What is the percentage calculation of water in a waterbut. I'm having a senior moment and am unable to work out a simple formula for the volume of water in my water-but, question below: I have a blue 120cm high water-but, it also has a ultrasonic sensor to measure where the water level is. When the water-but is full the sensor shows there is 5cm between it and the water (this is 100% full). When the water-but is empty the sensor shows there is 86cm between it and the water (this is 0% full). So when the water-but is full and starts emptying what is the calculation to work out the percentage of water remaining? Thanks in advance.

If the sensor is showing $s$ cm then the proportion full is $$\frac{86-s}{86-5}=\frac{86-s}{81}$$

Multiply by $100$ to get a percentage