Why the plate figure is almost never the answer
A condensing unit is charged at the factory for itself plus a stated length of pipe. Run 40 ft of liquid line where the instructions assumed 15 and you have 25 ft of unaccounted pipe. At 0.6 oz per foot that is 15 oz. The defaults on this page work out to 9 lb 13.1 oz accounted against a 6 lb 8 oz plate, which is 53.1 oz of difference — more than three pounds on a small system, and about 60 dollars of refrigerant at 18 dollars a pound.
The number that moves the total most is rarely the one people check. Charge per foot depends on the liquid line diameter, and one size up roughly doubles it. Reading the wrong pipe size off the table, if it moves the figure by 0.1 oz per foot, is 4 oz over a 40 ft run — small next to the 1 lb 14.8 oz sitting in the receiver on the defaults, but it compounds with every foot you add.
The receiver is the part that will not sit still
The line set holds what it holds. The receiver holds whatever the system is not currently using, and that changes with ambient, with load and with how much is in the evaporator. A receiver a quarter full at 8 a.m. and half full at 3 p.m. has moved 1 lb 14.8 oz on the defaults here, because ten percentage points of level is 12.3 oz. That is not an error in the accounting — it is the receiver doing its job, which is why the fill fraction is a field with a note rather than a constant.
If you are accounting for a system across a season, run it twice with the two levels you actually saw and treat the pair as the range.
What this page will not tell you
It will not tell you whether the charge is correct. Correct charge is settled by superheat and subcooling against the charging chart for that specific system at the conditions on the day, and this page has no chart, no target and no refrigerant property in it. Every such figure here — charge per foot, liquid density, the length the plate covers — is something you read off a document that came with the equipment.
It also will not tell you how to add or remove anything. A sealed refrigerant circuit is federally regulated work that requires certification, venting is illegal, and nothing above is a procedure.
Getting the line length right
Pipe length, not distance. The rise up the wall, the drop into the rack, the offsets around the beam and the loop at the compressor are all pipe, and on a plant run they add up to a quarter of the total more often than not. Measure the run when it is being installed and write it on the panel next to the plate; nobody enjoys measuring a finished pipe run through a ceiling.
Questions people ask
How much refrigerant does a foot of liquid line hold?
It depends on the pipe diameter and the refrigerant, and this page deliberately does not tell you — the figure comes from the line set table in the installation instructions for that unit. What the page does is show you what an error in it costs: with 40 ft of liquid line, being out by 0.1 oz per foot moves the total by 4 oz.
Do I subtract the line length the plate already covers?
Yes, or you count it twice. The page takes the plate charge, removes the refrigerant sitting in the length the instructions say that charge covers, and treats what is left as the unit on its own. Then it adds the pipe you actually installed. Put 0 in the covered length if the plate charge is stated as the unit only.
Why does the receiver need a fill percentage?
Because a receiver exists to hold a varying amount. The level rises in cool weather and on light load and falls when the system needs the liquid, so there is no single right answer. On the defaults, ten percentage points of level is 12.3 oz. Run the calculation at the level you actually saw and note the time of day next to it.
Can I use this to work out how much to add?
No. This is an inventory of volumes and lengths, and it says nothing about what is in the system now or what should be. Working on a sealed circuit is certified work, venting is illegal, and neither this page nor any part of the site gives a charging, recovery or evacuation procedure.
What if my system has no receiver?
Leave the receiver volume at 0 and the term drops out. Direct expansion systems with a fixed orifice and no receiver are accounted with just the unit, the pipe and whatever the drier and accumulator data sheets list as adders.