What a radiant floor layout has to satisfy at once
A radiant floor design is four constraints solved together, and the tubing length is only the first of them. You need enough tube in the floor to spread the heat evenly, few enough feet in each loop that the pump can push water through it, enough flow per loop to keep the water from cooling too far between the start and the end, and a floor surface temperature that stays comfortable under bare feet.
Tubing quantity is pure geometry. Lay tube on 12-inch centres and every square foot of floor gets one linear foot of tube; lay it on 6-inch centres and every square foot gets two. The factor is 12 divided by the spacing in inches, and nothing else about the design changes it.
Loop count is set by pressure drop, expressed as a maximum length. Manufacturers publish a maximum loop length for each tube size because head loss climbs with length and a circulator has to move design flow through the longest loop on the manifold. Typical published maximums are around 250 ft for 3/8-inch PEX, 300 ft for 1/2-inch, 400 ft for 5/8-inch and 500 ft for 3/4-inch — check the figure for your own tubing, because it depends on wall thickness and on the design flow assumed.
The leaders count, and they are easy to forget. Every loop makes a round trip from the manifold to the zone, so a manifold 20 ft away consumes 40 ft of every loop before a single foot of floor is heated. That is 13% of a 300 ft loop gone, and it is the reason a zone that seems to need two loops on paper sometimes needs three on site.
Working the four constraints in order
Start with the field tubing. Multiply the heated area by 12 divided by the spacing. A 400 ft² room at 12-inch spacing needs 400 ft of tube in the floor; the same room at 8-inch spacing needs 600 ft.
Then find the loop count, which has to be solved rather than divided. Adding a loop adds another pair of leaders, so the total length grows with the loop count and a single division can undershoot. The rule is to take the smallest whole number of loops n for which (field tubing + 2 × n × leader) divided by n still fits under the maximum. For 800 ft of field tube with 20 ft leaders and a 300 ft maximum, three loops would average (800 + 120) ÷ 3 = 306.7 ft and fail; four loops give (800 + 160) ÷ 4 = 240 ft and pass.
Flow comes from the same sensible-heat equation as any other hydronic circuit. Zone flow is the load divided by 499.8 times the design temperature drop, and each loop carries an equal share when the loops are equal length — which is exactly why designers keep loops within about 10% of each other in length. The hydronic GPM calculator covers that equation, including the correction when the circuit is filled with glycol rather than water.
Finally, work backwards from the room to the water. The floor has to deliver Q ÷ A BTU per hour per square foot. Radiant practice approximates the output of a heated floor as about 2 BTU/h·ft² for every °F the surface sits above the room, combining radiation and natural convection into one coefficient, so the required surface temperature is the room temperature plus half the required output. Push heat up through the covering and the pour above the tube — a resistance of R — and the water has to be q × R warmer still. That gives the average water temperature; the supply is half the design drop above it.
Worked example: a 400 ft² room losing 12,000 BTU/h
A 400 ft² living room with a design heat loss of 12,000 BTU/h, tube on 12-inch centres in a slab, manifold 20 ft away, 1/2-inch PEX at a 300 ft maximum loop, 20 °F design drop, room held at 68 °F, and a finished floor plus pour above the tube worth R-1.0.
- Tube per square foot. 12 ÷ 12 = 1.000 ft of tube per ft².
- Field tubing. 400 × 1.000 = 400 ft.
- Loop count. One loop would be 400 + 2 × 20 = 440 ft, over the 300 ft limit. Two loops give 400 + 2 × 2 × 20 = 480 ft total, or 240 ft each, which fits. So 2 loops of 240 ft, and 480 ft of tubing to buy before waste allowance.
- Zone flow. 12,000 ÷ (499.8 × 20) = 1.2005 GPM, so each loop carries 1.2005 ÷ 2 = 0.60 GPM. That sits comfortably in the 0.5–0.8 GPM range where 1/2-inch loops are usually balanced.
- Output per square foot. 12,000 ÷ 400 = 30 BTU/h·ft².
- Floor surface temperature. 68 + 30 ÷ 2 = 83 °F. That is under the 84 °F ASHRAE Standard 55 limit for occupied floors, but only just — there is one degree of margin in this design.
- Average water temperature. 83 + 30 × 1.0 = 113 °F.
- Supply water temperature. 113 + 20 ÷ 2 = 123 °F, with the return at 103 °F. Both are low enough to keep a condensing boiler in condensing mode all season.
Now change one thing: put carpet with a pad over the slab, worth about R-2.0 instead of R-1.0. The surface temperature does not move, because the room still needs 30 BTU/h·ft². But the average water temperature becomes 83 + 30 × 2.0 = 143 °F and the supply becomes 153 °F. The same floor, the same tubing and the same flow now demand a boiler running 30 °F hotter and losing condensing operation. Floor covering is the single biggest lever in a radiant design, and it is usually chosen by someone who is not in the room when the heat loss is calculated.
Reading the results and knowing when the design fails
Check the floor surface temperature first, because it is a hard ceiling. ASHRAE Standard 55 puts the comfortable range for floor surfaces in occupied spaces at 66 to 84 °F for people with normal footwear. At the 2 BTU/h·ft²·°F approximation, a room at 68 °F cannot get more than about 32 BTU/h·ft² out of its floor without breaching that. If your zone needs more, no amount of tubing will fix it — you reduce the heat loss, or you add another emitter. Start with the envelope, using the envelope heat loss calculator.
Then look at flow per loop. Below roughly 0.3 GPM a loop is hard to balance and the water can cool noticeably between the start and the end of the run, leaving a visible warm patch near the manifold end. Above roughly 1.5 GPM a 1/2-inch loop is running fast enough that head loss and noise start to matter. Both problems have the same two fixes: change the loop count, or change the design temperature drop, which moves flow in exact inverse proportion.
