How much memory is free?

Using `top` to determine how much free physical memory a machine has available has always been confusing to me. It seems that you need a math degree and why doesn't it show values in MB instead of KB? I have found the `free -m` tool to be much easier to understand. In the image below I have circled the two numbers that really matter. The other numbers don't mean much unless you are a kernel developer. You want the number circled under the "free" column to be high and the number circled under the "used" column to be low. The first one is the amount of physical memory available. The second one is the amount of swap space used.