Radiant floor heating calculator

How much PEX tubing do I need for radiant floor heat?

Radiant floor tubing length depends on three numbers: the area you are heating, the on-center spacing of the loops, and a small waste factor for bends and the run to the manifold. The formula is straightforward: linear feet per square foot equals 12 divided by your spacing in inches. Tighter spacing puts more PEX under the same floor, which means higher heat output but also more material and labor cost.

• 6 inch spacing: 2.0 linear ft of PEX per sqft (high-load, tile bathrooms)
• 8 inch spacing: 1.5 linear ft per sqft (cold-climate kitchens, main living areas)
• 9 inch spacing: 1.33 linear ft per sqft (typical residential default)
• 12 inch spacing: 1.0 linear ft per sqft (well-insulated rooms, low heat load)

A 400 sqft tile bathroom at 6 inch spacing needs 800 ft of PEX. The same room at 12 inch spacing needs 400 ft. The tighter layout doubles the tubing cost and labor but lets the floor deliver up to 40 BTU/hr per sqft instead of about 20. In cold climates with high heat loss, you usually need the tighter spacing whether you want it or not.

PEX loop length limits: why 300 feet is the magic number

A single radiant loop runs from the manifold, snakes through the floor, and returns to the manifold. The longer the loop, the more pressure drop the circulator pump has to overcome. Past a certain length the water moves so slowly that the end of the loop runs noticeably cooler than the start, producing visible cold strips in the floor surface. Industry maximum loop lengths by PEX size:

• 3/8 inch PEX: 200 ft maximum, used for small bathrooms and narrow runs
• 1/2 inch PEX: 300 ft maximum, the standard residential size for almost every install
• 5/8 inch PEX: 400 ft maximum, used for large open areas and basements

A 1,200 sqft basement at 9 inch spacing needs 1,600 ft of 1/2 inch PEX, which means at least 6 separate loops (1,600 ÷ 300 = 5.3, round up). Six loops require a 12 port manifold, six supply and six return. The calculator above runs this math automatically and rounds to the next standard manifold size. Tip: keep all loops in a manifold roughly equal in length so the flow stays balanced. If one loop is 280 ft and the next is 150 ft, the short loop steals most of the flow.

Tubing spacing by floor type: tile vs hardwood vs carpet

Floor finish controls how much heat actually reaches your feet. Tile and bare concrete transfer radiant heat very efficiently. Engineered hardwood adds an insulation layer. Hardwood and carpet limit how hot the floor surface can run before they get damaged or feel uncomfortable. The combination of finish + spacing determines whether radiant floor can fully heat the room or whether you need supplemental heat.

• Tile floor: up to 40 BTU/hr per sqft delivered, run 6 to 9 inch spacing
• Bare concrete slab: up to 38 BTU/hr per sqft, run 9 to 12 inch spacing
• Engineered hardwood: up to 25 BTU/hr per sqft, run 8 to 9 inch spacing
• Solid hardwood: up to 22 BTU/hr per sqft, run 6 to 8 inch spacing
• Carpet over PEX: up to 18 BTU/hr per sqft, run 6 inch spacing always

Carpet over radiant is the toughest case. The carpet R-value blocks heat transfer, the floor surface temperature cap (about 85°F per ASHRAE comfort standards) leaves you with very little headroom, and unless you tighten spacing to 6 inches you usually cannot meet the room load. For most carpet-over-radiant projects, contractors recommend either switching to a low pile area rug instead of wall-to-wall, or accepting that the radiant is a comfort booster rather than the primary heat source.

Radiant floor heat output: 25 BTU per sqft is the design target

A correctly sized radiant floor system delivers somewhere between 20 and 30 BTU/hr per sqft on a design day, with the comfort sweet spot around 25 BTU/hr per sqft. Above 30 the floor surface gets uncomfortable on bare feet. Below 15 the system is oversized and never reaches full output, which wastes pump energy. To translate your home's heat loss into a floor output target:

• Well-insulated home in Zone 4: typically 15 to 20 BTU/sqft load, easy radiant target
• Average home in Zone 5: 25 to 30 BTU/sqft, fits radiant comfortably
• Older home in Zone 6: 35 to 45 BTU/sqft, radiant alone usually cannot keep up
• Poorly insulated home anywhere: 45+ BTU/sqft, fix insulation before installing radiant

If your load is above what the floor finish + tightest practical spacing can deliver, you have three options. Tighten the building envelope (cheapest long-term), add supplemental baseboard or panel radiators for the shortfall, or accept higher boiler water temperatures that shorten the equipment lifespan and run less efficiently with a heat pump.

