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The system's nameplate cooling/heating capacity at the manufacturer's base (included) line length.
This is the capacity published for the unit at or below the manufacturer's base line length allowance — before any derate for extra length or vertical rise is applied.
The horizontal refrigerant line run beyond the manufacturer's base length allowance.
Only the portion of the horizontal run that exceeds the manufacturer's included base allowance counts here — check the installation manual for that base length before entering this value.
The percentage of rated capacity lost per foot of horizontal line beyond the base allowance.
This rate is published in the specific manufacturer's engineering or installation manual for the exact model — it varies significantly between manufacturers and product lines.
The total vertical elevation change between indoor and outdoor units.
Vertical rise typically carries a steeper capacity derate than horizontal run because of the added oil-return and pressure-drop demands on the compressor.
The percentage of rated capacity lost per foot of vertical rise.
This rate is also published in the specific manufacturer's engineering or installation manual — it is typically higher than the horizontal derate rate for the same model.
Derated system capacity
19,300 BTU/hr
The horizontal and vertical derate rates MUST come from the specific manufacturer's installation/engineering manual for your exact model — they vary significantly between manufacturers and are not standardized. This calculator only applies the rates you supply; it does not know or assume any specific product's actual derate schedule.
- Total capacity derate
- 19.75 %
They open the calculator with your figures already in it
Mini-Split/VRF Line Set Length Derating Calculator: 19,260 BTU/hr — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Manufacturers of mini-split and VRF systems publish a base line length included at 100% rated capacity, then apply a percentage capacity derate per foot of additional horizontal run and per foot of vertical rise beyond that allowance — this calculator applies user-supplied derate rates (from the specific manufacturer's engineering/installation manual) rather than a universal constant, since these rates vary significantly by manufacturer and model.
Inputs used
- Manufacturer's Rated Capacity
- 24000 BTU/hr
- Additional Horizontal Line Length Beyond Base Allowance
- 19.5 ft
- Manufacturer's Horizontal Derate Rate (% per ft)
- 0.5
- Vertical Rise
- 10 ft
- Manufacturer's Vertical Derate Rate (% per ft)
- 1
Intermediate steps
- Total capacity derate
- 19.75 %
Confidence note: The horizontal and vertical derate rates MUST come from the specific manufacturer's installation/engineering manual for your exact model — they vary significantly between manufacturers and are not standardized. This calculator only applies the rates you supply; it does not know or assume any specific product's actual derate schedule.
What this calculation does not cover
- A long line set needs more refrigerant, and that quantity is not computed here. Systems ship pre-charged to a stated line length, and past it charge is added at a published rate per unit of extra length — get that wrong and the capacity loss dwarfs the derate above, starving the evaporator when short and returning liquid to the compressor when long. The top-up is weighed in, not gauged in.
- A derate schedule has an end and this arithmetic does not. Manufacturers publish a maximum total line length and a maximum height difference, and beyond either the system is outside its listing no matter what percentage the rates would produce. Direction counts too: permitted lift is usually different with the outdoor unit above the indoor one than below it, and oil traps at intervals up a long riser belong to the same rule — the field above takes one rise with no sign on it.
- The nameplate being derated is a laboratory figure. Rated capacity is measured at standard test conditions, and on a design summer afternoon at a higher outdoor temperature — or in heating at a low one, where defrost cycles take their own share — the machine delivers less than its rating before line length is counted at all. Comparing a derated nameplate against a load computed at design conditions compares two numbers taken on different days.
Add the equipment this sizes
This result is a specification — 19,300 BTU/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Manufacturers of mini-split and VRF systems publish a base line length included at 100% rated capacity, then apply a percentage capacity derate per foot of additional horizontal run and per foot of vertical rise beyond that allowance — this calculator applies user-supplied derate rates (from the specific manufacturer's engineering/installation manual) rather than a universal constant, since these rates vary significantly by manufacturer and model.
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