Calculate how many feet of PEX tubing your radiant floor needs from area and tube spacing, plus loop count, leader tails, and rolls to buy.
Radiant Floor Heating Tubing Length Formula
Estimate the active tubing inside the heated floor from its area and center-to-center tube spacing. With area in square feet and spacing in inches:
Enter the maximum installed circuit length approved for the exact tubing product and system design. The estimate divides the active tubing among equal circuits, each with supply and return tails to the manifold and any extra installed routing allowance. The limit must exceed those tails and routing. Actual layouts, bends, flow and pressure drop still need design checks:
Variables:
- Lfield is the estimated active tubing in the heated floor (ft), before separately entered tails and extra routing
- A is the heated floor area (ft²), after subtracting cabinets, tubs, and other blocked spots
- S is the tube spacing, center to center (in)
- N is the estimated number of circuits; each circuit uses one supply-and-return manifold port pair
- Lmax is the approved maximum installed circuit length for the selected product and design (ft), including tails and extra routing; nominal tube size alone does not establish it
- D is the one-way routed distance from the manifold to the heated zone, including vertical travel (ft). E is extra installed bend or obstacle routing per circuit (ft), excluding footage already counted
- B is the chosen spare purchase allowance (%); this stock remains outside the installed circuits
- Linstalled is the estimated tubing actually installed (ft). Ltotal is the purchase budget including uninstalled spare stock (ft)
The length mode accepts net heated area directly or as length and width, spacing, and an approved design loop limit. Optional manifold tails and extra routing count as installed tubing; a separate spare percentage increases the purchase budget. Results include estimated circuits, average installed cut length and roll plans. Optional water volume uses the installed length and the selected product fill reference, excluding spare stock and other system components. Coverage mode instead treats T as tubing on hand and H as the percentage of that stock held back for tails and waste. It estimates field coverage from the remainder; this holdback is not the inverse of the length mode’s purchase markup and does not verify individual circuits:
Tube Spacing and Roll Planning Values
The approximate active tubing per square foot is 12 divided by the spacing in inches. The room examples below are illustrative, not spacing recommendations; heat loss, floor construction, water temperature and the selected system determine suitable spacing. The 9 in row is rounded:
| Spacing | Tubing per ft² | Tubing per 100 ft² | Typical use |
|---|---|---|---|
| 6 in | 2.00 ft | 200 ft | Bathrooms, high heat loss rooms, cold perimeters |
| 8 in | 1.50 ft | 150 ft | Kitchens, entries, aluminum plate systems |
| 9 in | 1.33 ft | 133 ft | Standard living areas with normal insulation |
| 12 in | 1.00 ft | 100 ft | Slabs, basements, bedrooms |
| 16 in | 0.75 ft | 75 ft | Garages and low load slabs |
The roll plan assumes each installed circuit is one continuous cut from the manifold and back. A roll must fit the full cut; short remnants cannot be joined to satisfy this planning assumption. This is not a universal prohibition on manufacturer-approved repair fittings, which remain subject to product instructions and local requirements. This table shows complete cuts per roll; its sample cut lengths are not approved circuit limits:
| Roll size | Full cuts at 250 ft per loop | Full cuts at 300 ft per loop | Full cuts at 330 ft per loop |
|---|---|---|---|
| 300 ft | 1 | 1 | 0 |
| 500 ft | 2 | 1 | 1 |
| 1000 ft | 4 | 3 | 3 |
Two 500 ft rolls can therefore be the wrong buy for a plan with three 330 ft loops even though 1000 ft covers the total, while a single 1000 ft roll handles it. The calculator checks equal installed cuts and enough total stock for the separate spare purchase allowance. It reports purchased footage and the amount left uninstalled.
Radiant Floor Tubing Length Example Problems
Example 1: tubing to order for a room. Assume a 200 ft² heated area, 9 in spacing, an approved 300 ft installed-circuit limit for this example, a 10 ft one-way manifold route, no extra routing and a chosen 10% spare purchase allowance. Field tubing is 200 * 12 / 9 = 266.667 ft. Each circuit can hold 300 – 2 * 10 = 280 ft of field tubing, so the estimate needs 1 circuit. Its tails add 20 ft, giving an installed cut of 286.667 ft. The separate purchase budget is 286.667 * 1.10 = 315.333 ft, using unrounded values. A 300 ft roll fits the installed cut but falls short of that total purchase budget. One 500 ft roll leaves 213.333 ft uninstalled, including the 28.667 ft planned spare allowance and 184.667 ft beyond the purchase budget. These figures are rounded to three decimals; they do not prescribe spacing or a loop limit for another installation.
Example 2: coverage from tubing on hand. You have one 300 ft roll and assume 12 in spacing with 10% of the stock held back for tails and waste. Usable field tubing is 300 – 30 = 270 ft, and at 1 ft of tubing per square foot that estimates 270 * 12 / 12 = 270 ft² of floor. This separate holdback model assumes the reserved 30 ft is sufficient; it does not check circuit limits, actual tails, bends or heat output.
Radiant Floor Tubing FAQs
What spacing should I use for radiant floor heating? Use the spacing selected for the room’s heat loss, floor construction and coverings, water temperature and the manufacturer’s system design. Room type alone does not determine it. Closer spacing uses more tubing and can improve heat transfer and temperature uniformity, but this quantity estimator does not calculate heat output, response time or floor temperature.
How long can each loop be? Enter the installed-circuit limit approved for the exact product and design. It depends on tube diameter, required flow, heat load, temperature drop and fluid composition, including glycol. Count supply and return tails and extra routing within that limit. Similar circuit lengths can help balancing; the equal-length estimate here is not a pressure-drop, pump or manifold-capacity calculation.
Why should I order more tubing than the math says? First include planned manifold tails and extra bend or obstacle routing in the installed estimate and loop checks. Then choose any additional uninstalled stock for scrap, offcuts or uncertainty. No fixed percentage guarantees enough tubing for every layout. The roll plan keeps circuits continuous and allows for the selected spare budget; optional tubing water volume excludes all uninstalled stock. Follow the product’s bend-radius and installation requirements.
