How much PEX does your radiant floor need?

In a hydronic radiant floor, warm water runs through PEX tubing laid under or in the floor. How much tubing you need, how many loops it must be cut into and how much water must flow through them follow from a few numbers you already have: the floor area, the spacing of the tubes and the heat the room needs.

What the number means

Radiant floor tubing has to do two things at once: cover the floor evenly and keep every loop short enough for the circulator to push water through it. The total length is the first and the number of loops is the second. This calculator gives both, together with the flow and pressure loss you need to pick the pump and the manifold, and checks the output against the one limit that has a clear published source, the one for wood floors.

Method and variables

Tubing in the floor = area × 12 ÷ spacing (inches). Each loop also needs a leader, the round-trip run to the manifold, so the loops are chosen as the smallest number n with area tubing ÷ n + leader no more than the loop limit. Total flow = load ÷ (500 × ΔT), split equally, and the head lost in a loop follows from the Hazen-Williams equation with C = 150 for plastic and the PEX bore. The result also gives the speed of the water and the volume of water in the loops, for sizing the expansion tank (expansion tank calculator).

Worked example: 400 square feet at 9 inch spacing

The floor holds 533 feet of ½ inch PEX (1.33 feet per square foot). With a 40 foot leader per loop and a 300 foot limit, 1 loop would be too long and 3 loops of 218 feet each fit: 653 feet in all. A 12,000 Btu/h load with a 20 °F drop needs 1.20 gpm, 0.40 gpm per loop. Water moves at 0.69 ft/s and loses 1.3 feet of head in a loop. The loops hold 6.3 gallons. The output is 30 Btu/h per square foot.

Limits of this method

This page sizes the tubing, not the system. It does not find the heat loss of the room (use the heat loss calculator), the water temperature that delivers the output through your floor build-up, or the manifold, pump and boiler. The loop limits are the figures published by manufacturers and sellers, and they differ, so check the manual of the tubing you buy. We did not open the Uponor Radiant Design Manual or the ASHRAE Handbook, so we give no floor surface temperature estimate and no maximum water speed. Check your local code and the manufacturer's installation guide, including the minimum bend radius.

How much PEX do I need per square foot of floor?

Twelve divided by the spacing in inches: 12 inch spacing needs 1 foot of tubing per square foot, 9 inch spacing needs 1.33 feet and 6 inch spacing needs 2 feet. That is geometry, not a rule of thumb: tubes s inches apart put 12 ÷ s feet of tube across each square foot. On top of that comes the leader, the run from the manifold to the heated area and back, which is not in the floor. For the 400 square foot example at 9 inches, 533 feet lie in the floor and 653 feet are bought.

What is the maximum length of a radiant loop?

Published figures vary by source and tubing size. Radiant Made Simple says a PEX loop should not exceed 300 feet. Uponor's blog gives under 300 feet for ½ inch and under 400 feet for ¾ inch. A seller of PEX, PEX Universe, lists 200, 300, 400 and 500 feet for ⅜, ½, ⅝ and ¾ inch. Because the sources disagree on ¾ inch, this calculator takes the limit as an input and starts from 300 feet, which the sources agree on for ½ inch. The limit exists because a longer loop loses more pressure; the result shows the head lost in a loop so you can see how close you are.

How do I find the flow rate for a radiant floor?

For water, heat output in Btu per hour equals 500 × flow in gallons per minute × the temperature drop in °F. The 500 comes from 8.33 pounds per gallon × 60 minutes × 1 Btu per pound per °F. For 12000 Btu/h and a 20 °F drop the flow is 1.20 gpm, shared by 3 loops: 0.40 gpm each. A smaller drop, say 10 °F, needs twice the flow (2.40 gpm) and more pump work. The 500 factor is for water; antifreeze mixes change it.

How many Btu per square foot can a floor give?

It depends on the floor covering and the water temperature, and the limit that is easiest to cite is for wood. The National Wood Flooring Association's radiant guidelines limit the finished wood floor surface to 80 °F; at 70 °F room air that delivers about 20 Btu/h per square foot, so heat loads above that need supplemental heat. Our example needs 30 Btu/h per square foot, so under wood the floor alone is not enough. For a floor of 400 square feet at the NWFA limit, the load would be 8000 Btu/h and the flow 0.80 gpm.

What tube spacing should I use?

Radiant Made Simple lists 6, 9 or 12 inches on center. Closer spacing puts more tubing in the floor and spreads the heat more evenly, at a cost: for the example, 960 feet in 4 loops at 6 inches, 653 feet in 3 at 9 inches and 480 feet in 2 at 12 inches. Tubes spaced closer let the floor run at a lower water temperature for the same output, but the amount of the effect depends on the floor build-up and is something the manufacturer's design manual gives; this page does not estimate it.

Does the tube diameter matter?

Yes, for pressure drop. The inside diameter of PEX tubing is the outside diameter times 7/9 (SDR 9). ½ inch tubing has a 0.486 inch bore and ¾ inch tubing 0.681 inch. In the example, each of the 3 loops loses about 1.3 feet of head in ½ inch tubing; in ¾ inch tubing at the same flow the loss falls to 0.26 feet, so longer loops become possible. A larger tube also holds more water: 12.3 gallons against 6.3.

Can I use this for snow melting or a garage slab?

The arithmetic for tubing length is the same. The design is not: snow melting needs a much higher output per square foot, a glycol mix and its own loop limits. The Uponor article mentions 12 inch spacing as standard for residential snowmelt, but the loop limits, the heat output and the fluid are set by a snow melt design guide, not by this page.

Sources: Radiant Made Simple, “PEX layout for radiant floor heat” (radiantmadesimple.com); Uponor, blog “Uponor PEX systems: your top questions answered for radiant heating and plumbing” (uponorus.com); PEX Universe, “Radiant heating floor output based on PEX tubing sizes and circuit lengths” (a seller; vendor figures); National Wood Flooring Association, Technical Publication “Radiant Heating Systems Requirements” (2019); ASTM F876 for the PEX dimensions (via the SDR 9 ratio, not opened); NFPA 13 Hazen-Williams coefficients as in our friction loss calculator; all read October 27, 2026. The Uponor Radiant Design Manual and the ASHRAE Handbook were not opened. Related: friction loss calculator, pipe volume calculator, BTU calculator, expansion tank calculator. How we check our calculators.