Guidance, not a calculator
Foundations and footings
How deep and wide a foundation should be depends on your ground and what it carries. That's for an engineer or building control, not a calculator.
What the work involves
A foundation spreads a building's weight into the ground. Its size depends on the soil, nearby trees and drains, groundwater, slopes and the load above. Getting it right usually starts with trial holes or a soil survey, before any design.
Why it's regulated
Foundations for extensions and new buildings are checked under building regulations in the UK, France and Spain, and France requires a soil study for new builds in many areas. A foundation that's too shallow or too narrow can crack the whole building, and putting it right costs far more than the survey.
Who to ask
A structural engineer, with building control in the UK. For a garden wall or a small outbuilding, ask your building control office or town hall what it needs before you dig.
No figures here, on purpose. We don't give sizes, rules of thumb or typical values for this work. Only a qualified professional who has seen your building can.