Free Concrete Footing & Deck Post Load Calculator

Calculate tributary column loads in pounds, verify soil bearing capacity, and size round sonotubes or square footing pads with zero guesswork.

🏗️ Deck Post Load & Soil Condition

Soil Type & Bearing Capacity

Tributary Length (ft) Joist span / 2

Tributary Width (ft) Beam span / 2

Deck Live Load (psf) Residential code = 40 psf

Dead Load (psf) Framing & boards = 10 psf

Footing Depth Below Frost Line (Inches) Local frost depth requirement

📊 Recommended Footing Size & Concrete

Total Column Load on Soil

3,200 lbs

64 sq ft tributary area @ 50 psf design load

Round Sonotube Size

18" or Bell Base

Min 2.13 sq ft base area

Square Pad Sizing

18" × 18" Min

Pour 8" thick pad at bottom

Concrete Volume (12" Pier): 2.36 cu ft (0.09 cu yd)

80 lb Pre-Mix Bags: 4 Bags (80 lb each)

60 lb Pre-Mix Bags: 6 Bags (60 lb each)

Recommended Tools & Equipment

Tested hardware and components for high reliability

100% Free Tool Zero Sign-Up

Sizing Deck & Column Footings

A concrete footing acts like a snowshoe for your structure. If the downward load concentrated on a deck post exceeds the allowable soil bearing pressure, the footing will sink into the ground, causing deck sag, cracked ledger boards, and door jams.

Tributary Area & Sizing Equations

\text{Tributary Area } (A_{\text{trib}}) = \text{Span}_L \times \text{Span}_W \quad (\text{sq ft})

\text{Design Load } (P) = A_{\text{trib}} \times (\text{Live Load} + \text{Dead Load}) \quad (\text{lbs})

\text{Required Base Area } (A_{\text{req}}) = \frac{P}{\text{Soil Capacity (psf)}} \quad (\text{sq ft})

\text{Minimum Diameter } (D) = 2 \cdot \sqrt{\frac{A_{\text{req}}}{\pi}} \cdot 12 \quad (\text{inches})

Why Belled-Bottom Footings Save Concrete

If soft clay soil requires a wide 18" or 24" bearing footprint, filling a solid 24" diameter cylinder down 48 inches into the ground requires a massive 15 cubic feet (nearly 25 bags!) of concrete. Instead, smart builders use a plastic bell footing base (like BigFoot or RediFoot) to spread load at the bottom, topped with an inexpensive 10" or 12" Sonotube column stem, cutting concrete volume by over 60%.

Frequently Asked Questions

What is a typical residential deck design load?

The International Residential Code (IRC) specifies a minimum uniform live load of 40 psf for decks (accounting for occupants, furniture, and light snow) plus 10 psf dead load (weight of lumber, fasteners, and composite decking), totaling 50 psf.

How do I know my local soil bearing capacity without a geotech engineer?

In the absence of a professional geotechnical soil test, building codes (IRC Table R401.4.1) allow assuming a conservative presumptive soil bearing capacity of 1,500 psf for common clay or silt soils.

Why must footings extend below the frost line?

When soil water freezes in winter, it expands into ice lenses, exerting thousands of pounds of upward hydraulic pressure (frost heave). Setting the bottom of the footing at least 6 to 12 inches below the local frost depth guarantees the bearing surface rests on unfrozen ground.