How to calculate additional refrigerant charge
Split systems and heat pumps ship precharged for a set line length. Run the lines longer than that and you owe the system extra refrigerant, calculated as extra length times the charge rate for your liquid line size. This tool derives the charge rate from the liquid line inside diameter and the density of liquid refrigerant rather than reading it off a single brand table.
The physics is simple: the liquid line runs full of liquid, so the rate per foot is internal cross-section area times liquid density. A 3/8 in liquid line carrying R-410A works out to roughly 0.56 oz/ft, so a 100 ft run on a unit precharged for 25 ft needs about 42 oz, or 2.6 lb, of additional charge. Suction line length is not counted because the refrigerant in it is vapor and weighs very little by comparison.
In the field, the manufacturer installation instructions always govern. Many systems add a per-indoor-unit allowance, a correction for branch kits, and hard limits on total line length and vertical separation. Overcharging raises head pressure and risks compressor damage, while undercharging costs capacity and can cause floodback, so always weigh the charge in with a scale. Refrigerant recovery and charging must be done by a technician certified under EPA Section 608 rules.
Frequently Asked Questions
Refrigerant in the suction line is vapor, with a density a fraction of the liquid, so its mass barely moves the total. Manufacturer tables list charge rates by liquid line size for the same reason.
Follow the manufacturer. Their table folds in indoor unit allowances and oil retention, so treat this calculation as a planning estimate for ordering refrigerant and checking a quote.