Deck Footing Calculator

1. Enter your deck length and width in feet.
2. Set post spacing along the length and across the width.
3. Use a suitable design load, or keep the default.
4. Enter the soil bearing value from tests or local guidance.
5. Choose a footing thickness based on your installation depth.
6. Click Calculate to see footings count, size, and volume.
7. Use CSV for records and PDF for sharing on-site.

Footings and outdoor stability

Garden decks rely on footings to transfer loads into soil without excessive settlement. Each footing must resist vertical load, small uplift, and seasonal movement. This calculator estimates a conservative plan using area, spacing, and soil bearing. Typical residential platform loads range from 40 to 60 psf, including live and dead load. Always confirm frost depth and minimum embedment for your climate zone for long-term surface stability.

Key inputs that drive results

Footing sizing depends on deck length, width, and how posts are arranged. Post spacing along the length and row spacing across the width define a tributary rectangle for each post. Larger spacing increases tributary area and raises the design load per post. A safety factor adds margin for uncertain soil, construction tolerances, and unbalanced loading from furniture, planters, or gatherings on one edge during everyday use.

Soil bearing in irrigated landscapes

Allowable soil bearing varies widely with texture and moisture. Dense gravel may exceed 3,000 psf, while soft clay can be below 1,000 psf. Saturated garden soils lose strength after irrigation or heavy rain, so conservative inputs reduce risk. If you have a site report, use the tested allowable bearing. When unsure, use a lower value and enlarge the footing diameter near slopes, tree roots, and fills.

Spacing, tributary area, and post count

Post count is estimated with a simple grid: columns equal ceil(length/spacing) plus one, and rows equal ceil(width/spacing) plus one. This approach ensures end posts are included for rim support. The calculator assumes each post carries the full spacing rectangle, which is conservative for corner and edge posts. If beams and joists create different tributary areas, adjust spacing to match your framing layout to reflect the structure.

Concrete ordering and field checks

Concrete volume is calculated from the recommended diameter and thickness, then multiplied by total footings. Convert cubic feet to cubic yards for ordering, and add waste for spillage and uneven excavation. In the field, check excavation width, base compaction, and level bearing. A wider footing may be safer than a deeper one in loose garden fill, but embedment rules still apply and cure properly before loading.

FAQs

Does this calculator replace local building requirements?

No. It supports early planning and budgeting. Verify frost depth, minimum embedment, reinforcement, and inspection rules with your local authority before you excavate or pour concrete.

What soil bearing value should I enter if I have no test data?

Use a conservative value such as 1,500 psf, or lower if the ground is wet, organic, or recently filled. When in doubt, increase footing diameter and consult a professional for site-specific guidance.

Why does changing spacing affect footing diameter so much?

Spacing changes tributary area. A larger tributary area means each post supports more deck load, so required bearing area increases. Bigger bearing area leads to a larger calculated footing diameter.

How should I handle heavy planters, hot tubs, or masonry features?

Increase the design load and consider tighter spacing under the heavier zone. You may also need deeper embedment and reinforcement. For concentrated loads, get a site-specific check rather than relying on uniform load assumptions.

Should I use thickness to represent frost depth?

No. Thickness here represents the pad depth used for volume. Frost depth relates to embedment below grade and varies by region. If you need deeper excavation, increase thickness for estimating concrete and confirm code-required depth separately.

Why does the tool recommend a standard diameter above the minimum?

Standard sizes are easier to form, excavate, and inspect. Rounding up provides practical construction tolerance and added capacity when soil conditions vary across the garden area.