Deck Footing Spacing Calculator

Calculate post distance and footing diameter based on deck loads

Joist Span (ft) The distance the joists travel from the house to the beam.

Beam Configuration Double 2×8 Double 2×10 Double 2×12 Triple 2×10 Triple 2×12 Select the size of your structural beam.

Soil Bearing Capacity (psf) 1500 (Clay/Silt/Silty Sand) 2000 (Sand/Gravelly Sand) 3000 (Sedimentary Rock/Hard Clay) Standard soil is typically 1500-2000 psf.

Total Load (psf) Typically 40 (Live) + 10 (Dead) = 50 psf.

Recommended Specifications:

Max Spacing Between Posts 0 ft

Required Footing Diameter 0 in

Load Per Footing 0 lbs

Tributary Area 0 sq.ft

How Deck Footing Spacing Works

Designing a safe deck requires understanding how weight is distributed through the structure. The footings are the final point of contact with the earth, and they must be spaced appropriately to prevent the beam from sagging and sized correctly to prevent the posts from sinking into the ground.

Key Engineering Concepts

Tributary Area: This is the square footage of the deck that a single footing is responsible for carrying. It is calculated as (Joist Span / 2) × Post Spacing.
Total Design Load: Building codes usually require a deck to support 40 pounds per square foot (psf) of live load (people, furniture) and 10 psf of dead load (the weight of the wood itself).
Soil Bearing Capacity: Different soils can hold different weights. If your soil is soft clay, you need wider footings to distribute the weight than you would if you were building on compacted gravel.

Example Calculation

If you have a 10-foot joist span and you space your posts 8 feet apart, the tributary area for a center footing is 40 square feet (5ft × 8ft). At a 50 psf design load, that single footing must support 2,000 lbs. In soil with a 1,500 psf capacity, you would need a footing with an area of 1.33 square feet, which translates to roughly a 16-inch diameter round footing.

Pro Tip: Always check your local building codes. Many jurisdictions have specific span tables that supersede general calculations, especially in areas with high snow loads.

function calculateDeck() { var joistSpan = parseFloat(document.getElementById('joistSpan').value); var beamFactor = parseFloat(document.getElementById('beamSize').value); var soilCap = parseFloat(document.getElementById('soilCapacity').value); var designLoad = parseFloat(document.getElementById('totalLoad').value); if (isNaN(joistSpan) || joistSpan <= 0 || isNaN(designLoad) || designLoad 12) maxSpacingVal = 12; // Tributary Area (Assuming spacing is max spacing) var tributaryArea = (joistSpan / 2) * maxSpacingVal; // Load per footing var loadPerFooting = tributaryArea * designLoad; // Required Area in square feet var requiredAreaSqFt = loadPerFooting / soilCap; // Diameter in inches: Area = Pi * r^2 -> r = sqrt(Area/Pi) -> D = 2 * r * 12 (to inches) var diameterInches = Math.sqrt(requiredAreaSqFt / Math.PI) * 2 * 12; // Standard footing sizes are usually in 2-inch increments (round up) var recommendedDia = Math.ceil(diameterInches / 2) * 2; if (recommendedDia < 8) recommendedDia = 8; // Update Display document.getElementById('maxSpacing').innerText = maxSpacingVal.toFixed(1) + " ft"; document.getElementById('footingDia').innerText = recommendedDia + " in"; document.getElementById('footingLoad').innerText = Math.round(loadPerFooting).toLocaleString() + " lbs"; document.getElementById('tribArea').innerText = tributaryArea.toFixed(1) + " sq.ft"; document.getElementById('results').style.display = 'block'; }