20 lb Propane tank ( Typical Forklift Size Tank )

I brought one in for re-fill yesterday and it registered 6 gallons, I thought they might hold a little more than 6, but I have no way of knowing, I'll look on the tank and see if any info is there ( they were new mfr. a few years ago ).

The relief valve was open and usually when the vapor starts to come out, it's full, sometimes it sputters a bit, but once you have a steady flow, it should be full, not the case yesterday at the filling station. I did run it until it was empty and hardly any pressure in this tank, last time mowing, not much pressure when I opened the relief valve. Not to overfill or be a pain to the supplier, but I'd like to be sure, that I'm getting my $20 worth, it certainly runs and is much better on fuel ( a Ford 850 LP ) since I re-built the vaporizer and replaced the safety valve.

Seems to run longer ( had a leak ) and I don't have to switch to the spare I keep full as often, nice to have at least 2 tanks in rotation.

Mahalo,
doogdoog

So possibly 8 gallons should be about right if empty, not sure if even after the tractor starts to sputter, meaning it's out, is there any other volume to consider, even if the relief valve shows little pressure while standing up right ?

The kid at the place is kind of new, he's a little "funky" around that big nurse tank, and it just seems I'm not getting filled to the correct level, but I'm not one to say something unless I really know what the deal is and it's nice to go to the field with a full tank when brushogging, this time of year is when I do most of it, as the re-growth is not so much.