Calculator
Radiant Floor Loop Calculator
A radiant loop that runs longer than its tube size's maximum does not trip an alarm, leak, or fail an inspection on day one. It just quietly starves — friction loss along the extra length of tube eats into the flow a residential circulator can push, so the far end of the loop gets a trickle of hot water instead of a proper flow, and the last few feet of that room never come up to temperature. By the time anyone notices, it is a cold patch in finished flooring with no visible cause, and the actual cause is a number that was wrong before the gypcrete or tile ever went down.
This calculator does not stop at total tube footage. It splits the floor area into loops that respect the real maximum length for the tube size you chose, counting the leader run's full round trip to the manifold and back against that limit, and reports how many loops you need and how close each one runs to the ceiling — the check that has to happen before the floor is covered, not after a room stays cold and nobody can explain why.
Calculate your quantity
BTU/hr per sq ft
Tubing needed
720 ft
Loops
3 loops
- Tube per square foot
- 1 ft/sq ft
- Tube in the floor
- 600 ft
- Maximum loop length
- 300 ft
- Average loop length
- 240 ft
- Manifold ports
- 3 ports
- Design heat output
- 15,000 BTU/hr
- Exceeding the 300 ft maximum for 0.5 in tube does not fail loudly — it just starves the far end of the loop, so the far side of the room never warms up and nobody can find the reason.
Shopping summary
- • 3 x 300 ft rolls of 0.5 in PEX
- • A 3-port manifold with balancing valves
This is an estimate — confirm structural work with a professional.
How this calculation works
Feet of tube per square foot is simply 12 divided by the spacing in inches: 12 inch spacing is one foot of tube per square foot of floor, 6 inch spacing is two feet, because at half the spacing you fit twice as many passes across the same area. Multiply by the room's area and you have the field tube — the length actually laid in the floor pattern, before the manifold connection is added.
Each tube size has its own real maximum loop length — 3/8 inch tube tops out around 200 feet, 1/2 inch around 300, 5/8 inch around 400 — because a larger inside diameter lets a given flow travel farther before friction loss becomes too much for a typical circulator to overcome. The leader run to the manifold counts against that limit twice, once out and once back, so a distant manifold eats into the usable budget before any field tube is laid at all. Loop count is the field tube divided by what's left of the maximum after the leader, rounded up — because a fraction of a loop over what whole loops can carry needs one more loop, not a single loop stretched past its limit.
- Exceeding the maximum does not fail loudly. It shows up as a room that never quite reaches temperature at one end, discovered only after the floor covering is already down.
- This page uses one flat design-output figure in BTU/hr per square foot for the whole area entered. A real house needs that figure set room by room from an actual heat-loss calculation, not one number applied everywhere.
- Tighter spacing is not free extra output — halving spacing roughly doubles both the tube used and, often, the loop count, which changes manifold size and pump head, not just the material list.
- Manifold, circulator and boiler sizing for the whole system is separate, professional work this page does not do. It sizes tube and loops for one zone at a time.
The formula
tubePerSqft = 12 / spacing; fieldTube = area x tubePerSqft; usablePerLoop = maxLoop - leaderLength x 2; loops = ceil(fieldTube / usablePerLoop); totalTube = fieldTube + loops x leaderLength x 2
- tubePerSqft (12 / spacing)
- Derived on this page from the spacing you set, not looked up: at 12 inch spacing one linear foot of tube covers one square foot of floor; tighten the spacing and this rate rises directly.
- maxLoop
- The longest a single loop of the chosen tube size can run before friction loss outpaces what a typical residential circulator can push — 200, 300 or 400 feet depending on whether the tube is 3/8, 1/2 or 5/8 inch.
- leaderLength x 2
- The supply-and-return run from the loop out to the manifold, counted twice because the leader travels the distance in both directions and both directions consume the loop's length budget.
- loops = ceil(...)
- Field tube divided by what remains of the maximum loop length after the leader is subtracted, rounded UP — a remainder of even one foot over what whole loops can carry forces another full loop and another manifold port.
