PEX Radiant Floor Calculator (5/8 in Tubing Sizing)

Room heat loss uses the same sourced, IECC-zone-based heating BTU/sqft formula this site's AC & Furnace Size Calculator uses, based on the room's full (gross) footprint — walls and windows lose heat regardless of what sits on the floor below them. Active heated area subtracts any excluded area (cabinets, tubs, islands, fixed fixtures) you entered, and the room's full heat loss must be delivered through that smaller active surface, which is why heat loss per active sq ft (not gross sq ft) drives the covering screening below. Floor covering's maximum achievable output is a planning-stage range cross-checked across multiple published guides (bare/polished concrete or stone slab 45, tile/stone 35, engineered wood/LVP 25, carpet with pad 18 BTU/hr per sq ft) — a PRELIMINARY SCREENING only, not a substitute for a full hydronic design procedure (Uponor's and Caleffi's own published design procedures account for installation assembly, covering resistance, tube spacing, average water temperature, flow, and head loss together — this screening checks floor covering alone, and does not claim spacing or installation method are irrelevant to real output). Active tubing length (what's actually laid in the active floor area) is Area x 12 / spacing (the exact geometric relationship, confirmed against a manufacturer's own published multiplier table) — a required minimum, rounded UP, never down. Each loop also carries its own leader run — a supply-and-return connection back to the manifold, twice the one-way manifold distance you entered — and that leader length counts against the SAME maximum circuit length as the loop's active tubing (Uponor's own design manual calculates total loop length as active length plus leader length this same way, and the 2024 IMC Table 1209.6.1 max-loop-length limits apply to the complete circuit, not active tubing alone). Required loops therefore reserve each loop's leader length out of the selected tube diameter's maximum recommended loop length (3/8" 250 ft, 1/2" 300 ft, 5/8" 400 ft, 3/4" 500 ft, per the Radiant Professionals Alliance) before dividing the remaining budget into the active tubing length, rounded UP to a whole loop count. Ordering length totals the active tubing plus every loop's leader run, plus a 10% allowance for return bends and connection waste on top of that — the practical "how much to buy" figure. This is a planning estimate — actual water temperature (which varies significantly by assembly — a bare slab with low heat loss can run as cool as the mid-90s°F, while other assemblies need substantially higher temperatures), manifold design, and floor assembly details should come from a licensed hydronic heating contractor's full design before purchasing or installing.