Use this calculator to find the percentage change from an old number to the new number.
Here's where we'll show the work as it's being done by your computer in an easy to read format.
( ( 0 - 0 ) / 0) * 100 = 0%
Subtract the latest number from the older number. Divide by that older number. Multiply by one hundred. "Old Number" meaning the number from the start.
If the water level of the lake were at 23 foot when you first measured it, that's the old number. But you're about to leave for home and want to see what the water level is before you leave. You take a "new" measurement. Whether higher or lower, that's the latest height of the lake. Assuming it dropped ten feet that would be 43% of the original level of the lake. If it went up by five feet, then it's 22% higher or 122% of the original water level.