Then look at supply water temperature. Anything at or below about 120 °F keeps a condensing boiler in its best efficiency range and lets a heat pump work at reasonable capacity. Once you are pushing past 140 °F, the design has usually gone wrong upstream — a high-R floor covering, a heat loss that is too large for the floor area available, or spacing so wide that the surface cannot deliver evenly.
Finally, sanity-check the loop lengths against each other. Loops on the same manifold should be within about 10% of each other in length, because equal lengths at equal valve settings carry roughly equal flow. A manifold with a 120 ft loop beside a 280 ft loop will starve the long one unless every circuit is individually balanced with a flow meter.
Tubing quantity and coverage by tube spacing
| Spacing on centre | Tube per ft² of floor | Tube per 100 ft² | Field area per 300 ft loop | Where it is normally used |
|---|---|---|---|---|
| 6 in | 2.00 ft | 200 ft | 150 ft² | Bathrooms, entries, high-loss perimeter bands |
| 8 in | 1.50 ft | 150 ft | 200 ft² | Rooms with large glazing areas |
| 9 in | 1.33 ft | 133 ft | 225 ft² | General-purpose slab spacing in cold climates |
| 12 in | 1.00 ft | 100 ft | 300 ft² | The usual default for slabs in ordinary rooms |
| 16 in | 0.75 ft | 75 ft | 400 ft² | Low-loss spaces, garages, joist-bay staple-up |
| 18 in | 0.67 ft | 67 ft | 450 ft² | Snow melt aprons and outbuildings only |
Wider spacing buys less tubing but produces a larger temperature swing across the floor surface between a tube run and the midpoint between two runs, which is felt most through tile and least through carpet.
Where the 2 BTU/h·ft²·°F floor coefficient comes from
The output of a heated floor is the sum of thermal radiation to the surrounding surfaces and natural convection to the room air, and both terms are non-linear in the surface-to-room temperature difference. Radiant design practice collapses them into a single combined coefficient of roughly 2 BTU/h·ft² per °F, which is accurate enough for layout work over the 75–85 °F surface range that comfort limits confine you to. This calculator uses that approximation, and you should treat the surface and water temperatures it returns as design guidance rather than as a performance guarantee. For a final submittal, run the panel output correlations in the ASHRAE Handbook—HVAC Systems and Equipment, or use the tubing manufacturer's own output tables, which are indexed by tube spacing, floor construction and covering R-value and account for the fin effect of the slab.
Mistakes that show up after the concrete is poured
- Forgetting the leaders. A manifold 30 ft from the zone eats 60 ft of every loop. On a 300 ft maximum that is a fifth of the loop gone before it heats anything.
- Ordering the exact calculated length. Buy coils, not feet. PEX comes in fixed coil lengths, a loop must be one unbroken piece from manifold to manifold, and an offcut of 60 ft is not a loop. Order whole coils and plan which loops come out of which.
- Tubing the whole floor area including under cabinets. Heat put under a fixed cabinet or a built-in never reaches the room usefully. Subtract those areas, then recheck that the remaining area can still carry the load within the surface-temperature limit.
- Choosing the floor covering after the design. R-2.0 carpet instead of R-0.05 tile raises required water temperature by q × 1.95 — nearly 60 °F at 30 BTU/h·ft². Fix the covering before sizing the heat source.
- Mixing loop lengths on one manifold. Equal valve settings give equal flow only for equal lengths. Keep loops within about 10% of each other, or specify balancing valves with flow meters and actually set them.
- Designing without a room-by-room heat loss. A watts-per-square-foot guess makes every downstream number decorative. The load drives flow, surface temperature and water temperature alike.
- Ignoring downward loss. This calculator sizes upward output. A slab on grade needs edge and under-slab insulation, or a substantial fraction of the heat you paid for leaves through the ground rather than the room.
Where this sits in the rest of the system design
Loop layout is the middle of a radiant design, not the start. The order that works is: calculate the room-by-room heat loss, decide the floor construction and covering, lay out the tubing here, check the surface temperature against comfort limits, then size the flow, the mixing arrangement and the heat source.
The mixing arrangement matters because a radiant floor wants much cooler water than a boiler is happy making. A 123 °F supply from a non-condensing boiler needs a mixing valve or an injection circuit to protect the boiler from sustained flue-gas condensation, whereas a modulating condensing boiler can simply be set to the design supply temperature and reset it against outdoor temperature. Either way, take the flow from this calculator to the hydronic pump head calculator to select the circulator against the longest loop's pressure drop, and to the boiler size calculator for the heat source.
Radiant floors are not always the right emitter, and the surface-temperature ceiling is the reason. A room that needs more than about 32 BTU/h·ft² from its floor cannot be heated by the floor alone at 68 °F. Options in order of preference: reduce the heat loss, add radiant to a wall or ceiling, or supplement with a second emitter such as fin-tube — see the baseboard heater length calculator for sizing that. Panel radiators on the same low-temperature water are another good fit, since they can be sized for 120 °F supply if you accept a larger panel.
Two limits of this calculation are worth stating. It sizes upward output only and does not model downward or edge losses, which insulation under and around the slab controls; and it assumes tubing is laid at a uniform spacing across the whole zone. Real layouts often tighten the spacing to 6 inches in a perimeter band along exterior walls and open up to 12 inches in the interior; to model that, run the band and the field as two separate zones and add the tubing quantities. Assemblies and R-values feed in from the R-value to U-value calculator.