Hydronic radiant floor cost per square foot: $7 to $17 installed

The installed cost depends mainly on how the tubing is attached to the structure and whether the floor is new construction or a retrofit. Cross-checked against current installed-quote data:

• New slab embed (in concrete pour): $6 to $12 per sqft
• Thin-slab overpour (1.5 inch gypcrete on existing subfloor): $8 to $16 per sqft
• Staple-up under existing subfloor: $7 to $14 per sqft
• Aluminum plate system under subfloor: $10 to $22 per sqft
• Add boiler, manifold, pump, and controls: $4,000 to $9,000 above the per-sqft number

A typical 2,000 sqft whole-house hydronic radiant retrofit with a thin-slab overpour and a new high-efficiency boiler runs $30,000 to $50,000 fully installed. New construction with in-slab radiant is much cheaper because the tubing goes in during the pour. A single bathroom warm-floor add-on with electric radiant runs $1,200 to $3,500 installed and is usually the right call for one-room comfort projects rather than whole-house hydronic.

Supply water temperature: why low-temp design matters for heat pumps

The supply water temperature your boiler or water heater delivers to the manifold is the most important variable for long-term operating cost. Lower supply temperatures are more efficient because the boiler returns to condensing mode (95+ percent AFUE), and they let you pair the radiant loop with an air-to-water heat pump (Sanco2, Arctic Heat Pump, Chiltrix) that cannot deliver high water temperatures.

• Under 100°F supply: heat pump compatible, very efficient, low output per sqft
• 100 to 115°F supply: heat pump still compatible, typical tile floor at 9 inch spacing
• 115 to 130°F supply: condensing boiler only, hardwood floors usually need this range
• 130 to 140°F supply: non-condensing or carpet retrofits, lower system efficiency

If you might ever electrify with a heat pump, design the radiant system for supply temps under 115°F from day one. That means tile or bare slab finish, tighter 6 to 9 inch spacing, and a building envelope tight enough to keep the heat load below 30 BTU/sqft. The calculator flags the recommended supply temperature so you can see whether your design works with the boiler or heat pump you have in mind.

Manifold sizing: how many ports do you need?

The manifold is the brass or polymer header that distributes water from the boiler to each PEX loop. Each loop needs one supply port and one return port, so a 6 loop system needs a 12 port manifold. Manifolds come in 2 to 12 port sizes; above 12 you split into multiple manifolds tied together with a primary loop. Standard residential manifold count by home size:

• Single bathroom (50-100 sqft): 1 loop, 2 port manifold (often sold as a single-zone kit)
• Master bath plus closet (200-300 sqft): 1 to 2 loops, 4 port manifold
• Whole first floor (800-1,200 sqft): 4 to 6 loops, 8 to 12 port manifold
• Whole house 2,000+ sqft: 8 to 14 loops across one or two manifolds, often zoned

Each manifold needs a circulator pump and ideally a separate zone valve or actuator per loop so individual rooms can be controlled independently. Uponor, Watts Radiant, Viega, and Caleffi are the main residential manifold brands. Expect to pay $300 to $800 per manifold fully outfitted with valves, flow meters, and balancing valves.

Wet vs dry vs staple-up radiant install methods

The install method depends entirely on whether you have access to the subfloor from below and whether the floor is new or retrofit. Each method has a clear use case.

• New slab embed: PEX laid on rigid foam and rebar before the concrete pour. Cheapest per sqft and most thermally efficient. New construction or major renovation only.
• Thin-slab overpour: PEX stapled to existing wood subfloor, then 1.25 to 1.5 inches of gypcrete poured on top. Adds floor height and weight (8 lb per sqft). Best for whole-house retrofits where you can live with the height change.
• Staple-up: PEX stapled to the underside of the subfloor with insulation below. No floor height change, but heat transfer is reduced by the wood layer above. Usually paired with aluminum heat-transfer plates to compensate.
• Aluminum plate system: PEX runs through pre-cut aluminum plates that lock under the subfloor. Best heat transfer of any retrofit method but the priciest at $10 to $22 per sqft.

For most retrofit projects in homes with accessible floor joists, the aluminum plate system delivers the best long-term performance per dollar. The thin-slab overpour wins on whole-house new-floor projects where the height change is acceptable.

Air-to-water heat pump or condensing boiler: what feeds the radiant?

Radiant floors work with any heat source that can deliver hot water at the design supply temperature, but the operating cost varies dramatically by source. A 95% AFUE condensing gas boiler at $1.35 per therm delivers 100,000 BTU for about $1.42. An air-to-water heat pump at COP 3.5 and $0.17 per kWh delivers the same 100,000 BTU for about $1.43. They are roughly tied today, but the heat pump benefits from federal and state rebates that the gas boiler does not qualify for, and electricity costs less than gas in many western and northeastern markets.

The constraint: most air-to-water heat pumps cannot deliver supply temperatures above 120°F. If your radiant design targets 130°F+, you are stuck with a condensing boiler. If it targets under 115°F, you have your choice. Design for low temperature from the start and you keep your options open for electrification later. Pair this calculator with the boiler sizing calculator to get the matching heat source BTU rating.