); var D = 2 * r; var DRounded = Math.ceil(D / 0.25) * 0.25; areaActual = Math.PI * Math.pow(DRounded / 2, 2); dimStr = 'Diameter (D)' + DRounded.toFixed(2) + ' ft (' + (DRounded * 12).toFixed(1) + ' in)' + 'Actual Footing Area' + areaActual.toFixed(2) + ' ft²'; }
// --- Net Bearing Pressure Check --- var netPressure = totalLoad / areaActual; // ksf var utilisation = (netPressure / bearKsf) * 100;
// --- Convert back to SI for display --- var areaReqM2 = areaReqSqFt * 0.092903; var areaActM2 = areaActual * 0.092903; var netPresKPa = netPressure / 0.020885;
resultDiv.style.display = 'block'; resultDiv.innerHTML = '### ✓ Footing Sizing Results ' + '' + 'ParameterValue' + 'Column / Wall Load' + loadKips.toFixed(2) + ' kips (' + (loadKips * 4.44822).toFixed(1) + ' kN)' + 'Footing Self-Weight (' + swPct + '%)' + (loadKips * swPct / 100).toFixed(2) + ' kips' + 'Total Design Load' + totalLoad.toFixed(2) + ' kips' + 'Allowable Bearing Capacity' + bearKsf.toFixed(3) + ' ksf (' + (bearKsf / 0.020885).toFixed(1) + ' kPa)' + 'Required Footing Area' + areaReqSqFt.toFixed(2) + ' ft² (' + areaReqM2.toFixed(2) + ' m²)' + dimStr + 'Net Bearing Pressure' + netPressure.toFixed(3) + ' ksf (' + netPresKPa.toFixed(1) + ' kPa)' + 'Capacity Utilisation' + utilisation.toFixed(1) + '%' + (utilisation > 100 ? ' ⚠ EXCEEDS CAPACITY' : utilisation > 90 ? ' ⚠ Near limit' : ' ✓ OK') + '' + ''; }
#### Formulas Used
1. Total Design Load
Ptotal = Pcolumn × (1 + SW%/100)
where SW% is the estimated self-weight of the footing as a percentage of the column load.
2. Required Footing Area
Areq = Ptotal / qallow
where qallow is the allowable soil bearing capacity.
3. Footing Dimensions
- Square: B = √Areq → rounded up to nearest 3-in increment
- Rectangular (2:1): B = √(Areq/2), L = 2B → each rounded up to nearest 3-in increment
- Circular: D = 2√(Areq/π) → rounded up to nearest 3-in increment
4. Net Bearing Pressure Check
qnet = Ptotal / Aactual ≤ qallow
#### Assumptions & References
- The allowable soil bearing capacity (qallow) must be determined by a geotechnical engineer from a soil investigation report; typical values range from 1–4 ksf (50–200 kPa) for cohesive soils and 2–8 ksf (100–400 kPa) for granular soils.
- References: ACI 318-19 (Building Code Requirements for Structural Concrete); ASCE 7-22 (Minimum Design Loads); Das, B.M. — Principles of Foundation Engineering, 9th ed.
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