How it works
In a radiant floor the tubing lies in a serpentine pattern at a fixed spacing, so the length in the floor is simply the area divided by the spacing. Tighter spacing puts more tubing, and more heat, into the same floor.
Tubing is run in loops of limited length, because pressure drop rises with length. Each loop also needs a lead to and from the manifold, and that lead uses up part of the length budget. The calculator divides the floor tubing into the fewest loops that fit under your maximum, then adds the leads. Heat output divided by 500 × the temperature drop gives the water flow.
Worked example
A 400 ft² slab with 12 inch tube spacing, a 300 ft maximum loop, 20 ft leads, at 25 BTU/hr per ft² and a 20°F drop:
- Tubing in the floor = 400 ft² × 12 ÷ 12 in = 400 ft
- Loops = ⌈400 ÷ (300 − 2 × 20)⌉ = 2
- Total tubing = 400 + 2 × 20 × 2 = 480 ft
- Flow = 10,000 BTU/hr ÷ (500 × 20°F) = 1 gpm
| Input | Value |
|---|---|
| Heated floor area | 400 ft² |
| Tube spacing (on center) | 12 in |
| Maximum loop length (total, including leads) | 300 ft |
| Average one-way lead to the manifold | 20 ft |
| Tube inside diameter | 0.475 in |
| Design heat output | 25 BTU/hr·ft² |
| Loop temperature drop | 20 °F |
| Result | Value |
|---|---|
| Tubing in the floor | 400 ft |
| Number of loops | 2 loops |
| Length of each loop, including leads | 240 ft |
| Total tubing to buy | 480 ft |
| Total heat output | 10,000 BTU/hr |
| Total flow | 1 gpm |
| Flow per loop | 0.5 gpm |
| Fluid volume in all the tubing | 4.4 gal |
Assumptions and limits
- Assumes the floor area is all covered by tubing at the spacing you chose. Subtract areas under cabinets and fixtures.
- Maximum loop length depends on tube size, flow, and the circulator. Use the tubing maker's recommendation.
- Flow uses water (500 factor). Glycol mixes need more flow; use a lower factor from the fluid data.
- Heat output per square foot depends on the floor covering, water temperature, and spacing. This tool does not predict it; use your heat load and the manufacturer's output charts.
Common questions
How much PEX do I need per square foot?
At 12 inch spacing, 1 foot of tubing per square foot; at 9 inches, 1.33 feet; at 6 inches, 2 feet. Add the leads on top.
How long can a radiant loop be?
It depends on the tube size and pump. 300 ft is common for 1/2 inch PEX, and larger tubes allow longer loops. Follow the manufacturer's data.
What spacing should I use?
Use tighter spacing (6 or 8 inches) for higher heat output in cold rooms or over hard floor coverings, and 12 inches for well-insulated floors with modest loads.
How do I size the circulator?
You need the flow from this tool and the pressure drop of the longest loop plus the manifold and piping. Use the pump total dynamic head calculator to add it up.
Sources
- Geometry: tubing length = area ÷ spacing.
- Hydronic flow equation for water: BTU/hr = 500 × gpm × ΔT (ASHRAE Handbook, HVAC Systems and Equipment).
Updated 2026-09-30