Pergola Footing Calculator

Units

Switching units keeps your current values.

Pergola length (ft)

Pergola width (ft)

Number of posts

Load is shared across posts.

Dead load (psf)

Includes beams, slats, light roofing.

Live / snow load (psf)

Use local design values if known.

Extra total point load (lb)

Hanging swings, planters, dense vines, etc.

Load factor (1.0–2.0)

Accounts for framing layout and imbalance.

Safety factor (1.1–3.0)

Higher values add conservatism.

Soil bearing capacity (psf)

Typical: 1500–3000 psf (70–140 kPa).

Soil reduction factor (0.50–1.00)

Lower for loose, wet, or disturbed soil.

Frost depth (in)

Bottom should go below frost line.

Footing type

Piers use the rounded diameter.

Pad thickness (in)

Used for pad volume estimate only.

Extra depth below frost (in)

Adds a buffer under the frost line.

Round diameter step (in)

Example: 2 in (or 5 cm).

Concrete bag size (lb)

Bag count is an approximate estimate.

Example data table

Length	Width	Posts	Loads	Soil	Suggested diameter	Suggested depth
12 ft	10 ft	4	8 + 20 psf	2000 psf (0.80 factor)	16–18 in	≥ 30 in
16 ft	12 ft	6	10 + 25 psf	2500 psf (0.85 factor)	14–16 in	≥ frost + 6 in
4.0 m	3.0 m	4	0.4 + 1.0 kPa	100 kPa (0.80 factor)	40–45 cm	≥ 75 cm

Examples are illustrative. Always check local requirements.

Formula used

Plan area = length × width
Total uniform load = (dead + live) × area
Per-post service load = (total ÷ posts) × load factor
Per-post design load = service load × safety factor
Required footing area = design load ÷ (soil bearing × soil factor)
Circular diameter = √(4 × area ÷ π)
Concrete volume = area × thickness (pad) or cylinder volume (pier)

This is a sizing estimate. Lateral loads and uplift need separate checks.

How to use this calculator

Enter pergola length, width, and number of posts.
Set dead and live loads using local design guidance.
Add extra point load for swings, planters, or vines.
Enter soil bearing capacity and choose a reduction factor.
Provide frost depth and a small extra depth buffer.
Select pad or pier footing and set pad thickness.
Press calculate to see rounded diameter and depth.
Use CSV or PDF downloads to save your results.

Professional guidance

Footing sizing starts with load paths

A pergola transfers roof, beam, and occupancy loads into each post, then into soil. This calculator converts area loads into a total force, distributes it by post count, and applies a load factor for uneven framing. A safety factor then produces a conservative per-post design load that the footing must support without excessive settlement. Treat the output as a vertical bearing check, not a full structural design.

Soil bearing and reduction factors

Soil capacity is rarely uniform across a garden. Granular soils may tolerate higher pressure than wet clays or recently filled beds. The soil reduction factor intentionally derates the published bearing value to reflect uncertainty, moisture changes, and disturbance from excavation. Using a lower factor increases required footing area and usually improves long-term performance. When unsure, test with a hand auger and note groundwater or loose layers.

Frost depth and embedment decisions

In cold climates, frost heave can lift shallow footings and crack posts. Setting the footing bottom below the local frost depth reduces this risk. The extra depth buffer accounts for grading changes and imperfect excavation. Even in mild climates, deeper embedment can improve stability where irrigation keeps soil saturated for long periods. Add gravel only where it improves drainage without reducing bearing area.

Choosing pad versus pier footings

Spread pads work well for firm soils and larger tributary areas because they spread pressure over a wider footprint. Round piers can be faster in tight spaces, and the cylindrical shape resists minor soil movement. The calculator reports a rounded diameter for construction practicality; for pads, thickness affects concrete volume but not bearing area. For wind or shade-sail uplift, consider anchors and uplift capacity separately.

Concrete quantity and build checkpoints

Concrete volume depends on footing geometry and depth. Bag counts are estimates, so add contingency for spillage and over-excavation. Confirm post base hardware, drainage at the bottom of the hole, and proper consolidation of the pour. Use reinforcement appropriate to the footing type, keeping steel clear of soil contact. After curing, verify post plumb and recheck alignment before installing beams and slats and final bracing.

FAQs

What soil bearing value should I enter?

Use a site value from local guidance or a geotech report when available. If unknown, choose a conservative number and apply a lower soil reduction factor. Softer, wetter soils generally require larger diameters.

Does the calculator include wind uplift or lateral loads?

It focuses on vertical bearing pressure and concrete quantity. Wind, shade sails, and bracing can create uplift and lateral forces. Check hardware ratings, embedment, and bracing details separately for your exposure and pergola style.

How do I choose pad versus pier?

Pads spread load over a wide area and can reduce settlement on firm soils. Piers suit narrow locations and simple augered holes. Select the option that matches your construction method, then review diameter, depth, and reinforcement notes.

Why is there a load factor and a safety factor?

Load factor accounts for uneven load sharing from beams, cantilevers, and framing layout. Safety factor adds conservatism for uncertainties in soil, materials, and workmanship. Increasing either raises the recommended footing size.

How accurate is the concrete bag estimate?

It is a planning estimate based on calculated volume and typical bag yield. Over-excavation, bell bottoms, and spillage can increase usage. Buying one extra bag per footing often prevents delays on pour day.

What depth should I use if there is no frost?

Use a depth that reaches undisturbed soil and provides stability, often at least 18 inches (about 45 cm). Add extra depth if soil is loose, irrigated frequently, or sloped, and ensure proper drainage at the base.