Foundation calculator
Check soil pressure, concrete compression, and simple frost-depth limits for rectangular isolated footings.
Load and soil
Total service load applied to the footing.
Pick the closest simplified soil class for a quick bearing check.
Geometry
Longer footing side in plan.
Shorter footing side in plan.
This simplified tool uses the entered value as block depth and for the frost-depth check.
Material
Characteristic concrete class for the footing block.
Choose plain concrete or reinforced concrete.
Local frost depth used for the simplified depth check.
Foundation basics
An isolated footing does one job: it spreads a concentrated load into the soil below. If the footing is too small, the soil pressure rises too far and the support starts to settle or rotate.
This tool is built for early checks on rectangular footings under posts and similar supports. It helps you compare load, soil class, footing size, concrete class, and a simplified frost-depth limit before you move into detailed design.
What a footing does
The footing is the transition between the structure and the ground. The post or wall brings the load down; the footing spreads that load over a larger area so the soil can carry it.
A simple way to picture it is a heavy cabinet on a soft floor. A small base sinks faster than a larger one. The same idea drives footing sizing: more loaded area means lower soil pressure.
Where isolated footings are common
- Posts and columns: Timber or steel supports for canopies, carports, balconies, and pergolas often sit on individual footings.
- Small retaining or garden structures: Freestanding walls, gate posts, and heavier fence elements still need stable support below grade.
- Equipment supports: Heat pumps, tanks, and similar point loads need a base that does not settle unevenly.
Main foundation types
- Isolated footing: A single footing below one post or one concentrated load. That is the case this calculator covers.
- Strip footing: A continuous footing below a wall. Loads are distributed linearly instead of at one point.
- Slab foundation: A full-area slab below the structure. It is used when loads are broad or soil conditions are less favorable.
Key terms
Soil pressure
Soil pressure is the load divided by the loaded footing area. The unit here is kN/m2.
When you increase footing area, soil pressure drops. That is the first lever when a footing check fails.
Allowable bearing capacity
Different soils carry different pressures. Dense gravel is usually more forgiving than soft clay or loose sand.
This calculator uses simplified preset values. If you have a soil report, those project values should govern.
Frost depth
If the founding zone freezes, the soil can expand and lift the footing. That can crack or misalign the structure above.
This tool compares your entered footing depth directly with a local frost-depth value. It is a simplified check, not a full geotechnical frost design.
When a footing check matters
- When the footing supports a post that carries a roof, balcony, or deck load.
- When soil is soft, layered, wet, or otherwise uncertain.
- When loads are high enough that trial-and-error sizing is no longer reliable.
- When permit or engineering review requires a traceable basis for the support design.
Typical site workflow
- Check the support load: Start with the vertical reaction from the beam, roof, or post check.
- Assess the soil class: Pick the closest simplified soil category or use the bearing value from the soil report.
- Size the footing: Choose a trial footing width and length that keep soil pressure in range.
- Check depth: Set a depth that suits frost protection and the intended construction method.
- Choose the concrete class: Use a class that matches the exposure and project standard.
- Decide on reinforcement: Small block footings may work unreinforced. Larger or flatter footings usually move into reinforced design.
- Prepare the excavation: Remove loose material, level the base, and provide a clean working surface.
- Place and compact concrete properly: Keep the base clean, respect cover, and compact the concrete so voids do not remain.
- Cure before full load: Do not treat early-age concrete like finished structural concrete.
Typical mistakes to avoid
- Sizing the footing only by habit, without checking soil pressure against the actual load.
- Using poor or disturbed soil at the base of the excavation.
- Treating footing thickness and founding depth as if they were always the same thing.
- Skipping reinforcement even when the footing gets wide, flat, or heavily loaded.
- Pouring concrete into standing water or onto an unprepared base.
From roof load to footing size
For timber structures, footing sizing is usually the last step in the chain. The load comes from the structure above, not from the footing itself.
Use the beam tool to estimate section size, bending, shear, and deflection for the spanning members.
Use the post tool to check axial compression and buckling for the support below the beam.
Use this footing tool to size the concrete support under that post and compare the result against soil pressure, concrete compression, and simple depth limits.
A cleaner load path gives you a cleaner footing input. If the beam or post assumptions change, revisit the footing too.