About Radiant Floor Tubing Calculator (PEX Loop Length)
The radiant floor tubing calculator estimates how much PEX a radiant heating job needs and how to split it into balanced manifold loops. Enter the heated area of each room, the tube spacing, and the manifold distance: the tool computes the active tubing from the coverage identity the Uponor Complete Design Assistance Manual (CDAM) tabulates — 12-inch on-center spacing lays 1.0 ft of tube per square foot, 9-inch lays 1.33, 6-inch lays 2.0 (exactly 12 ÷ spacing) — then adds leader runs at twice the one-way manifold distance and splits the total into equal-length loops.
Loops must be split because pressure loss grows with loop length — push one loop too long and its flow starves. The CDAM derives each project's loop limit from a head-loss design rather than one fixed number, so the calculator makes the target maximum loop length an editable input: Uponor's installation handbook notes 250–300 ft loops are workable with 3/8″ PEX and caps 5/16″ Quik Trak loops at 250 ft including leaders, while 1/2″ and 5/8″ tubing tolerate longer runs at the same head. Loops on a manifold are balanced to equal lengths, matching Uponor's manifold guidance that loop lengths stay within a few percent of each other.
How It Works
- Enter the heated floor area of each room the manifold serves (up to 10). Subtract areas that get no tubing — under cabinets, tubs, and fixed built-ins.
- Pick the tube spacing. 12″ on-center is the common field for living spaces, 9″ raises output for higher-load rooms, and 6″ serves bathrooms, slab edges, and high-loss zones — the spacing your heat-loss design calls for governs. Feet of tube per square foot is exactly 12 ÷ spacing, the identity behind the CDAM's printed multipliers.
- Enter the one-way distance from the manifold to the heated area, including the drop from the manifold to the floor. Each loop's leader is twice that (supply + return), exactly as the CDAM's worked example computes it.
- Set the target maximum total loop length. The default 300 ft is the top of Uponor's published 3/8″ guidance range; tighten it toward 250 ft for small tubing or generous head margins, or stretch it where your hydronic design (flow, ΔT, pump head) proves a longer loop out. The calculator divides the active tubing into the fewest equal loops that keep every loop, leader included, at or under the target.
- Read the takeoff: loop count (= manifold ports), balanced active and total length per loop, and total tubing to buy. Order whole coils and assign loop lengths from each coil to minimize waste — and remember this is a layout estimate: supply temperature, flow rate, and spacing itself must come from a heat-loss-based radiant design.
Worked Example
A 600 ft² open basement zone runs 12″ on-center from a manifold 10 ft away. Active tubing is 600 × (12 ÷ 12) = 600 ft; each loop carries a 2 × 10 = 20 ft leader. Against a 320 ft target loop length, each loop can hold 300 ft of active tube, so the job needs ceil(600 ÷ 300) = 2 balanced loops of 300 ft active — 320 ft total each, right at target — and 600 + 2 × 20 = 640 ft of PEX on a 2-port manifold. The CDAM's own Bedroom-1 example checks the identity at 9″ spacing: 136 ft² × 1.33 = 181 ft active plus a 35 ft leader = 216 ft total loop.
Formulas
- Active tubing from coverage
L_active = A × (12 / s)- Leader and loop split
L_leader = 2 × d; n = ceil(L_active / (L_max − L_leader)); L_loop = L_active/n + L_leader
Standards & References
- Uponor Complete Design Assistance Manual (CDAM), 4th ed., Chapter 8 "Radiant Floor System Design," Step 9 — loop length method: area multipliers by on-center distance (12″ 1.0 · 10″ 1.2 · 9″ 1.33 · 8″ 1.5 · 7″ 1.7 · 6″ 2.0 ft/ft²), leader = 2 × manifold distance, total loop = active + leader
- Uponor Residential Radiant and Hydronics Installation Handbook — 3/8″ PEX loop guidance (250–300 ft workable) and 5/16″ Quik Trak 250 ft maximum including leaders
- Uponor CDAM manifold guidance — loop lengths balanced (equal, within ~3% on non-balancing manifolds)
Frequently Asked Questions
How much PEX tubing do I need per square foot of radiant floor?
Exactly 12 ÷ spacing feet per square foot: 1.0 ft/ft² at 12″ on-center, 1.33 at 9″, and 2.0 at 6″ — the multipliers printed in the Uponor CDAM. A 1,000 ft² zone at 12″ spacing needs about 1,000 ft of active tubing before leaders; add 2 × the manifold distance for every loop.
How long can a radiant floor loop be?
It depends on tubing size, flow, and the pump — pressure loss, not heat output, is the limit. Uponor's handbook says 250–300 ft loops are workable with 3/8″ PEX and caps 5/16″ Quik Trak loops at 250 ft including leaders; 1/2″ and 5/8″ tubing run longer at the same head. The CDAM sizes each project's loops through its head-loss procedure, which is why this calculator leaves the maximum as an editable design target rather than pretending one number fits all systems.
Why do all the loops on a manifold need to be the same length?
Water follows the path of least resistance: a short loop steals flow from a long one on the same manifold. Uponor's guidance is to design equal loop lengths (within about 3% on manifolds without balancing valves) so every loop delivers its share of heat without valve-throttling gymnastics. This calculator always splits the total into equal loops for exactly that reason.
Does this calculator design my radiant heating system?
No — it is a tubing layout and quantity estimate. Tube spacing, supply water temperature, flow per loop, and pump head must come from a room-by-room heat-loss calculation and a hydronic design per the CDAM (or software like LoopCAD). Run the heat loss first, let it set the spacing and loop limit, then use this tool to count tubing and manifold ports.
What size PEX should I use for radiant floor heating?
Per Uponor: 1/2″ is the residential workhorse — flexible, cheap, and equal in heat output to larger sizes since the floor, not the tube, is the emitter. 3/8″ suits joist-bay retrofits but drops pressure faster, so loops run shorter; 5/8″ buys longer loops at the cost of stiffness and price; 5/16″ is for Quik Trak panels. Larger tubing does NOT deliver more heat per square foot.