Calculator Inputs
Formula Used
- Bearing area: A = (P × SF) ÷ q, where P is vertical load and q is allowable soil bearing.
- Overturning check (optional): M = L × H and the calculator iterates a square plan so qmax = P/A + 6M/B³ ≤ q.
- Plan size: square uses B = √A; circular uses D = √(4A/π), then rounds up.
- Concrete volume: volume = plan area × depth, plus overbreak and bell allowances.
How to Use This Calculator
- Choose units and select a footing shape.
- Enter vertical load and an estimated soil bearing capacity.
- Add lateral load and height if wind push is expected.
- Set frost depth and any minimum depth you must follow.
- Adjust rounding, overbreak, and bell/flare allowances as needed.
- Press calculate, then download CSV or PDF for records.
Example Data Table
| Use case | Units | Vertical load | Soil bearing | Shape | Recommended size (example) |
|---|---|---|---|---|---|
| Fence corner post | Metric | 2.0 kN | 150 kPa | Circular | 0.25 m diameter × 0.75 m depth |
| Pergola post | Metric | 6.0 kN | 120 kPa | Square | 0.30 m × 0.30 m × 0.85 m depth |
| Trellis post | Imperial | 600 lbf | 1500 psf | Circular | 10 in diameter × 30 in depth |
| Gate post | Imperial | 1200 lbf | 2000 psf | Square | 12 in × 12 in × 36 in depth |
Soil Bearing Capacity Benchmarks
Footings work by spreading the post load into soil. Use site data when available, but typical planning values help: soft clay 75–100 kPa, firm clay 100–150 kPa, sandy silt 150–200 kPa, compact sand or gravel 200–300 kPa. A quick check is to observe excavation walls; if they crumble or seep water, treat the soil as weaker. If you cannot verify soil, choose the lower value and raise the safety factor to reduce settlement risk.
Vertical Load and Factored Demand
The calculator multiplies vertical load by a safety factor, then divides by allowable bearing pressure to get required area. Include the post weight, supported framing, hanging planters, and wet soil loads from trellises. A conservative safety factor of 1.5–2.0 is common for garden projects, especially when soil moisture varies through the year.
Lateral Load and Overturning Effects
Wind on screens, vines, and shade cloth can create side force. When you enter lateral load and height above grade, the tool estimates overturning moment and increases plan size until peak soil pressure stays below the allowable value. This reduces rotation at the surface and keeps posts plumb. For gates and corner braces, lateral demand is often higher than vertical demand.
Embedment Depth and Frost Considerations
Depth is set by the larger of frost depth, your minimum depth, and a simple embedment rule based on post height. Deeper footings resist uplift and reduce wobble. In cold regions, keep the bottom below frost depth to limit heave. In warm climates, depth still matters for stability in loose backfill. Drain standing water before pouring.
Concrete Quantity and Waste Allowances
Concrete volume is computed from the rounded plan size and depth, then adjusted for overbreak and optional bell/flare allowance. Overbreak accounts for irregular auger holes and sloughing. Use the bag yield to estimate bags, or enter ready-mix pricing to approximate material cost. Ordering 5–10% extra helps avoid cold joints. If you hand-mix, stage water and aggregate so each batch matches strength and workability.
Frequently Asked Questions
How do I estimate vertical load for a garden post?
Add the weight carried by that post: beams, pergola rafters, screens, and any hanging items. Include a cushion for wet timber and occasional loads. If unsure, round up and use a higher safety factor.
What soil bearing value should I use if I have no test report?
Use a conservative default and confirm with local guidance. Weak clay can be near 75–100 kPa (about 1500–2000 psf). Compact sand or gravel is often higher. When uncertain, select the lowest reasonable value.
Do I need to enter a lateral load?
Enter lateral load when wind, gates, or bracing can push the post sideways. If you leave it at zero, the tool sizes for vertical bearing only, which may underestimate rotation for tall or exposed posts.
Why does the calculator add overbreak allowance?
Hand-dug and augered holes are rarely perfect cylinders. Overbreak covers sloughing, oversized diameters, and cleanup at the base. It helps you order enough concrete without pausing mid‑pour.
Should my footing bottom be below frost depth?
Yes, in freezing climates. Placing the base below local frost depth reduces heave and tilting. If frost depth is unknown, ask a local builder or authority, or use a conservative estimate.
Can I use a bell or flared base for better resistance?
A bell increases bearing area and improves uplift resistance, useful for trellises and wind-loaded screens. Enable the bell option to add a volume allowance. Always keep the neck wide enough to place concrete properly.