Deck Footing Calculator
Calculate the number and size of deck footings from deck size and soil bearing capacity. Enter values for instant results with step-by-step formulas.
Formula
Total Footings = Beam Lines x Footings per Line; Capacity = Soil Bearing x Footing Area
Beam lines are determined by dividing deck width by beam spacing. Footings per line are determined by dividing deck length by footing spacing. Each footing capacity equals soil bearing capacity times the circular footing area (pi x r squared). The safety factor is capacity divided by actual load per footing.
Worked Examples
Example 1: Standard 16 x 20 Deck
Problem: Calculate footings for a 16 x 20 foot deck with 8-foot beam spacing, 8-foot footing spacing, 12-inch diameter footings at 42 inches deep on 2,000 PSF soil.
Solution: Beam lines = floor(16/8) + 1 = 3\nFootings per line = floor(20/8) + 1 = 3\nTotal footings = 3 x 3 = 9\nFooting area = pi x 0.5^2 = 0.785 sq ft\nCapacity per footing = 2,000 x 0.785 = 1,571 lbs\nTotal load = 320 sq ft x 50 PSF = 16,000 lbs\nLoad per footing = 16,000 / 9 = 1,778 lbs\nSafety factor = 1,571 / 1,778 = 0.88 (UNDERSIZED - need larger footings)
Result: 9 footings needed | 12-inch may be undersized | Consider 16-inch diameter
Example 2: Large Deck with Good Soil
Problem: A 24 x 20 foot deck on 3,000 PSF gravel soil, 6-foot beam spacing, 6-foot footing spacing, 12-inch footings at 36 inches deep.
Solution: Beam lines = floor(20/6) + 1 = 4\nFootings per line = floor(24/6) + 1 = 5\nTotal footings = 4 x 5 = 20\nCapacity per footing = 3,000 x 0.785 = 2,356 lbs\nTotal load = 480 x 50 = 24,000 lbs\nLoad per footing = 24,000 / 20 = 1,200 lbs\nSafety factor = 2,356 / 1,200 = 1.96 (ADEQUATE)\nConcrete = 20 x 2.36 = 47.1 cu ft = 1.75 cu yd
Result: 20 footings | 1.75 cu yd concrete | Safety factor 1.96 - adequate
Frequently Asked Questions
How do I determine the number of deck footings I need?
The number of deck footings depends on your deck size, beam spacing, and footing spacing along each beam. First, determine how many beam lines run across the deck width by dividing the width by your beam spacing (typically 6 to 8 feet) and adding one. Then determine how many footings per beam line by dividing the deck length by your footing spacing (typically 6 to 8 feet) and adding one. Multiply beam lines by footings per line for the total. A 16 by 20 foot deck with 8-foot spacing typically needs 9 footings arranged in a 3 by 3 grid pattern.
What size deck footings do I need?
Footing size depends on the load each footing must carry and the soil bearing capacity. For most residential decks on average soil (2,000 PSF bearing capacity), 12-inch diameter footings are sufficient. Heavy-duty applications or poor soil may require 16 to 24 inch footings. The footing must be large enough so that the load per footing divided by the footing area does not exceed the soil bearing capacity. A 12-inch round footing has an area of 0.785 square feet, providing 1,570 pounds of capacity on 2,000 PSF soil. Double the diameter to quadruple the capacity because area scales with the square of the radius.
How deep should deck footings be?
Deck footings must extend below the frost line in your area to prevent heaving during freeze-thaw cycles. Frost depths vary significantly by region: 12 inches in the southern United States, 24 to 36 inches in the mid-Atlantic, 36 to 48 inches in the northern states, and up to 60 inches in parts of Minnesota and Alaska. Check your local building code for the exact frost depth requirement. Footings that do not reach below the frost line will heave upward when soil freezes, causing the deck to shift and potentially separate from the house. Most building departments require inspection before pouring concrete.
How much concrete do I need for deck footings?
Concrete volume per footing is calculated using the cylinder formula: pi times radius squared times depth. For a 12-inch diameter footing at 42 inches deep, the volume is 3.14 times 0.5 squared times 3.5, which equals 2.75 cubic feet per footing. A standard 60-pound bag of premixed concrete yields approximately 0.45 cubic feet, so you would need about 7 bags per footing. For multiple footings, consider ordering ready-mix concrete from a truck if you need more than 1 cubic yard total, as mixing dozens of bags by hand is extremely labor intensive. Always add 10 percent extra for spillage and waste.
What is a Sonotube and do I need one for each footing?
A Sonotube is a brand name for cardboard form tubes used to create cylindrical concrete footings. They keep the hole walls from caving in, ensure a consistent diameter, and create a smooth finish above grade. Sonotubes are highly recommended for any footing deeper than 12 inches because digging a clean, consistent hole is difficult without them. They come in 8, 10, 12, 16, 20, and 24 inch diameters. Set the tube in the hole, level it, backfill around the outside, then pour concrete inside. The cardboard eventually biodegrades underground. Cost ranges from 8 to 20 dollars per tube depending on diameter.
Should I use rebar in my deck footings?
Rebar reinforcement is recommended but not always required by code for residential deck footings. A single piece of number 4 rebar placed vertically in the center of each footing significantly increases resistance to lateral forces and provides an anchor point for the post base hardware. In areas with expansive clay soils or seismic activity, rebar is typically required. The rebar should extend from the bottom of the footing to near the top surface with at least 3 inches of concrete cover on all sides. Some builders use a J-bolt instead of rebar to anchor the post bracket directly into the wet concrete.