Tree Foundation Calculator: Size & Cost Guide

This calculator works out the recommended size and concrete volume for a foundation that supports a tree house or a heavy planter. You enter the total load (tree house weight plus people and equipment), the soil bearing capacity, and the number of support posts, and you get back the minimum footing diameter and the concrete needed per footing. It uses the standard bearing-pressure formula, so you can check a structural engineer’s numbers or get a rough starting point for a DIY project.

Formula

The calculator uses the bearing pressure equation: Area = Load / Bearing Capacity. For a circular footing, the diameter is derived from that area: d = √(4 × Area / π). Here, the load per post is the total weight (in kN, converted from kg by multiplying by 9.81 and dividing by 1000) divided by the number of posts. The concrete volume assumes a footing thickness of 0.3 m (about 12 inches), which is typical for a small to medium tree house foundation.

Variables:

• t – total load on the foundation (kg). This includes the structure, people, furniture, and any stored items.
• b – soil bearing capacity (kN/m²). This is the maximum pressure the soil can safely support without excessive settlement.
• n – number of support posts (dimensionless).
• d – minimum footing diameter (m).
• v – concrete volume per footing (m³), based on a 0.3 m thick footing.

Worked Example

Using the default values: total load = 2000 kg, soil bearing capacity = 150 kN/m², number of posts = 4.

1. Convert load to kN: 2000 kg × 9.81 / 1000 = 19.62 kN.
2. Load per post: 19.62 kN / 4 = 4.905 kN.
3. Required area per post: 4.905 kN / 150 kN/m² = 0.0327 m².
4. Diameter: √(4 × 0.0327 / π) = √(0.0416) = 0.204 m.
5. Concrete volume per footing: π × (0.204/2)² × 0.3 = 0.0098 m³.

The calculator rounds the diameter to 0.20 m and the volume to 0.01 m³ per footing. In practice, you would not pour a footing that small; the minimum practical size is usually 0.3 m diameter, and local building codes may require larger.

When to Use This Calculator

Use this tool when you are planning a tree house, a raised deck, or a heavy planter that sits on posts. It gives a quick estimate of the footing size needed to avoid sinking into the soil. You might also use it to check whether a proposed design is reasonable before paying for a structural engineer.

It is not a substitute for professional design. Local building codes, frost depth, and soil conditions vary widely, and a real foundation often needs to be larger than the bare bearing-pressure calculation suggests.

How to Choose Soil Bearing Capacity

Soil bearing capacity is usually determined by a geotechnical survey, but you can use typical values for preliminary planning:

• Soft clay or loose sand: 50–100 kN/m²
• Medium clay or compacted sand: 100–200 kN/m²
• Hard clay or dense gravel: 200–400 kN/m²
• Rock: 500+ kN/m²

If you are unsure, use the lower end of the range for your soil type. The bearing capacity is the maximum pressure the soil can take before it fails or settles too much. It is not the same as the concrete strength—concrete is almost always stronger than the soil.

Common Mistakes and Misconceptions

One common mistake is mixing units. The calculator uses kN/m² for bearing capacity and kg for load. If you input pounds or psf, the result will be wrong. Always convert to the same unit system.

Another mistake is forgetting that the load is shared among posts. Dividing the total load by the number of posts is essential—if you use the full load for each post, you will get oversized footings.

A third misconception is that a larger footing is always better. While a larger footing reduces pressure, it also increases cost and excavation. The goal is to find the smallest safe size, not the biggest.

Also, this calculator ignores the weight of the footing itself. For a small footing, the concrete weight is minor, but for large footings, you may need to add that to the load. A typical reinforced concrete density is 2400 kg/m³.

Limitations of the Formula

The bearing-pressure formula assumes the soil is uniform and the load is centered on each footing. Real soil is rarely uniform, and loads can be eccentric, especially in a tree house where wind and movement create lateral forces. The formula also does not account for frost heave—in cold climates, footings must extend below the frost line, which often makes them much deeper than the 0.3 m assumed here.

For a tree house, the tree itself moves, so the foundation must accommodate that. Rigid footings attached directly to a tree can fail because the tree’s sway creates forces the footing was not designed for. Many tree house builders use flexible connections or dynamic supports, which this calculator does not model.

Finally, the calculator does not check for overturning or sliding. A tall, heavy structure on a small footing can tip over even if the bearing pressure is fine. That requires a more detailed structural analysis.

Practical Tips for Building a Tree House Foundation

If you are building a tree house, consider these general guidelines:

• Always consult a structural engineer or arborist before building, especially if the tree is valuable or the structure will be used by children.
• Use more posts than the minimum—four is typical, but six or eight can reduce the load per post and increase stability.
• Place footings away from the trunk to avoid damaging roots. A distance of at least 1.5 m from the trunk is often recommended.
• Use a footing that is wider at the bottom than at the top (a “spread” footing) to distribute load over a larger area.
• Reinforce the concrete with rebar, especially if the footing is large or the soil is weak.
• Check local building codes—many areas require permits for tree houses over a certain size or height.

Frequently Asked Questions

Q: Can I use this calculator for a deck or shed foundation?
Yes, the same bearing-pressure formula applies to any post-supported structure. Just enter the total load and the number of posts. For a deck, the load often includes people and furniture, so use a higher total load than the structure’s empty weight.

Q: What is a typical soil bearing capacity for residential soil?
For preliminary design, 150 kN/m² is a common conservative value for medium clay or compacted sand. However, actual capacity can vary widely, so a soil test is recommended for permanent structures.

Q: How deep should a tree house footing be?
This calculator only gives the diameter, not the depth. Depth depends on frost line and soil conditions. In cold climates, footings must go below the frost line (often 0.6–1.2 m deep). In mild climates, 0.3 m may be enough, but always check local codes.

Q: Do I need a permit for a tree house?
Many municipalities require a permit for structures over a certain size or height. Check with your local building department. Even if a permit is not required, following building codes is wise for safety.

Q: What if the tree is not strong enough to support a tree house?
This calculator assumes the foundation is on the ground, not on the tree. If you are attaching the tree house to the tree itself, you need an arborist to assess the tree’s health and structural capacity. A tree house supported only by the tree is a very different engineering problem.

Q: Can I use this for a concrete pad instead of a footing?
This calculator is for circular footings under posts. For a rectangular pad, you would use the same area formula but calculate the dimensions differently. The principle is the same: area = load / bearing capacity.

Related

• Talent Tree Calculator: How It Works & How to Use It
• Rift Soul Tree Calculator: Plan Your Build
• Poe Genesis Tree Calculator: How It Works

in t | Total load | kg | 2000 | 0
in b | Soil bearing capacity | kN/m² | 150 | 0
in n | Number of posts | - | 4 | 1
out d | Footing diameter | m | sqrt((t * 9.81 / 1000) / (b * n) * 4 / pi) | 2
out v | Concrete per footing | m³ | pi * (d / 2)^2 * 0.3 | 2