Baseboard heater calculator: what size baseboard heater do I need?
1 heater: 1,500 W, 6 ft
1,440 W needed at 10 W per sq ft, King K Series at 240 V · 6.3 A: 1 circuit on a 15 A breaker (12 A at 80%) · for a 12 × 12 ft room
Run your own numbers
Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
1 heater: 1,500 W, 6 ft
1,440 W needed at 10 W per sq ft, King K Series at 240 V · 6.3 A: 1 circuit on a 15 A breaker (12 A at 80%)
Source: Cadet, Installing electric heaters: 6 to 10 watts per square foot, read October 31, 2026. Read the source
A 12 × 12 ft room at 10 watts per sq ft, the top of Cadet's 6 to 10 range, needs 1,440 W: one 1,500 W, 6 ft baseboard heater at 240 V, 6.3 A, under the 12 A Cadet allows per 15 A breaker.
A worked example
A 14 ft 6 in × 21 ft living room at 9 W per sq ft, on a 208 V supply with 15 A breakers and King heaters, needs two heaters, 2,400 W, 8 ft (1,800 W at 208 V) + 1,500 W, 6 ft (1,125 W at 208 V), on two 15 A circuits. 14.5 × 21 ft is 304.5 sq ft, and at 9 W that is 2,741 W. On 208 V a King 240 V heater gives 25% less, so the 2,400 W and 1,500 W models give 2,925 W together: 14.1 A, over the 12 A that 80% of one 15 A breaker allows.
How many baseboard heaters on a 20 amp circuit?
As many as stay within 80% of the breaker, Cadet's limit: 16 A on a 20 A breaker, or 3,840 W of heaters at 240 V, such as a 2,000 W and a 1,500 W. A 15 A breaker takes 12 A, 2,880 W. A thermostat rated lower sets the cap.
How many watts per square foot for baseboard heat?
Cadet recommends 6 to 10 W per sq ft, by the home's construction, the region and whether the heaters are the only heat: 900 to 1,500 W for a 150 sq ft room.
How many watts is an 8 foot baseboard heater?
2,000 or 2,400 W in King's K Series and 2,000 or 2,500 W in Marley's 2500 Series, at 240 V. On 208 V they give 25% less: 1,500 or 1,800 W for King. A 6 ft heater is 1,500 W in both lines.
What else it is called
Also searched as a baseboard heat calculator, baseboard heating calculator, baseboard heater size or wattage calculator, or electric baseboard heat calculator. This one is for electric heaters; hot water (hydronic) baseboard is sized by Btu per foot of element.
Heaters by room size
| Room | At 6 W per sq ft | At 8 W per sq ft | At 10 W per sq ft |
|---|---|---|---|
| 10 × 10 ft (100 sq ft) | 750 W, 3 ft (3.1 A) | 1,000 W, 4 ft (4.2 A) | 1,000 W, 4 ft (4.2 A) |
| 10 × 12 ft (120 sq ft) | 750 W, 3 ft (3.1 A) | 1,000 W, 4 ft (4.2 A) | 1,250 W, 5 ft (5.2 A) |
| 12 × 12 ft (144 sq ft) | 1,000 W, 4 ft (4.2 A) | 1,250 W, 5 ft (5.2 A) | 1,500 W, 6 ft (6.3 A) |
| 12 × 14 ft (168 sq ft) | 1,250 W, 5 ft (5.2 A) | 1,500 W, 6 ft (6.3 A) | 2,000 W, 8 ft (8.3 A) |
| 14 × 16 ft (224 sq ft) | 1,500 W, 6 ft (6.3 A) | 2,000 W, 8 ft (8.3 A) | 2,400 W, 8 ft (10.0 A) |
| 16 × 20 ft (320 sq ft) | 2,000 W, 8 ft (8.3 A) | 2,000 W, 8 ft + 750 W, 3 ft (11.5 A) | 2,000 W, 8 ft + 1,250 W, 5 ft (13.5 A) |
Heaters from the King K Series 240 V list (500 W 27 in; 750, 1,000, 1,250 and 1,500 W at 3, 4, 5 and 6 ft; 2,000 and 2,400 W at 8 ft), the fewest pieces that reach the watts; amps at 240 V. Cadet recommends 6 to 10 W per sq ft and at most 80% of the breaker on one circuit: 12 A on 15 A, 16 A on 20 A.
Where the numbers come from
- Watts per sq ft: Cadet (Glen Dimplex Americas), Installing electric heaters: "we recommend somewhere between 6 and 10 watts per square foot depending on your home's construction, if the heater is your only source of heat and what part of the country you live in." Read October 31, 2026. Cadet on how many watts you need
- Circuit: Cadet Owner's Guide, Electric Baseboard Heater (720001 BB, October 2023): heaters go on a circuit of their own, and the load on one breaker is limited to 80% of its rating or to the thermostat's maximum amps, whichever is lower. Read October 31, 2026. Cadet baseboard owner's guide (PDF)
- Heaters: King Electrical K Series, 240/208 V models: 500 W at 27 in, 750 W at 3 ft, 1,000 W at 4 ft, 1,250 W at 5 ft, 1,500 W at 6 ft, 2,000 and 2,400 W at 8 ft; dual-rated heaters give 25% less wattage at 208 V. Read October 31, 2026. King K Series models
- Heaters: Marley Engineered Products, Berko catalog, August 2026, 2500 Series 240/208 V models: 400 W at 24 in, 500 W at 30 in, 750 W at 36 in, 1,000 W at 48 in, 1,250 W at 60 in, 1,500 W at 72 in, 2,000 and 2,500 W at 96 in. Read October 31, 2026. Marley 2500 Series catalog (PDF)
- The rest is arithmetic: watts = length × width × watts per sq ft, rounded up; then the fewest catalog heaters whose watts at your voltage reach it; amps = watts ÷ volts.
Watts per square foot is the one sizing input: Cadet's range already spans construction, region and whether the heaters are the only heat. Watts and lengths are the catalog's 240/208 V models; 120 V models are left out. The 80% check counts the heaters alone, on circuits that feed nothing else.
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