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Mini-Split/VRF Refrigerant Line Set Length Capacity Derating Calculator

Estimate a mini-split or VRF system's derated capacity when the refrigerant line set exceeds the manufacturer's base allowance for horizontal and vertical length.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
19.69 ft9.84 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Derated system capacity

19275.59 BTU/hr

Check your inputs

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
19.69 %

With the figures above, the derated system capacity comes to 19276 BTU/hr. The method behind this is well established, though site conditions and material batches will move it somewhat. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Add the equipment this sizes

This result is a specification — 19,275.59 BTU/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • 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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Frequently asked questions

Where do the horizontal and vertical derate rates come from?
They must come from the specific manufacturer's installation or engineering manual for the exact model being installed — these rates vary significantly between manufacturers and are not standardized. This calculator only applies whatever rates you supply; it doesn't assume any product's actual derate schedule.
Why does vertical rise usually carry a steeper derate than horizontal length?
Manufacturers publish separate horizontal and vertical derate rates because vertical rise adds oil-return and pressure-drop demands on the compressor beyond simple line friction, which is why the vertical rate is commonly higher than the horizontal rate for the same system.
What counts as the horizontal length entered here?
Only the horizontal run beyond the manufacturer's published base allowance included at 100% rated capacity — check the installation manual for that base length first, then enter only the additional length beyond it.