How to convert foot to square foot?

This type of question is very typical on units conversion forum. People often ask how to convert linear meters to square meters, cubic yards to square yards, etc.

Related links for instant units conversion:
> length units (inch, meter, etc.)
> area units (acre, square meter, etc.)
> volume units (barrel, cubic meter, etc.)

The thing is linear something, square something and cubic something are three different measures. You can't convert between them simply because they measure different things.

Linear units (foot or meter) measure length or distance. Square units measure area. Cubic units measure capacity or volume.

Let's say you know the distance between two trees in your garden and it is 10 meters. You can't convert that to square meters because there is no such thing as "an area between the two points".

On the other hand, if you have a room that is 5 meters long and 4 meters wide, you can calculate its area. The area will be 5*4 = 20 square meters. But that's not conversion, that's a calculation, since you need to have two different length measurements to make a single area result. Similarly, to get the capacity of the room you will need three separate linear measurement: length, width and height. Knowing only one or two will not be enough. Calculations will be much more complex if your room is not rectangular.

Conversion is always direct: 12 inches always equal to 1 foot regardless of what you measure. Calculation often depends on other conditions and require extra information. A pint of water weighs 1.043 lbs, but it's only true for water. A pint of some other substance may weigh differently. Even a pint of water would not weigh the same if it were not on the Earth surface.

So the conclusion is -- many things can be calculated, but calculations can be complex and always require a lot of extra information. Conversions are simple, but only allow to convert between units that measure exactly the same things.