Short answer: HeatAlgo's reference house, a 102.2 m² (1,100 ft²) house calculated for a Warsaw design day of -20 °C (-4 °F), with radiant floor heat in every room at 15 cm (5.9 in, the usual 6-inch spacing), has 2,619 ft of 16 mm (5/8 in OD, the size of 1/2-inch PEX) tubing in the floor in 11 loops. That is 2.38 ft per sq ft, not the rule's 2.04. The materials list orders 2,834 ft (2.59 ft per sq ft), and because PEX comes in fixed-length coils, the purchase rounds up from there.
"How much PEX do I need?" is a thread title that keeps coming back, from a Garage Journal member with a 1,000 sq ft garage to whole-house builds. The usual answer is area divided by spacing, maybe plus 20%. That counts the loop field only, and overstates even that. It leaves out the water's route from the manifold to the room and back, once for every loop.
From the rule to the order
| Step | Feet of PEX | What it counts |
|---|---|---|
| Rule: area ÷ spacing | 2,235 ft | the floor area only |
| Laid in the floor | 2,619 ft | 2,020 ft in the rooms + 563 ft of leaders + 36.1 ft of reserve |
| Materials list | 2,834 ft | adds 215.6 ft so no room comes out short |
| Bought | whole coils | the order rounded up to fixed coil lengths |
The reference house has eight rooms but 11 loops: the 301 sq ft living room is split into three and the bedroom into two, because one loop would be too long. Each loop has its own leaders, 51.2 ft there and back with the manifold 26.2 ft from the rooms, so the house pays for leaders eleven times, not eight. That is 563 ft the rule never sees.
The tubing in the loop field alone is 1.71 to 1.86 ft per sq ft in this house, less than the rule, because the tubing does not run right against the walls. Why is in the guide to radiant floor tubing spacing and loop length.
Why the list orders more than is laid
HeatAlgo's materials list works out each loop so that it never orders short. It takes the larger of two values: the drawn loop length, or the area divided by the spacing. Then it adds the leaders and 3.3 ft of reserve. In this house area ÷ spacing wins in every room, so the list adds 215.6 ft on top of what was drawn. That is a margin for cutting and on-site changes.
| Room | Loops | Drawn | Area ÷ spacing | Leaders and reserve |
|---|---|---|---|---|
| Living room, 301 sq ft | 3 | 560.4 ft | 612.5 ft | 163.4 ft |
| Kitchen, 108 sq ft | 1 | 192.6 ft | 218.8 ft | 54.5 ft |
| Bedroom, 173 sq ft | 2 | 316.6 ft | 352.0 ft | 108.9 ft |
| Room 1, 128 sq ft | 1 | 237.5 ft | 260.2 ft | 54.5 ft |
| Room 2, 128 sq ft | 1 | 237.5 ft | 260.2 ft | 54.5 ft |
| Bathroom, 87 sq ft | 1 | 160.4 ft | 177.2 ft | 54.5 ft |
| Hall, 110 sq ft | 1 | 204.4 ft | 223.1 ft | 54.5 ft |
| Mechanical room, 65 sq ft | 1 | 110.2 ft | 131.2 ft | 54.5 ft |
| Total, 1,100 sq ft | 11 | 2,019.7 ft | 2,235.2 ft | 599.1 ft |
The order is the "Area ÷ spacing" column plus "Leaders and reserve": 2,235.2 + 599.1 = 2,834 ft. Calculated in metric (6 in is 15 cm) and converted.
Why you buy more than 2,834 ft
Loops in a slab are never spliced from pieces: each one runs as a single length from the manifold and back. So PEX is bought in coils, and the loops are allocated so each fits inside one. Coils come in fixed lengths, so the purchase rounds up. In the metric example HeatAlgo chose a 600 m coil and two 200 m coils, 1,000 m (3,281 ft) in all, with seven loops in the big coil and two in each small one. Those are European coil sizes; your supplier's will differ.
That is the list to take to the supply house: the length of every loop, so you can ask for coils that fit them rather than "the usual amount".
How to work out your own house
The figures above are for HeatAlgo's reference house, a European house, not a US one. The tubing quantities come from loop geometry, so they apply to any floor plan; the floor output HeatAlgo calculates is for tubing embedded in a slab or thin slab. Staple-up, joist-bay and heat transfer plate systems are not calculated.
For one room, the radiant floor heating loop calculator gives the tubing in the room, the leaders and the number of loops. For a whole house, HeatAlgo builds the materials list from your radiant floor design: tubing, coils, manifolds and insulation.
Count the PEX for your houseA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.