I use a ultrasonic Compumeter PRO by RYOBI.
Measurement on one side, calc/conversion comp on the other.
Brilliant for domestics and small commercials, accurate for upto 99%measurement accuracy. I do check with the trusty tape measure and it's not too far out, not enough to make a significant differnce in the price of the job.
good upto 10m, over that then I use the tape measure.
Chris