Short answer: a radiant floor loop is measured including the leaders to the manifold and back, and its maximum length is the tubing manufacturer's figure, not a universal rule. The quick "square feet ÷ spacing × 1.1" formula skips the leaders: on one 138 sq ft room at 6 in on center it came up 41.7 ft short, enough to turn "one loop" into "two loops". How many loops rooms from 129 to 431 sq ft need is in the table below.
Many radiant floor heating layout guides quote the same shortcut: floor area divided by the tubing spacing, times 1.1 for good measure. It is quick and easy to remember. The trouble is that it leaves out two things that decide whether a loop fits the limit at all, and it is wrong in two opposite directions at the same time.
In short
Building or remodeling and comparing radiant floor heat bids? You do not need to lay out the loops yourself. It does help to know what the numbers in a design or a quote mean.
- Loop length is all the tubing in one loop. That is the PEX in the room plus the leaders: the supply and return lines between the room and the manifold. The area formula leaves the leaders out.
- Tubing spacing comes from a calculation, not from floor area. It depends on how much heat the room needs, the floor covering and the supply water temperature.
- The maximum loop length comes from your tubing manufacturer. It depends on the tubing size and the brand. The tables below assume 100 m (about 330 ft) per loop as a stand-in; use your manufacturer's number for your project.
- One loop is not a design. A complete design connects every loop to the manifold and checks that the floor in each room delivers the heat it needs.
If you are still collecting plans and details, start with the radiant floor heating quote checklist. If you already have a design, see the radiant floor heating design guide. The rest of this article is more technical: where the length limit comes from, and how far off the popular formula is.
Where the loop length limit comes from
The limit is not a convention, it is a consequence of friction. Water flowing through tubing loses pressure, and the loss grows with length. A loop that is too long means three things: the circulator has to push against more resistance, the temperature drop between supply and return grows, and balancing that loop against the others on the same manifold gets hard or impossible.
That is why tubing manufacturers publish a maximum loop length, usually a different one for each tubing size. Check it in your own tubing's data. The figures passed around online differ from one another by tens of feet for the same size: a Hearth.com member once asked whether "400ft 1/2" pex 1-loop" could work in a big slab, and the replies pointed back to pressure drop. Whatever figure applies, count the leaders in it, because the water flows through the leaders exactly as it flows through the floor.
What the formula leaves out
The setback from the walls
A loop is not run right up to the wall. HeatAlgo's calculator keeps the tubing 10 cm (3.9 in) from the walls. In the radiant floor module it is the Pipe distance from wall project setting, 100 mm (3.9 in) by default and adjustable per section. Your tubing's installation instructions give the distance they recommend. The area the tubing actually covers is therefore smaller than the room, and the formula counts the whole room. This error overstates the answer.
The leaders to the manifold
A pair of lines runs from the manifold to the room: supply and return. If the manifold is 49.2 ft (15 m) away, that is about 98 ft of PEX that never appears in the loop field, and the formula does not contain it. Some guides add it separately as twice the distance to the manifold, and they are right to. This error understates the answer, and the farther away the manifold, the bigger it gets.
Turns
At the end of every run the tubing turns back, and it has to reach the leaders. Those stretches add a few percent to the area divided by the spacing. The bend itself does not lengthen the tubing: an arc is shorter than a square corner. This is the smallest of the three corrections. Guides describe the 1.1 multiplier as a margin for bends, fittings and offcuts. In the table below it adds 10%, more than the turns add.
What the loop length includes
In this article, in the calculator and in HeatAlgo's module, loop length means all the tubing in one loop: the tubing in the loop field plus the leaders to the manifold and back. That total is what gets compared with the manufacturer's limit. It is not the tubing for the whole house: with several loops, each one has its own leaders. For the whole-house order, see how much PEX for radiant floor heating. Nor does the length tell you whether the spacing is right or whether the floor will heat the room. That is checked separately, by comparing the loop's output with the room's heat loss.
