PEX Radiant Floor Heating Layout Calculator

How to Use This PEX Radiant Floor Heating Layout Calculator

Enter the room length and width, subtract any area that will not get tubing (cabinets, a tub, a fixed island), and set the tube spacing with the slider. Choose the PEX size and the coil length you can buy, then enter how far the manifold sits from the room. The calculator returns total tubing, the number of loops, the length of each loop and how many coils to order.

The max loop length box fills in the usual limit for the tube size you choose. You can lower it, but values above the limit for that size are capped. Results update when you change a setting after the first calculation.

Why This Matters

Tubing is the cheap part of a radiant floor and the impossible part to fix. Once it is under a slab or a gypsum pour, a loop that is too long, a coil that comes up short or a missing circuit on the manifold means ripping it out. A 270 ft² living room at 9 in spacing needs roughly 380 ft of heated tube before leaders, which already exceeds one 300 ft coil and rules out a single loop.

Loop length is the number that causes most comfort complaints. Water loses heat as it travels, and a long loop in small-diameter tube also adds pressure drop that a standard circulator may not overcome. The far end of an overlong 1/2 in loop runs noticeably cooler, giving a floor with warm and cold stripes. Equal loops, each within the limit for its tube size, give even heat and balance easily at the manifold.

Contractors, heating installers and DIY builders all use this step to price the job, size the manifold (one port pair per loop) and check that coils will actually cut into legal loops. Loops cannot be spliced under a floor, so leftover tubing in each coil is wasted unless a shorter loop elsewhere can use it.

How It's Calculated

All steps below are exactly what the tool computes:

1. Heated area = length × width − unheated area.
2. Heated tube = heated area × 12 ÷ spacing (in) × 1.05. At 12 in spacing that is 1 ft of tube per ft²; at 6 in it is 2 ft. The 1.05 is a fixed 5% allowance for turns, obstacles and layout error.
3. Leaders per loop = 2 × the one-way run from the manifold.
4. Loop limit = the smaller of your max loop length and the limit for the tube size. Tube available for heating in each loop = limit − leaders.
5. Loops = heated tube ÷ available tube, rounded up. Each loop = heated tube ÷ loops + leaders, so all loops are equal and none exceeds the limit.
6. Total tubing = heated tube + loops × leaders, rounded up to whole feet.
7. Coils: each loop is cut as one unspliced piece (length rounded up). Loops per coil = ⌊coil ÷ loop⌋, and coils = ⌈loops ÷ loops per coil⌉.

Serpentine and counterflow layouts use the same tube length per square foot at the same spacing. The pattern changes only how evenly the floor surface warms, so it affects the diagram and not the footage.

The loop limits are common manufacturer rules of thumb, not code requirements. Local codes vary, and the manufacturer's design guide for your tubing governs.

Minimum U-turn spacing assumes a conservative bend radius of 6 × tube outside diameter, so a 180° turn needs spacing of at least 12 × OD. Some PEX-A tube allows tighter bends. Check the manufacturer's data for yours.

Worked Example

A 20 ft × 15 ft room has 30 ft² of built-in cabinets and a tub. Tubing is 1/2 in PEX at 9 in spacing, the manifold is 10 ft away, the max loop is 300 ft, and 300 ft coils are available.

1. Heated area = 20 × 15 − 30 = 270 ft².
2. Heated tube = 270 × 12 ÷ 9 × 1.05 = 360 × 1.05 = 378 ft.
3. Leaders per loop = 2 × 10 = 20 ft. Limit = min(300, 300) = 300 ft, so available per loop = 280 ft.
4. Loops = ⌈378 ÷ 280⌉ = ⌈1.35⌉ = 2 loops. Each loop = 378 ÷ 2 + 20 = 209 ft, covering 135 ft².
5. Total tubing = 378 + 2 × 20 = 418 ft.
6. Coils: ⌊300 ÷ 209⌋ = 1 loop per coil, so 2 coils of 300 ft (600 ft ordered, 182 ft of offcuts). A single 500 ft coil would also work.
7. Spacing check: 9 in ≥ 7.5 in minimum U-turn spacing for 1/2 in tube, so no bend warning.

Tips & Common Mistakes

• Never splice under the floor. Each loop must be one continuous piece, which is why the coil count can exceed total feet ÷ coil length.
• Keep loops equal. Loops within about 10–15% of each other balance easily. Do not make one 300 ft loop and one 60 ft loop.
• Do not forget the leaders. A manifold in the next room adds tube to every loop. Count the run in both directions.
• Do not use tight spacing to chase output. Spacing below the tube's bend limit kinks the tube. Heat output also depends on water temperature and floor covering, which this tool does not calculate.
• Pressure-test before covering. Hold the system under test pressure while the slab or underlayment is placed, following your tubing maker's instructions and local code.

Frequently Asked Questions

How much PEX do I need per square foot for radiant floor heating?

The tube length is 12 ÷ spacing in inches: 1 ft per ft² at 12 in, 1.33 ft at 9 in and 2 ft at 6 in. This calculator adds 5% for turns and layout error, plus the leaders from the manifold for each loop.

How long can a single PEX loop be?

Common manufacturer rules of thumb are about 200 ft for 3/8 in, 300 ft for 1/2 in, 400 ft for 5/8 in and 500 ft for 3/4 in, counting the leaders. The tool caps your max loop length at the limit for the size you choose.

Is a serpentine or a counterflow spiral layout better?

They use the same tubing at the same spacing. Serpentine is faster to install, but one end of the room gets hotter water. Counterflow spiral puts supply and return tubes side by side for more even surface temperature, at the cost of more complex installation.

Can I join two pieces of PEX under the floor?

Avoid it. Fittings buried in a slab or underlayment are a leak risk and many manufacturers and codes forbid or restrict them. Plan each loop as one length of tube and use the coil result to order enough.

Does this calculator tell me the heat output?

No. Heat output depends on room heat loss, supply water temperature, floor covering and surface-temperature limits. Use a heat loss calculation and the tubing manufacturer's output charts to choose spacing, then use this tool to lay out the loops.

Why does the tool say I need more coils than total feet suggest?

Because loops cannot be spliced, leftover tubing in a coil cannot always be used. With 209 ft loops and 300 ft coils, each coil yields one loop and 91 ft of offcut. A longer coil size often reduces waste.

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