There are some traditional methods available of measuring the water level of a cistern tank, however, they tend to be much less accurate that modern technological options. The stick method consists of dipping a long stick or pole into your tank to see how deep the water level is. Others simply guess based on time and familiarity with their usage.
Our recommendation would be the PTLevel which can accurately measure your water level and be tracked on your phone. This allows to you access real-time measurements, and order the exact quantity of water you need.