How far off it is, calculated
A room 10.5 ft (3.2 m) wide and 13.1 ft (4.0 m) long, so 137.8 sq ft (12.8 m²). The manifold is 49.2 ft (15 m) away, and the leaders run from it in a straight line to the middle of the longer, 13.1 ft wall. The order of the dimensions matters: entered the other way round they give a different loop and different numbers. The "formula" column is (room area ÷ spacing) × 1.1; the "total tubing" column is the result from real loop geometry including the leaders.
HeatAlgo's geometry engine calculated the table on 23 September 2026 in metric: a 10 cm (3.9 in) setback from the walls, bends of 50 to 80 mm (2.0 to 3.1 in) radius depending on spacing, and straight leaders. Spacings are the metric ones, with the nearest common US spacing in brackets. It is a calculation, not a site measurement.
| Spacing on center | Formula | In the room | Leaders | Total tubing |
|---|---|---|---|---|
| 10 cm (3.94 in, about 4 in) | 461.9 ft | 377.3 ft | 97.4 ft | 475.1 ft |
| 15 cm (5.91 in, the usual 6 in) | 308.1 ft | 252.6 ft | 97.1 ft | 349.7 ft |
| 20 cm (7.87 in, about 8 in) | 231.0 ft | 195.2 ft | 97.1 ft | 292.3 ft |
| 25 cm (9.84 in, about 10 in) | 184.7 ft | 153.5 ft | 97.4 ft | 251.0 ft |
| 30 cm (11.81 in, about 12 in) | 153.9 ft | 128.6 ft | 97.4 ft | 226.0 ft |
At 4 and 6 in on center a single loop is over the 330 ft limit, so this room needs two loops. The "total tubing" column still shows it as one unsplit loop. After the split each loop has its own leaders, so the room takes more tubing in total.
There is a pattern here. In every row the formula overstates the tubing inside the room: by 84.6 ft at 4 in, by 25.3 ft at 12 in. And in every row it understates the total, because the leaders are missing from it. The two errors point in opposite directions, so they partly cancel: at tight spacing the wall-setback error nearly makes up for the missing leaders and the formula looks about right, while at wide spacing the leaders dominate and the formula is clearly too low.
The row that matters most is 6 in on center. The formula gives 308.1 ft; the real loop is 349.7 ft. The formula lands 41.7 ft short. Against a 330 ft limit the formula says "one loop is enough" and the correct answer is two. Against any other manufacturer's limit the same error can put you on the wrong side of it just as easily, because what is missing is the entire set of leaders, and here they are 97.1 ft.
The difference is not academic: it is a decision about the number of loops, made on site from a number that leaves out the distance to the manifold.
Lay out your own roomHow to choose the tubing spacing
Spacing is not a matter of taste, nor of floor area alone. It follows from how much heat the floor has to deliver into the room. Tubing laid tighter gives more Btu/h per square foot at the same supply water temperature, and wider spacing needs hotter water to deliver the same output.
Hence the practical order:
- Start from the room's heat loss. Without it, choosing a spacing is guesswork. That means a room-by-room heat loss calculation, not a Btu-per-square-foot rule of thumb. In the US, the contractor's room-by-room load calculation is the number to design from.
- Check the floor can carry that output at the intended supply water temperature and with the chosen floor covering. Tile conducts heat far better than wood or carpet.
- Consider a tighter spacing in an edge zone, along exterior walls and under large windows where losses are highest. In HeatAlgo an edge zone is part of the same loop, laid tighter.
- Check the length only now, because only now do you know the spacing.
In the radiant floor module, Apply the recommended spacing picks the widest spacing from 200 mm (7.87 in) down to 100 mm (3.94 in) at which the floor covers the room's heat loss.
The lower the supply water temperature, the more a tighter spacing pays off, and that is the direct link to a heat pump, which runs more efficiently when it can make cooler water: delivering the same output at 30 °C (86 °F) rather than 40 °C (104 °F) asks less of it. How water temperature, floor covering and spacing together set a floor's output is explained in the guide to radiant floor heating water temperature.