- totalTube
- Field tube plus every loop's own leader run added back in — the actual length to buy, which always runs longer than the field tube alone once leaders are counted.
Where these numbers come from
- 200 / 300 / 400 ft maximum loop length (3/8, 1/2, 5/8 in tube)
- Manufacturer-published: figures commonly published across PEX and hydronic system design guides for typical residential flow rates and circulator capacity. Exact maximums vary a little by manufacturer, fitting losses and the specific pump chosen — treat these as the widely used planning figures, not a single universal spec.
- 12 / spacing (tube per square foot)
- Derived arithmetic, not a looked-up figure: it follows directly from laying tube at a fixed spacing across a flat area, and it is exact rather than approximate for any spacing you enter.
- 20-40 BTU/hr per sq ft design output range
- Trade convention from hydronic design guides: a typical range for radiant floor output depending on flooring type, subfloor insulation, and how close the water temperature runs to the floor's practical maximum. 25 BTU/hr/sqft here is a common mid-range default, not a room-specific calculation.
- 300 ft PEX roll length
- Manufacturer-published: a common stock roll length PEX tubing is sold in, used here to convert total tube footage into a purchasable roll count.
Worked examples
A small bathroom zone at tight spacing with a distant manifold
| Floor area | 80 sq ft |
|---|---|
| Tube spacing | 6 in (high output, perimeter) |
| Tube size | 3/8 in (200 ft max loop) |
| Leader run to the manifold | 15 ft |
| Design output | 30 |
| Tubing needed | 190 ft |
|---|---|
| Loops | 1 loops |
| Tube per square foot | 2 ft/sq ft |
| Tube in the floor | 160 ft |
| Maximum loop length | 200 ft |
| Average loop length | 190 ft |
| Manifold ports | 1 ports |
| Design heat output | 2400 BTU/hr |
80 square feet at 6 inch spacing is only 160 feet of field tube — small enough that it looks like it could never come close to a limit. But the leader run to the manifold, 15 feet each way, subtracts 30 feet from the 200 foot maximum before the field tube is even considered, leaving a 170 foot budget. One loop still fits, at 190 feet total including the leader — but that is 95% of the 200 foot ceiling for 3/8 inch tube, with almost no margin left.
This is the case worth flagging before it is covered in tile: a small room can still run close to its tube size's limit if the manifold sits far away, and the leader run is exactly the part of the number a quick mental estimate skips.
A large open room split across multiple loops
| Floor area | 900 sq ft |
|---|---|
| Tube spacing | 9 in |
| Tube size | 1/2 in (300 ft max loop) |
| Leader run to the manifold | 25 ft |
| Design output | 25 |
| Tubing needed | 1450 ft |
|---|---|
| Loops | 5 loops |
| Tube per square foot | 1.33 ft/sq ft |
| Tube in the floor | 1200 ft |
| Maximum loop length | 300 ft |
| Average loop length | 290 ft |
| Manifold ports | 5 ports |
| Design heat output | 22500 BTU/hr |
1,200 feet of field tube divided by a 250 foot usable budget per loop (300 foot maximum for 1/2 inch tube, minus a 50 foot leader round trip) comes out to 4.8 — which rounds up to 5 loops, not 4 with one running slightly over. That fifth loop is the difference between every loop staying under the maximum and one loop quietly exceeding it.
Spread across 5 loops, the average loop length lands at 290 feet — again close to the 300 foot ceiling, and again a reason not to treat 'under the maximum' as a wide margin rather than a hard limit that this floor plan is running right up against. A 5-port manifold is the real purchase this number drives, not a 4-port one squeezed to fit.
Common mistakes
- Estimating loop length from field tube alone and forgetting the leader run's round trip to the manifold — a distant manifold can consume a meaningful share of the maximum before any tube is even laid in the floor.
