Radiant Floor Heat Calculator

How Radiant Floor Sizing Works

Hydronic radiant floor heating circulates warm water through PEX tubing embedded in the floor. The tubing is laid in a serpentine or spiral pattern at a specified "on-center" spacing (6", 9", or 12"). Output depends on three things: water temperature, tube spacing, and the floor covering's resistance to heat flow. The calculator uses industry-standard output values for 1/2" PEX at 110-120 °F supply water — the typical range for modern low-temperature hydronic systems.

Typical Output by Floor Covering

Max floor surface temperature in occupied spaces is 85 °F per ASHRAE for foot comfort. At 35 BTU/hr/ft² output, surface temp is about 85 °F with 65 °F room air — the upper limit. Higher outputs require tighter spacing and higher water temp but may overshoot the comfort limit.

Tube Spacing Quick Reference

PEX length per sq ft = 12 / spacing (inches), plus 10% for bends and headers. Practical numbers for common installations:

• 6" OC: 2.2 ft PEX per sq ft. Use in bathrooms, mudrooms, high-heat-loss perimeters, and where highest output is needed.
• 9" OC: 1.47 ft PEX per sq ft. Standard for typical rooms with normal heat loss.
• 12" OC: 1.1 ft PEX per sq ft. Supplemental or warm-floor-only applications in well-insulated spaces.

Loop Layout Rules

Keep 1/2" PEX loops under 300 ft each to maintain reasonable head loss (under ~4 ft of water column per loop at 0.5-1 gpm). If your calculated total tube exceeds the max loop length, the calculator splits it into equal loops automatically. Every loop runs from and returns to a manifold, which balances flow and isolates each zone with its own thermostat and actuator.

PEX Quantities for Common Room Sizes

Total PEX tube (with 10% waste) and number of 1/2" loops for typical residential areas at 9" OC spacing:

Heat output is per sq ft × surface factor: 32 BTU/hr/ft² bare concrete, 27 for tile, 22 for wood, 14 for carpet. For a 200 sq ft tile bathroom with a 5,000 BTU/hr heat loss, radiant floor at 9" OC (5,400 BTU/hr) covers the load as the sole heat source. For a poorly insulated 400 sq ft room losing 15,000 BTU/hr, radiant alone (10,800 BTU/hr) is not enough — pair with baseboard or a panel radiator.

Frequently Asked Questions

How deep should PEX be embedded in concrete?

2-3 inches below the finished floor surface. Stapled to a foam board or wire mesh before pouring. Keeps the tube away from edges where it would be damaged.

What water temperature do I need?

110-120 °F supply for modern systems. Old high-temp designs used 180 °F but this risks cracking the concrete from thermal expansion.

Do I need a manifold?

Yes — all loops start and return to a manifold for balancing, air removal, and zoning. Stainless or brass manifolds last decades; plastic is OK for 10-15 years.

Can radiant floor handle 100% of heating load?

In well-insulated homes at design heat loss under 30 BTU/hr per sq ft, yes. In leaky older homes you may need supplemental baseboards or panel radiators for peak cold snaps.

How much does radiant floor cost?

$5-$15 per sq ft installed for retrofit (staple-up or Warmboard), $3-$8 per sq ft for new construction in a concrete slab. Add manifolds, pump, mixing valve, and controls ($800-$1,500 for a single zone).