Layout Breakdown
Full Calculation Table
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How to Use the Radiant Floor Pipe Calculator
Enter the floor area and the target heat output per square metre, pulled from a room-by-room heat loss calculation in your load calculation. Higher output requires tighter tube spacing.
1/2 inch PEX is the standard size for most residential radiant floors. Tighter spacing increases output per square metre but uses more tube and adds more loops per manifold. Compare PEX against other materials on the pipe material comparison tool if you have not settled on tubing yet.
Enter the flow rate you plan to run through each circuit, typically set by the manifold's balancing valves. Use the GPM calculator if you are starting from a heat output number instead.
Click Calculate Loop Layout to see total tube length, the number of circuits needed, and whether velocity and circuit length fall within typical manufacturer limits.
Laying Out PEX Loops for Radiant Floor Heating
Radiant floor design comes down to fitting enough PEX tube into a room at the right spacing to deliver the target heat output, then splitting that tube into circuits short enough to keep flow even and pressure drop manageable. Unlike baseboard or fan coil systems, where the pipe sizing calculator picks one size to move a flow rate efficiently, radiant floor tubing size is fairly standardized at 1/2 inch, and the real design variable is spacing and circuit length. The target heat output itself should come from an actual room-by-room heat loss calculation rather than a rule of thumb; the heat loss slab calculator is a reasonable starting point for a slab-on-grade room specifically.
Tube Spacing Sets Heat Output
Tighter spacing puts more tube into the same floor area, which raises heat output per square metre and improves floor surface temperature uniformity, since the warm tube runs are closer together. Looser spacing covers the same area with less tube and lower cost, but delivers less output and can create slightly cooler strips of floor between runs if spacing gets too wide relative to the floor covering's heat spreading ability. Bathrooms and other high heat-loss rooms often get tighter spacing for this reason, while a low heat-loss room might use wider spacing without issue.
Why Circuit Length Has a Limit
A single PEX circuit that gets too long develops two compounding problems: pressure drop climbs because friction loss is roughly proportional to length, and the supply water cools as it travels the loop, so the far end of a long circuit delivers noticeably less heat than the near end. Most manufacturers cap a single 1/2 inch circuit around 100 to 120 metres for this reason, a limit consistent with the circuit length table in a real Canadian engineering specification, Metrolinx's technical master specification for hydronic radiant floor heating, which spells out matching pipe size and circuit length limits for transit facility projects. A large room or open floor plan that exceeds this length needs to be split into two or more shorter circuits fed in parallel from a manifold rather than one continuous loop, similar in spirit to how a hydronic distribution system splits into branches off a main rather than running one long pipe to every terminal unit.
Checking Velocity in a Radiant Loop
Radiant floor circuits run at relatively low flow rates compared to baseboard or fan coil loops, since the goal is even heat distribution rather than moving large amounts of heat quickly through one pipe. This calculator checks velocity using the same flow and area relationship as the flow velocity calculator, applied to the much smaller PEX tube sizes typical of radiant work. Radiant floors also pair naturally with lower-temperature heat sources than baseboard systems need, which is worth keeping in mind if the heat source being considered is a solar thermal collector loop or a heat pump rather than a conventional boiler.
A Note on Accuracy
This calculator estimates tube length and circuit count from area, spacing, and a simplified layout pattern. Actual radiant floor design should account for the specific room shape, fixed obstructions, and the manufacturer's published heat output curves for the floor covering involved, since carpet, tile, and exposed slab all transfer heat differently at the same water temperature. For permit-stamped mechanical drawings, confirm the design against the current CSA B214 installation code for hydronic heating systems, registered with the Standards Council of Canada.
Frequently Asked Questions
Most residential radiant floor designs use 150mm (6 inch) or 230mm (9 inch) spacing between PEX tubes. Tighter 150mm spacing delivers more heat per square metre and produces a more even floor surface temperature, which matters most in rooms with high heat loss like bathrooms with exterior walls. Wider 230mm spacing covers more area per length of tube and costs less, and works fine in rooms with lower heat loss or where floor temperature uniformity matters less.
Most manufacturers recommend keeping a single PEX circuit under 100 to 120 metres (300 to 400 feet) for 1/2 inch tubing, since longer circuits develop too much pressure drop and the temperature drops too much from the start of the loop to the end, creating an uneven floor surface temperature. Longer circuits should be split into multiple shorter circuits fed from a manifold instead of one long continuous loop.
Radiant loops generally run lower velocity than baseboard or fan coil branches, since the priority is even heat delivery across the loop length rather than moving a large amount of heat quickly. There is no strict minimum velocity requirement specific to radiant tubing, but extremely low flow can worsen the temperature drop along a long circuit. Check the result against the flow velocity calculator if you want a direct comparison to standard hydronic guidance.
Circuit count depends on total tube length divided by your maximum circuit length limit, rounded up. A room needing 150 metres of tube with a 100 metre circuit limit needs at least two circuits. Manifolds are sized by the number of circuit ports, so total circuit count across all rooms on one manifold determines the manifold size needed.
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