The only reliable way to determine if the tank needs to be pumped is to insert a Sludge Judge or a sock covered PVC pipe into the tank to discover the sludge level. It is easy to do.
24” white tube sock or similar cloth
Measuring tape or yardstick
8’ to 10” pvc pipe / stick or similar
To make the tool
To make the tool make a mark at 24” on the 8’ PVC pipe with the marker. Stretch the sock over the end of the PVC pipe beyond the 24” mark.
Tape the sock to the pipe.
See video How to make a tool to measure sludge and scum levels in a septic tank.
To use this tool see FAQ’s and related videos:
How do I know if the Septic tank needs pumping?
How to inspect a home Septic system.
Judging sludge quality