ToolwickHow to Use This Footing & Foundation Size Calculator
Choose the footing type (square pad, rectangular pad or continuous wall strip), then enter the dead and live load it must carry. Pick your soil type or type in an allowable bearing pressure from a soils report, and add the depth to the bottom of the footing, the thickness you want and the width of the column or wall above. Press Calculate to get the footing width, required thickness, concrete volume, bag count and minimum rebar, plus a table showing how the footing grows if the soil is weaker than expected.
Why This Matters
A footing spreads a structural load over enough soil that the ground does not settle or fail. Too small and a deck post, porch column or garage pier sinks unevenly and cracks whatever sits on it. Too large and you pay for concrete, digging and rebar you did not need. A 6,000 lb post on 2,000 psf soil needs roughly a 24 in square pad, but the same post on 1,500 psf clay grows to about 28 in, and on 3,000 psf gravel it shrinks to about 20 in. A one-story wall carrying 1,500 to 2,500 lb per linear foot on 2,000 psf soil typically needs a strip footing 12 to 18 in wide.
Homeowners use a concrete footing size calculator to plan a shed, carport, porch or addition before talking to a supplier. Contractors use it to order the right amount of concrete and steel. Because soil is the biggest unknown on most jobs, the comparison table is the most useful part: it shows the cost of being wrong about the ground before a single hole is dug.
How It's Calculated
The calculator uses a service-load (allowable stress) method, the same approach behind the presumptive soil values in IRC Table R401.4.1.
1. Net bearing pressure. Concrete weighs about 150 lb per cubic foot and backfill about 100, so the footing and the soil above it use up part of the soil's capacity:
2. Required area. Divide the load by the net pressure, then round each dimension up to the next even inch (minimum 12 in, and at least the column width plus 4 in):
3. Thickness. The projection is p = (L − c) / 2. For plain concrete, IRC R403.1 limits the projection to the footing thickness, so T ≥ p. For reinforced footings this tool uses the conservative rule of thumb T ≥ p/2. Either way the minimum is 8 in. If your thickness is too small, the calculator raises it and re-runs the weight calculation until the answer is stable.
4. Rebar. Minimum steel is the ACI 318 shrinkage ratio for Grade 60 bars, in each direction, with 3 in of cover and bars no more than 18 in apart:
5. Concrete. Pad volume is B × L × T ÷ 1,728 cubic feet. A wall footing is B × T ÷ 144 cubic feet per foot of length. Divide by 27 for cubic yards. One 80-lb bag yields about 0.6 cubic foot, and the bag count is rounded up.
This follows IRC R401.4.1 presumptive bearing values, IRC R403.1 footing rules of thumb and ACI 318 minimum steel. Local codes vary. The results are a preliminary estimate, not engineering, permit or inspection approval.
Worked Example
The default inputs describe a square pad under a post carrying 8,000 lb dead and 4,000 lb live load, on 2,000 psf soil, with the bottom 36 in down, 12 in thick and an 8 in post.
- Total load P = 8,000 + 4,000 = 12,000 lb.
- Footing weight = 150 × 12 ÷ 12 = 150 psf. Soil above = 100 × (36 − 12) ÷ 12 = 200 psf.
- Net bearing = 2,000 − 150 − 200 = 1,650 psf.
- Required area = 12,000 ÷ 1,650 = 7.27 sq ft, so B = √7.27 = 2.70 ft = 32.4 in, rounded up to 34 in square.
- Projection = (34 − 8) ÷ 2 = 13 in. Reinforced needs T ≥ 6.5 in and the 8 in minimum, so 12 in is fine.
- Actual pressure = 12,000 ÷ 8.03 + 150 + 200 = 1,845 psf, about 92% of the allowable 2,000 psf.
- Concrete = 34 × 34 × 12 ÷ 1,728 = 8.03 cu ft (0.30 cu yd). With 10% waste, 8.83 cu ft, or 15 bags of 80-lb mix.
- Rebar: As,min = 0.0018 × 34 × 12 = 0.73 sq in. With #4 bars (0.20 sq in), that is 4 bars each way at 9.3 in on center.
Tips & Common Mistakes
- Do not guess the soil. Dig a test hole or get a soils report. Wet clay, fill and organic topsoil can carry far less than the table values, and 1,500 psf is the safer assumption when unsure.
- Go below frost depth. The footing bottom must be at least 12 in below undisturbed ground and below your local frost line, or heaving can lift it every winter.
- Count every load. Include snow, the weight of the posts and beams themselves, and any future additions, not just what you can see today.
- Dig to firm, undisturbed soil. Never place a footing on loose fill or topsoil. Over-digging and backfilling loosely wastes the bearing capacity you calculated.
- Order a little extra. Holes are never perfectly shaped. A 10% waste allowance is normal for hand-dug pads, and more if the sides are ragged.
Frequently Asked Questions
How big should a concrete footing be for a deck post?
It depends on the load and soil. A post carrying 6,000 lb on 2,000 psf soil needs roughly a 24 in square footing. Use the Deck Footing Size & Frost Depth Calculator for deck-specific tributary loads.
How thick should a footing be?
IRC R403.1 sets a 6 in minimum, but this calculator never goes below 8 in. For plain concrete the thickness must be at least the projection beyond the column or wall. Reinforced footings can be thinner, but a structural engineer should confirm shear.
How deep should a footing be?
The bottom must sit at least 12 in below undisturbed ground and below your local frost depth, whichever is deeper. Frost depth ranges from nearly zero in the Deep South to 48 in or more in northern states, so check with your building department.
What soil bearing capacity should I use?
Use the value from a soils report if you have one. Otherwise IRC R401.4.1 gives presumptive values: 1,500 psf for clay, 2,000 psf for sand, 3,000 psf for gravel and 4,000 psf for sedimentary rock. When unsure, choose the lower number.
Does a footing need rebar?
Not always. Small plain-concrete wall footings that meet IRC projection limits can be unreinforced, but pads, tall projections and weak soil benefit from steel. This tool shows minimum rebar for reinforced footings and states when none is assumed.
How much concrete or how many bags do I need?
The results show cubic yards, cubic feet with your waste allowance and the number of 80-lb bags. For other bag sizes, the Concrete Bags Per Cubic Yard Calculator converts the volume.