When comparing or combining an area or volume it is important to measure the objects using the same units. Without converting the areas into the same unit, it might be difficult to tell, for example, ...

I consider myself pretty conversant in metric units. I know that 20 kilograms is 44 pounds (double it and add 10%) and that five kilometers is a little over three miles (I’ve, uh, run a lot of 5Ks).