- Rounding a loop count down to save a manifold port when the field tube total is only slightly over what whole loops can carry. That fractional remainder does not disappear — it becomes the length that pushes one loop past its maximum.
- Treating a loop calculated right at the maximum length as having the same margin as one calculated well under it. A number sitting at the printed ceiling has no room left for real-world fitting losses or a circulator that underperforms its rated head.
- Using one flat BTU/hr per square foot design figure for an entire house instead of a room-by-room heat-loss number. A bathroom over a slab and a great room with large windows do not lose heat at the same rate, even with identical flooring.
- Assuming tighter spacing is a free upgrade. Halving spacing roughly doubles tube usage and can push a zone from one loop to two or more, which changes manifold size and pump requirements, not just how much PEX to buy.
- Switching tube size mid-project without re-checking maximum loop length. 3/8, 1/2 and 5/8 inch tube each carry a different limit, and a loop plan that was safely under the maximum at one size can silently exceed it if the tube size changes and the loop count isn't re-run.
Shopping summary
Buy PEX in whole rolls to the total tube figure this page produces, plus a manifold with at least as many ports as the loop count — never fewer, since a loop count this page reports has already accounted for the maximum length constraint, and squeezing two loops onto one port defeats that math.
Replace the flat design-output default with a real per-zone figure from the heat loss calculator before finalizing tube spacing — that page's envelope-based BTU/hr number, applied room by room, is a materially better input than one flat figure guessed for the whole house.
If you are deciding between radiant floor heat and forced air for the same space, the duct size calculator shows what the forced-air alternative actually requires — the two systems solve heat delivery in genuinely different ways and are worth comparing on their own terms rather than assuming one is a drop-in substitute for the other. Subfloor or slab insulation below the tube also belongs in the same budget; check it against the insulation calculator.
FAQ
What actually happens if a loop ends up longer than the maximum for its tube size?
Nothing fails immediately or obviously. Friction loss along the extra length reduces the flow the circulator can push through that loop, so the far end receives less hot water than the near end. The room served by that loop develops a cold zone — often at the end of the loop farthest from the manifold — that is very hard to diagnose after the floor is finished, because nothing about the installation looks wrong.
Why did my loop count come out higher than area divided by maximum loop length would suggest?
Two reasons: the leader run to the manifold counts against the maximum twice, not zero times, which shrinks the usable length per loop below the tube size's printed maximum. And the loop count always rounds up — a field tube total that's even slightly over what whole loops can carry needs a full additional loop, not a single loop pushed past its limit.
Does tighter tube spacing always mean better heating performance?
There's a practical ceiling on what tighter spacing actually buys, tied to the floor covering rather than the tube pattern. Floor surface temperature has its own comfort and material limit — commonly well under 85°F for wood flooring, higher for tile — so once spacing is already close enough to reach that ceiling at your design water temperature, going tighter still just adds tube and cost without raising output any further. A carpeted room with insulating underlayment often hits that ceiling at a wider spacing than an exposed tile floor does, worth checking before assuming the tightest option always performs best.
Can I just use a bigger circulator pump instead of respecting the maximum loop length?
A stronger pump can push flow farther, which is part of why maximum loop lengths differ across tube sizes and system designs in the first place — but the figures on this page assume typical residential circulator capacity. Designing a longer loop around a specific, larger pump is a real option, but it is calculation an engineer or experienced installer does for that specific pump, not something this page's default figures cover.
Should every room use the same design output figure?
No. This page's BTU/hr per square foot input is a single default applied to whatever area you enter, useful for a first pass on one zone. A real design sets that figure per room from an actual heat-loss calculation, because rooms with different window area, exterior wall exposure, or slab-versus-framed-floor construction lose heat at genuinely different rates.
Can I mix tube sizes on the same manifold?
Physically, manifolds can accept different loop lengths and even different tube sizes across their ports, but each loop still has to respect its own tube size's maximum length independently — mixing sizes does not average the limits together. Run this calculation separately for any zone using a different tube size rather than assuming one result applies across a mixed system.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.