Staple-up and joist bays
A homeowner on HeatingHelp asked whether 1/2-inch PEX at 16 in on center, one run per joist bay, was standard. That is a staple-up layout under the subfloor, and it is a different system: the tubing heats the subfloor from below instead of sitting in concrete. HeatAlgo does not calculate staple-up, joist-bay, heat transfer plate or panel systems. Its floor output, and every spacing recommendation in this guide, is for tubing embedded in a slab or a thin slab. The loop length geometry still applies to any layout.
When to split into two loops
Usually for one of two reasons.
Length. One loop including the leaders exceeds the manufacturer's maximum. Split the field into two sections of similar area, because two loops of very different length are hard to balance on one manifold. Each section is then a separate loop with its own supply and return to the manifold: splitting divides the tubing in the room, never the leaders. So two loops need more tubing in total than one. Each has to fit the limit on its own, and long leaders can call for a third loop or more. If the leaders alone come close to the limit, splitting stops being the answer: move the manifold closer, or add a second one.
Shape. L and T shapes, narrow necks, alcoves and obstacles can make a single loop impossible to route without crossing tubing or leaving a patch uncovered. That is not a question of feet, it is geometry. The honest answer there is "split", not "draw something that cannot be laid".
HeatAlgo also suggests a split when the flow in a loop is above 1.5 l/min (0.40 gpm). That is the software's advisory threshold; whether to split is a decision for the design of the chosen system. What each loop should read on the manifold is in radiant floor manifold flow meter settings.
How many loops per room - six examples
The same calculation for six typical rooms. Assumptions: the manifold 26.2 ft (8 m) from the room, tubing kept 3.9 in from the walls, and a limit of about 330 ft (100 m) per loop standing in for the tubing manufacturer's figure. "One loop" is all the tubing if the room had a single loop, leaders included. Where it exceeds 330 ft the room gets more loops, each with its own leaders.
| Room | Size | One loop at 6 in, with leaders | Loops at 6 in | Loops at 8 in |
|---|---|---|---|---|
| 129 sq ft (12 m²) | 9.8 × 13.1 ft | 292.0 ft | 1 | 1 |
| 172 sq ft (16 m²) | 13.1 × 13.1 ft | 375.3 ft | 2 | 1 |
| 215 sq ft (20 m²) | 13.1 × 16.4 ft | 457.3 ft | 2 | 2 |
| 269 sq ft (25 m²) | 16.4 × 16.4 ft | 564.6 ft | 2 | 2 |
| 323 sq ft (30 m²) | 16.4 × 19.7 ft | 666.3 ft | 3 | 2 |
| 431 sq ft (40 m²) | 16.4 × 26.2 ft | 869.7 ft | 3 | 3 |
"6 in" here is 15 cm (5.91 in) and "8 in" is 20 cm (7.87 in). So a 323 sq ft living room takes three loops at 6 in and two at 8 in. The spacing still follows from the output the room needs, not from the loop count: a wider spacing gives less heat at the same water temperature. With a longer manufacturer's limit some of these rooms drop a loop; with a shorter one they gain one.
The check at the end
Loop length is not a result in itself. The result is the answer to whether this floor covers the room's heat loss. So the last step is comparing the loop's output against the room's heat loss, and if it falls short, you go back to the spacing or the supply water temperature before anything reaches the site.
That is also why the calculator handles one rectangular room and nothing more. A real project is several rooms on one manifold, each with its own heat loss and its own spacing, plus the balance of the whole. L and T shapes, sloped ceilings, obstacles and multiple floors are handled in the radiant floor heating module.
These figures come from the same engine that calculated HeatAlgo's reference house, a 102.2 m² (1,100 ft²) house calculated for a Warsaw design day of -20 °C (-4 °F), with every input published for checking.
Start your layout freeA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.