A 12 × 12 ft room (144 sq ft) with an 8 ft ceiling needs about 1,440 watts of electric baseboard heat at the usual 10 watts per square foot. That is one 6 ft, 1,500 W heater, which draws 6.25 amps on a 240 V circuit. Taller ceilings, weak insulation and big windows push the number up.
How baseboard heater wattage is calculated
Stelpro, a heater maker, gives the rule: "Multiply the area in feet by 10" for rooms with 8 ft ceilings. Above 8 ft it switches to volume, 1.25 W per cubic foot, which is the same as scaling 10 W per sq ft by height ÷ 8. The calculator does exactly that and then adds your allowance for poor insulation or large windows.
Watts = floor area (sq ft) × 10 W × (ceiling ÷ 8 ft) × (1 + extra %)
Amps = heater watts ÷ volts
Stelpro's own example: a 400 sq ft room with 9 ft ceilings needs 400 × 9 × 1.25 = 4,500 W. Enter 20 × 20 ft and 9 ft and the calculator shows the same 4,500 W.
How many feet of baseboard heater?
Heaters are sold by length and wattage. The Cadet F series product sheet lists these 240 V models (208 V output in brackets):
| Length | Model | Watts at 240 V (208 V) | Amps at 240 V |
|---|---|---|---|
| 24 in | 2F350 | 350 (262) | 1.5 |
| 30 in | 2F500 | 500 (375) | 2.1 |
| 36 in | 3F750 | 750 (563) | 3.1 |
| 48 in | 4F1000 | 1,000 (750) | 4.2 |
| 60 in | 5F1250 | 1,250 (937) | 5.2 |
| 72 in | 6F1500 | 1,500 (1,125) | 6.3 |
| 96 in | 8F2000 | 2,000 (1,500) | 8.3 |
| 96 in | 8F2500 | 2,500 (1,875) | 10.4 |
From 36 in up that is 250 W per foot, the figure most people quote, but the 30 in 500 W model shows it is not a law: pick from the table, not by dividing. The 120 V versions stop at 6 ft and 1,500 W (12.5 A).
The calculator splits the watts over the number of heaters you ask for and picks the shortest model that covers each share. For the 144 sq ft room, one heater means the 72 in 1,500 W model; two heaters (one under each of two windows) means two 36 in 750 W models, 1,500 W in total.
Worked example: 20 × 20 ft room, 9 ft ceiling
- Watts: 400 sq ft × 10 × 9 ÷ 8 = 4,500 W.
- One heater cannot do it (the biggest is 2,500 W), so the calculator uses at least two: 2,250 W each, two 96 in 2,500 W heaters, 5,000 W and 20.8 A at 240 V.
- Set three heaters instead and each share is 1,500 W: three 72 in 1,500 W heaters, 4,500 W and 18.75 A, spread along three walls.
240 V, 208 V or 120 V
Most baseboard heat runs on 240 V. In apartments and commercial buildings the supply is often 208 V, and Cadet notes that 240 V models "can be used at 208 volts. Wattage equals 75% of 240v rated wattage." The 144 sq ft room at 208 V therefore needs the 96 in 2,000 W model to reach 1,500 W, drawing 7.2 A. At 120 V the same room takes the 1,500 W 120 V model at 12.5 A, twice the current of the 240 V version, which is why larger rooms are almost always wired at 240 V.
Circuits and breakers
Heater current decides the circuit. Add every heater on the circuit, then size the breaker and wire for that total in the breaker size calculator and the wire size calculator. If you are adding heat to an existing panel, check the spare capacity first with the electrical load calculator. All Cadet baseboards need a thermostat, built in or on the wall, rated for the current it switches.
Where to put the heaters
Cadet's installation notes: under a window works best, keep at least 12 in from furniture and from drapes hanging above, and never mount a heater below an electrical outlet. The drawing under the result places each heater under its own window on your longest wall, to scale.
Watts to BTU
One watt is 3.412 BTU/h (EIA), so 1,500 W of baseboard heat is 5,118 BTU/h. To compare electric heat with a gas heater or a heat pump for the same room, use the heat tab of the BTU calculator, which sizes rooms by climate zone instead of a flat 10 W per sq ft.