How to use this calculator
- Enter post load. Use the gravity load carried by one footing or post location.
- Set load factor. Apply a project load or safety factor before comparing soil bearing.
- Enter soil bearing. Use local code, geotech or project allowable soil bearing.
- Check a selected footing. Choose round or square and enter the trial diameter or width.
- Review utilization. Compare selected soil pressure and utilization before moving to concrete quantity and detailing.
How it works
This deck footing size calculator is a bearing-area screen. It starts with the load on one post or pier, applies your entered load factor, then divides by the allowable soil bearing you supply:
A_req = P x F / q_allow
A round footing with that area has diameter:
D_req = sqrt(4 x A_req / pi). A square footing with that area has
width B_req = sqrt(A_req). The selected footing check then compares
actual pressure q_actual = P x F / A_selected with the entered
allowable bearing.
Use this before the deck footing concrete calculator. This page chooses preliminary bearing area; the concrete takeoff page turns final diameter/depth and bag yield into bags. For load-led searches, use the deck footing size by load page before moving into material takeoff.
Worked example
Verified against the live calculator
A deck post carries 4,000 lbf. With a load factor of
1.0 and allowable soil bearing of 1,500 psf, the
required area is 4000 / 1500 = 2.67 ft^2. A round footing with
that area needs diameter sqrt(4 x 2.67 / pi) = 1.84 ft, or about
22 in. A selected 24 in round footing has about
3.14 ft^2 of area and an actual pressure near
1,270 psf, so it is under the entered bearing value.
Frequently asked questions
How do you size a deck footing from load?
For a first-pass bearing screen, multiply the service load by your load factor, then divide by allowable soil bearing: A_req = P x F / q_allow. A round footing diameter is D = sqrt(4 x A_req / pi), and a square footing width is B = sqrt(A_req).
Where does allowable soil bearing come from?
Use the value required by your local code, a geotechnical report or project criteria. This calculator does not choose soil bearing capacity because soil type, frost, fill, groundwater, settlement and jurisdiction matter.
Is this the same as the deck footing concrete calculator?
No. This page sizes preliminary bearing area and checks a selected footing. The deck footing concrete calculator is a material takeoff for concrete volume and bag count after the footing diameter/depth are known.
Does this design reinforcement, thickness or frost depth?
No. Thickness/depth is used only for concrete quantity here. Final footing design needs frost depth, reinforcement, punching/shear, uplift, lateral load, post-base hardware, settlement and local permit review.
Method & assumptions
- Formula-only bearing area screen from user-entered load and allowable soil bearing.
- Round and square bearing areas only; bell bottoms, spread footings, eccentric posts and stepped pads need project-specific geometry.
- Selected thickness/depth is used for volume only. It is not a frost-depth, punching-shear, one-way-shear, settlement, uplift, lateral-load or reinforcement check.
- Final footing size, embedment, post base, connector, rebar, frost depth, uplift, lateral resistance, drainage, soil preparation and permit approval depend on local code and site conditions.
Source boundary
Formula-only bearing-area worksheet from user-entered load and allowable soil bearing. Verify final deck footings against local code, soil/geotechnical data and engineered drawings.
Related deck workflow
Set load paths with deck joist span, deck beam span and ledger fastener spacing. Use deck footing size by load when post reaction is the starting point. Then estimate material with deck boards, rebar spacing and rebar lap splice length, then footing concrete.