Deck Footing Size Calculator

Example Data Table

These examples show typical ranges. Always confirm site conditions and local requirements.

Formula Used

How to Use This Calculator

1. Enter deck length, width, and the number of posts.
2. Input live and dead loads that match your design assumptions.
3. Provide soil bearing capacity from tests or reliable guidance.
4. Pick a footing shape and set a practical minimum size.
5. Choose a safety factor, then press the calculate button.
6. Review the recommended size and thickness, then download outputs.

1) Define realistic design loads

Many residential decks use live load near 40 psf, while dead load is often 8–15 psf depending on framing, decking, and rails. If you expect planters, hot tubs, or snow, increase live load and re-run the sizing for each scenario.

2) Translate area loads into post reactions

The calculator converts deck area to total service load, then divides by the number of posts. Example: a 16 × 12 ft deck is 192 sq ft. At 50 psf total, service load is 9,600 lb before applying a safety factor. This assumes posts share load evenly, so confirm beam spans, cantilevers, and tributary areas.

3) Apply an appropriate safety factor

Safety factors around 1.15–1.50 help cover uncertainty in weights, occupancy, and load sharing. Higher factors may be reasonable for variable soils, poor drainage, or irregular layouts. Because the factor multiplies load, it also multiplies required bearing area.

4) Use defensible soil bearing values

Allowable bearing varies widely. Without test data, many builders assume 1,500–2,000 psf for typical granular soils and lower for soft clays. If you are unsure, start conservative and compare outcomes. Because required area equals post load divided by allowable bearing, conservative soil values increase footing size and can reduce settlement.

5) Choose square or round pads intentionally

For the same required area, round pads use diameter √(4A/π) and square pads use side √A. Round forms pair well with tubes; square pads are easy with lumber. Select what matches your forming method, excavation shape, and the footprint of post base hardware.

6) Respect minimums and constructability

Calculated sizes can be small on strong soils. Practical minimums improve placement tolerance and hardware fit. Many projects start around 12–16 in for light decks, then adjust for post load and soil. Use the “Minimum size” field to enforce a baseline, especially when aligning multiple footings.

7) Thickness and reinforcement considerations

Thickness affects stiffness and cracking resistance, not bearing area. Use the thickness estimate for early budgeting, then confirm minimum thickness, rebar, and cover for your conditions. Higher post loads, weak soils, and tall piers often justify thicker pads or added reinforcement to control cracking.

8) Frost depth and settlement checks

Footings should typically bear below local frost depth to reduce heave. Enter frost depth for an embedment reminder, then verify excavation depth on site. Also check drainage and compaction to limit settlement; bigger pads spread load but cannot fix weak subgrade. Where uplift or lateral forces matter, coordinate anchorage and bracing with your deck layout.

FAQs

1) Is this result a final design?

It is an estimate based on bearing pressure and simplified load sharing. Use it for planning, then confirm local minimums, post layout, reinforcement, and verified soil data before construction.

2) What soil bearing value should I enter?

Use a soil report when available. Otherwise choose a conservative value and follow local guidance. Lower allowable bearing increases footing size and helps reduce settlement risk.

3) Why does changing the number of posts matter?

Total factored load is divided by the number of posts. Fewer posts raise load per post, increasing required bearing area. More posts usually reduce footing size but add materials and labor.

4) Square pad or round pad—which is better?

Neither is universally better. Choose the shape that fits your formwork and excavation. Match the pad footprint to the required bearing area and the post base hardware.

5) How do I account for a hot tub or heavy feature?

Increase live load or model a dedicated load path near the feature. Heavy items can dominate post reactions. Re-run the calculator with higher loads and consider adding posts or beams.

6) Does thickness change the bearing area requirement?

Not directly. Bearing area comes from load and soil capacity. Thickness affects stiffness and cracking resistance. Use the estimate for budgeting, then verify minimum thickness and reinforcement for your site.

7) Why enter frost depth if size is based on bearing?

A properly sized pad can still heave if it is above frost depth. The frost input provides an embedment reminder so you can check excavation depth against local frost requirements.