Plan electric or hydronic radiant floor heating from the actual heated area
Radiant floor projects are often mis-sized because the room area is treated as the heating area. This calculator starts by separating gross floor area from the floor that will actually contain an electric heating element or hydronic tubing, then applies a different model for each system type.
- Electric mode estimates installed kW, current at the selected voltage, daily and seasonal kWh, and energy cost from your own tariff.
- Hydronic mode estimates active-area tubing, supply and return leaders, loop count, average loop length, manifold ports and full tubing coils.
- Metric and US customary inputs can be switched without rebuilding the calculation.
This is a project-planning calculator, not a heat-loss, electrical-code or hydraulic-design tool. It is intended to turn known product and layout assumptions into a transparent material and energy estimate.
Measure heated floor coverage before choosing a mat, cable or PEX layout
Enter the room length and width, then subtract permanent areas that the selected system should not heat. Electric cable manufacturers commonly restrict installation below fixed cabinets, appliances and fixtures without adequate clearance. A hydronic layout also needs a realistic occupied field rather than a gross rectangle. Follow the exact exclusion and clearance rules for the product you are installing.
Inputs used by each calculation mode
| Input | Electric | Hydronic | What it changes |
|---|---|---|---|
| Room size | Yes | Yes | Gross floor area |
| Excluded area | Yes | Yes | Actual heated coverage |
| Rated output | Yes | No | Installed watts and energy demand |
| Voltage | Yes | No | Calculated current |
| Schedule and duty cycle | Yes | No | Estimated kWh and running cost |
| Tubing spacing | No | Yes | Active tubing takeoff |
| Maximum planned loop length | No | Yes | Number of loops / manifold ports |
| Manifold distance | No | Yes | Supply and return leader length |
| Coil length | No | Yes | Whole coils to purchase |
Electric radiant floor heating: watts, amps and running cost
Electric mode does not guess the output of your product. Enter the rated W/m² or W/ft² from the mat, cable, foil or membrane documentation. The calculator multiplies that rating by heated area to find installed watts. Dividing by the selected supply voltage gives a planning current, while the schedule and thermostat duty cycle convert installed power into estimated energy use.
Installed power = heated area × rated output. Daily kWh = installed kW × scheduled hours × duty cycle. Cost = kWh × your electricity price.A bathroom has 80 ft² of actual heated floor and a 12 W/ft² electric system. What does the calculator show?
Answer: Installed load is about 960 W. At 120 V that is about 8 A before any code-specific circuit sizing is considered. If the thermostat schedule is 8 hours and the entered duty cycle is 50%, the energy model uses four equivalent full-load hours, or about 3.84 kWh per day.
Explanation: The duty-cycle field separates thermostat schedule from the amount of time the resistance element is actually energized.
Do not buy an oversized heating cable expecting to trim the excess. Many floor-warming cables cannot be shortened, and manufacturers require the product to be selected for the usable heated area. Circuit sizing, GFCI/RCD protection and wiring requirements must be verified for the actual product and jurisdiction.
Hydronic radiant floor tubing: spacing, leaders, loops and coils
For a regular serpentine or spiral field, the first-order tubing takeoff follows a simple coverage relationship: active tubing length is approximately heated area divided by tube spacing, with both values converted to consistent units. The calculator then adds the layout allowance you enter and adds two manifold leader runs per loop — one supply and one return.
Active tubing ≈ heated area ÷ spacing. Usable active length per loop = planned maximum loop length − 2 × one-way manifold distance. Loops = round active design tubing up to that usable length.The maximum loop length is deliberately editable. Pressure drop rises with flow and tubing length, and manufacturers do not treat one loop length as universal for every tube size and system. Use the limit from the hydronic design or the system documentation rather than treating the default as a code requirement.
How much tubing does a 600 ft² heated zone need at 12 in spacing with a manifold 10 ft away?
Answer: The base active-field takeoff is about 600 ft because 12 in spacing corresponds to one foot of tube per square foot of floor before allowances. The calculator applies your layout allowance, reserves 20 ft of supply-and-return leader for each loop, and splits the field against the maximum loop length you entered.
Explanation: A simple area-only estimate misses the tubing between the manifold and the heated field and may therefore understate both total footage and loop count.
Why loop count is not a boiler or pump sizing result
Loop count here is a material-layout result. It does not prove that a circulator can deliver the required flow or that the floor can meet the room load. A complete hydronic design also considers room-by-room heat loss, floor covering resistance, water temperature, mass flow, pipe diameter, circuit pressure drop and manifold balancing.
Choosing realistic inputs instead of generic rules of thumb
Where each important value should come from
| Value | Best source | Avoid |
|---|---|---|
| Electric W/m² or W/ft² | Heating product data sheet | A generic internet average |
| Excluded area | Measured floor plan + manufacturer clearances | Using the whole room automatically |
| Duty cycle | Your control strategy or measured history | Assuming scheduled hours equal full-load hours |
| Tube spacing | Heat-loss-based radiant design / system table | Picking spacing only to reduce tubing |
| Maximum loop length | Pressure-drop design / manufacturer guidance | Using 300 ft or 100 m as a universal maximum |
| Coil length | Actual product being purchased | Assuming every supplier sells the same coil |
Common radiant floor estimating mistakes
- Sizing the electric element from total tiled area instead of permitted heated area.
- Entering a rated watt density that does not match the actual product.
- Using 100% duty cycle for every hour on the thermostat schedule and then treating the cost as a prediction.
- Forgetting supply and return tubing between the manifold and the room.
- Treating maximum loop length as independent of tubing size, flow and pressure drop.
- Ordering tubing from the base grid length without a realistic allowance for bends and layout.
- Using tubing quantity as a substitute for a room heat-loss calculation.
Frequently asked questions
Is radiant floor heating the same as underfloor heating?
In everyday residential use the terms overlap heavily. “Radiant floor heating” is common in North America, while “underfloor heating” or UFH is especially common in the UK and Europe. Both may refer to electric resistance systems or water-based hydronic systems installed within or beneath the floor assembly.
How much PEX do I need per square foot?
The geometric multiplier depends on spacing. At 12 in on center the first-order field estimate is about 1.0 ft of tube per ft²; at 6 in it is about 2.0 ft/ft². Leaders, bends, edge conditions and layout allowance are additional, which is why this calculator asks for manifold distance and an allowance instead of stopping at the multiplier.
Does the calculator tell me whether radiant heat can heat the whole room?
No. That decision requires a design heat-loss calculation and a check that the proposed floor assembly can deliver the required output without exceeding product, floor-covering or comfort limits. This calculator intentionally separates material and electrical estimates from that design decision.
Use the result as a takeoff, then verify the system design
For electric floor warming, compare heated area, rated power and current with the exact product selection table and electrical design. For hydronic radiant heat, use tubing length, loop count and coil quantity as a purchasing estimate, then verify spacing, loop limits, flow and water temperature through a heat-loss and pressure-drop based design. That final verification is what turns a useful material estimate into a reliable heating system.