Calculate how much PEX tubing a radiant floor needs from heated area and tube spacing, with the number of loops at a 300 ft typical circuit limit.
Radiant Floor PEX Length Formula
L = (A * 12 / S) + (N * E)
N = ceil( (A * 12 / S) / (M - E) )
T = L * (1 + W / 100)
- L = tubing length before the waste allowance (ft)
- A = heated floor area (sq ft)
- S = tube spacing, center to center (in)
- N = number of loops, rounded up to a whole number
- E = leader length per loop, the round trip from the manifold to the heated area (ft)
- M = project-specific maximum installed circuit length, including leaders (ft)
- W = waste allowance (%)
- T = total tubing to order (ft)
The base term A * 12 / S converts your floor area into linear feet of tubing. Dividing 12 by the spacing gives the feet of PEX needed per square foot, so tighter spacing puts more tubing in the same floor. The calculator then splits that footage into loops so each equal circuit stays within your supplied total circuit limit, adds the leader run for each loop, and applies the waste allowance to get the total footage to order. It also reports the loop count, which equals the number of manifold ports you need, and compares illustrative stock-coil cutting plans for equal circuits. Actual cuts must remain continuous and fit the available coils; the result is not a hydraulic design.
The coverage mode removes the ordering allowance, compares the circuit counts immediately below and above the usable budget divided by your circuit limit, and subtracts the combined leaders for each candidate. It chooses the larger active length, which can leave unused tubing when another circuit would consume more in leaders. Coverage is A = active length * S / 12. This assumes a combined stock budget and equal circuits; separate short coils may cover less.
Spacing Multipliers and Coil Coverage
This table shows how much PEX each square foot of floor requires at common spacings. Multiply your heated area by the multiplier for a quick estimate before leaders and waste.
| Tube spacing | Feet of PEX per sq ft | Design consideration |
|---|---|---|
| 6 in | 2.00 | Higher tube density; verify heat-loss design |
| 8 in | 1.50 | Verify load, floor covering and supply temperature |
| 9 in | 1.33 | Quantity example, not a universal design spacing |
| 12 in | 1.00 | Verify heat output and floor temperature limits |
| 16 in | 0.75 | Lower tube density; verify heat-loss design |
PEX is sold in standard coil sizes, and buying the wrong mix of coils is one of the most common radiant floor mistakes. This table shows roughly how much floor each stock coil size covers with 1/2 in tubing, assuming 300 ft maximum loops, 10 ft of leader per loop, and a 10 percent waste allowance.
| Coil size | Coverage at 9 in spacing | Coverage at 12 in spacing | Loops supported |
|---|---|---|---|
| 100 ft | about 60 sq ft | about 80 sq ft | 1 |
| 300 ft | about 195 sq ft | about 260 sq ft | 1 |
| 500 ft | about 325 sq ft | about 435 sq ft | 2 |
| 1,000 ft | about 653 sq ft | about 870 sq ft | 3 |
Example Problems
Example 1: You are heating a 400 sq ft basement slab with 1/2 in PEX at 12 in spacing. The base tubing is 400 * 12 / 12 = 400 ft. With a 300 ft maximum loop and 10 ft leaders, each loop can hold 290 ft of floor tubing, so you need ceil(400 / 290) = 2 loops. Adding 2 * 10 = 20 ft of leaders gives 420 ft, and a 10 percent waste allowance brings the total to 462 ft. One 500 ft coil can supply two equal 210 ft installed circuits plus the stated allowance; confirm continuous cuts and connect the circuits to two manifold ports. The supplied 300 ft circuit limit is an example to be verified for the actual design.
Example 2: You already own a 300 ft coil of 1/2 in PEX and want to know how much bathroom and kitchen floor it can cover at 9 in spacing. Removing the 10 percent waste allowance leaves 300 / 1.10 = 272.7 ft. That fits in one loop, so subtracting one 10 ft leader leaves 262.7 ft in the floor. The coverage is 262.7 * 9 / 12 = 197 sq ft.
Radiant Floor PEX Length FAQ
How long can a single PEX loop be? Use the project-specific total circuit limit from the tube manufacturer or hydraulic design, including both supply and return leaders. Tube diameter alone does not establish a universal limit: flow, fluid temperature, glycol content and available pressure drop also matter. The calculator starts at 300 ft, a common limit for 1/2 in PEX; replace it with your project limit. Dividing footage into circuits does not itself verify manifold or pump capacity.
What tube spacing should I use? Select spacing from a room-by-room heat-loss and radiant-floor design, considering floor covering, supply temperature and manufacturer floor-temperature limits. Wider spacing uses less tubing; the quantity formula cannot prove that the floor meets the heat load. The initial 9 in spacing is an editable planning setting, not a recommendation for every floor.
Should I run tubing under cabinets and fixtures? No. Skip permanent obstructions like base cabinets, kitchen islands, tubs, and toilets, and subtract their footprint from your heated area before calculating. Heating under fixed cabinetry wastes tubing, can dry out or damage the cabinet bases, and does nothing for comfort. The exception is furniture that may move later, such as a bed or couch, which you should still tube under.
