Why these give different answers to other calculators
Most material calculators do the easy half of the sum and stop. Area times depth is trivial; the parts that decide whether you order the right amount are the waste allowance, the unit the material is actually sold in, and the geometric factor that turns a measurement into a surface.
So the roofing calculator applies the pitch multiplier rather than using the footprint. The wallpaper calculator works in drops and rounds each one up to the pattern repeat, instead of dividing area by roll coverage. The fence calculator counts bays and adds one for the end post. The concrete calculator converts to the bag sizes actually sold and tells you when bags stop making sense. Each of those is a real difference in the answer, not a presentational one.
Units
Every calculator works in imperial or metric and converts internally, so you can measure in feet and inches and order in litres, or the reverse. The toggle sits at the top of each calculator.
Where estimates end
These are planning tools. Coverage figures vary between products and the packaging is always the authority. Anything governed by building regulations — stair pitch, footing depth, fire-rated board, structural spans — needs confirming with your local building control, not with a website.