Radiant Floor Heating Calculator
Hydronic radiant floor heating circulates warm water through PEX tubing embedded in or under the floor, radiating gentle, even heat upward through the floor surface. The primary sizing calculation determines tubing length from floor area and tube spacing. The formula is: length = area divided by spacing. The estimated BTU output is then calculated from the tube length, spacing, and typical radiant output rate. This calculator helps you plan a zone's tubing requirements and check whether the output will meet the room's heat load.
Radiant floor tubing formula
Tubing length (ft) = Area (sq ft) ÷ (spacing (in) ÷ 12)
Circuits = ceiling(Tubing length ÷ 300 ft max per circuit)
Estimated BTU/h = Area (sq ft) × BTU output per sq ft
The tube spacing in inches is converted to feet for the division. A 12-inch (1-foot) spacing in a 200 sq ft room requires 200 feet of tubing. The 300-foot maximum circuit length is a practical guideline for 1/2-inch PEX at typical flow rates to avoid excessive pressure drop and temperature drop across the circuit. Larger PEX (5/8 or 3/4 inch) allows longer circuits.
Radiant system design considerations
- Insulate below the slab or subfloor to prevent heat loss downward. At minimum, R-5 under-slab insulation is recommended; R-10 is better in cold climates.
- Use 1/2-inch oxygen-barrier PEX for all hydronic radiant systems to prevent iron oxide corrosion in metal system components.
- A modulating circulating pump maintains consistent flow across all zones and prevents pressure imbalance between short and long circuits.
- Design supply water temperature to be as low as possible while meeting the heat load; lower supply temperature dramatically increases boiler or heat pump efficiency.
- Carpets and area rugs with padding act as thermal insulators; reduce expected BTU output by 30-50% for carpeted floors.
Radiant floor heating: frequently asked questions
How do I calculate tubing length for radiant floor heating?
Tubing length equals the heated area divided by the tube spacing (both in the same unit), times the number of passes. The formula is: length (ft) = area (sq ft) / spacing (ft) + perimeter allowances for supply and return connections. At 12-inch spacing, a 200 sq ft room needs about 200 linear feet of tubing plus connections.
What tube spacing should I use for hydronic radiant heat?
Standard residential spacing is 6 to 12 inches center-to-center. Closer spacing (6 inches) is used in slab-on-grade floors with high heat loss, cold climates, or when using lower water temperatures. Wider spacing (12 inches) works well in well-insulated homes or when using higher supply water temperatures. Tile floors conduct heat better than carpet and can often use wider spacing.
What water temperature does a hydronic radiant floor need?
Hydronic radiant systems typically operate at 85 to 120 F supply water temperature, far lower than forced-air or baseboard systems. Lower water temperature (85 to 100 F) improves efficiency significantly when paired with a condensing boiler or heat pump. Higher temperatures (110 to 120 F) are needed in less-insulated floors or colder climates.
How much BTU output does a radiant floor produce per square foot?
A typical hydronic radiant floor supplies 15 to 30 BTU per square foot depending on floor surface temperature, floor covering, and supply water temperature. Bare concrete or tile can deliver up to 30 BTU/sq ft; thick carpet greatly reduces output. The floor surface should not exceed 85 F (29 C) for comfort in occupied areas.
What is the maximum circuit length for PEX tubing in a radiant system?
For pressure-drop and flow-balance reasons, most radiant designers limit individual circuit lengths to 250 to 300 feet for 1/2-inch PEX and 350 to 400 feet for 5/8-inch PEX. Longer circuits cause uneven heating because the water cools significantly before returning. If a room requires more tubing, split it into two balanced circuits on the manifold.
Official sources
- U.S. DOE Energy Saver: Radiant Heating.
- ASHRAE: ASHRAE Handbook of HVAC Systems and Equipment (Radiant Heating chapter).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.