Radiant Floor Loop Length Calculator

Frequently Asked Questions

How long is a typical PEX radiant loop?

Loop length per zone ≈ area (sq ft) ÷ (tube spacing in inches ÷ 12) + supply/return runs. 200 sq ft at 9 in spacing ≈ 200 × 12/9 = 267 ft + 20 ft = 287 ft - under the 300 ft ½ in PEX practical cap.

What spacing should I use?

Standard residential: 12 in for whole-house slabs, 9 in for bathrooms and edges, 6 in for high-loss perimeters and snowmelt. Tighter spacing = lower supply temperature for the same output.

Why is 300 ft a common loop limit?

Beyond ~300 ft of ½ in PEX, friction loss exceeds typical circulator head (≈ 9–13 ft) and flow drops below the 0.6–1.0 GPM needed for even heating. Split larger zones into multiple loops on a manifold.

What supply temperature should I target?

Slab on grade: 90–110 °F. Light staple-up under wood floors: 130–150 °F. Higher mass = lower temperature = better condensing-boiler efficiency. Always confirm against a heat-loss calc, not rule-of-thumb.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A radiant-floor loop is a continuous run of PEX tubing buried in the slab or stapled under the subfloor that carries warm water and gives up its heat through the floor surface. Two design numbers govern layout: the tube spacing (typically 6 in for high-load perimeter zones, 9 in for living spaces, 12 in for low-load areas) and the maximum loop length, which is set by tube diameter because longer loops mean more pressure drop than a circulator can push at a useful flow. 1/2 in PEX caps near 300 ft per loop, 5/8 in near 400 ft, and 3/8 in near 200 ft. This tool turns area and spacing into total tubing required, then breaks it into the right number of equal-length loops on the manifold.

Formula / Method

Linear feet of tubing per square foot of floor = 12 ÷ spacing (in). So 6 in spacing needs 2 ft of tube per sq ft, 9 in needs 1.33 ft, and 12 in needs 1 ft. Multiply by zone area to get base tubing. Add roughly 20 ft of leader to each loop for the supply and return runs to the manifold. Number of loops = ceiling(total with leaders ÷ max loop length). The maximum loop length is enforced by the head-loss curve of the tubing - exceeding it forces the circulator off its design point, dropping flow until heat output collapses. Keep all loops on the same manifold within about 10% of each other's length so manual balancing valves can equalize flow without one short loop hogging all the water.

Worked Example

A 200 sq ft bathroom slab on 12 in centers with 1/2 in PEX needs 200 × (12 ÷ 12) = 200 ft of tubing in the floor, plus 20 ft of leader = 220 ft total. One loop of 220 ft sits comfortably under the 300 ft cap. Same bathroom on 6 in spacing (perimeter cold-floor design) takes 200 × 2 = 400 ft + 20 = 420 ft - that exceeds 300 ft so it must be two loops, each about 210 ft. A 600 sq ft living-area zone at 9 in spacing is 600 × 1.33 = 800 ft + 20 = 820 ft - three loops of roughly 275 ft each, all returning to the same manifold.

Common Mistakes

Running a single oversized loop - the circulator can't maintain flow, the far end runs cold, and the room never reaches setpoint.

Mixing loop lengths on one manifold that differ by more than 10–15%; balancing valves can only fight so much.

Using 12 in spacing under cold-floor materials like tile over concrete - the surface temperature ripples between tubes feel cold.

Forgetting expansion-loop relief at penetrations; PEX moves with temperature and can rub through metal edges.

Burying tube too deep in the pour - aim for 3/4–1 in below the finished surface for responsive feel.

Tips & FAQ

What spacing should I use? 6 in along exterior walls and under glass, 9 in in main living zones, 12 in in interior rooms or under carpet. Tighter spacing means more even surface temperature, faster response, and lower required water temperature. What supply water temperature? Slab on grade: 90–110 °F supply works well with mod-con or heat-pump boilers. Staple-up under subfloor: 130–150 °F because the floor is a poorer heat-spreader. Why 1/2 in instead of 5/8? 1/2 in is the most common, fits standard staple guns and snap panels, and 300 ft loops cover most rooms. 5/8 in is for large open areas where you want fewer loops and the manifold cost matters. Oxygen barrier: always use PEX-AL-PEX or PEX with an EVOH barrier; bare PEX lets oxygen diffuse into the loop and corrodes ferrous components. Manifold layout: one manifold per zone or floor; brass with built-in flow meters costs more but is night-and-day for balancing. Slab installation: tie tubing to remesh or use clip panels; pressure-test to 60–80 psi before the pour and leave it under pressure during placement to spot punctures. Insulation: R-10 under the slab edge minimum, R-20 in cold climates - uninsulated slabs lose heat downward as fast as upward. This is a planning estimate - final designs should be checked by a hydronic professional.

Radiant Floor Loop Length Calculator

Frequently Asked Questions

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