Space Heater Size Calculator
Heater watts, count, and amps from room area.
What it does
Space heaters size by watts per square foot: 10 W each covers typical rooms, and a 150 ft2 corner lands exactly on the standard 1500 W unit. Circuits decide the rest.
The tool above takes room area in square feet or meters. It returns watts, heater count with circuit notes, and amps.
The default 150 ft2 needs 1500 W from one heater on a dedicated circuit at 12.5 A. The 300 ft2 case needs 3000 W across two heaters at 25 A.
Heating guidance uses the 10 W rule for supplemental sizing, and electrical practice caps continuous loads at 80% of breaker rating. One 1500 W unit already fills a 15 A circuit alone.
Heaters supplement sealed rooms, never whole houses through one outlet. Close doors, clear 3 ft around every unit, and let tip over switches with timers guard the night.
How to use
- Type the room area.
- Read watts first.
- Read heater count beside it.
- Read amps last.
- Dedicate one circuit each.
- Keep 3 ft clear.
Formula
- area
- Room area in ft2 or m2.
- watts
- Heat load at 10 W per ft2.
- heaters
- 1500 W units on own circuits.
Energy department heating guidance frames supplemental heat sizing with the same watts per square foot planning math. Continuous loads cap at 80% of breaker rating, putting one 1500 W heater alone on a 15 A circuit. Metric mirrors convert square meters at 10.7639 first.
Worked example
Example: 150 sq ft room
Imperial defaults: 150 sq ft.
- Room: 150 sq ft living corner.
- Watts: 150 x 10 = 1500 W.
- Heaters: ceil(1500 / 1500) = 1 on a dedicated circuit.
- Amps: 1500 / 120 = 12.5 A on 120 V.
- Check: 80% of a 15 A breaker is 12 A continuous.
- Tool shows 1500 W, 1 heater (dedicated circuit), 12.5 A on 120 V.
Reference table
| Room | Watts | Heaters |
|---|---|---|
| 100 ft2 | 1000 W | 1 |
| 150 ft2 | 1500 W | 1 |
| 200 ft2 | 2000 W | 2 |
| 300 ft2 | 3000 W | 2 |
Limits
- Supplemental heat planning.
- Typical insulation assumed.
- 120 V circuits assumed.
- Drafts and ceilings shift loads.
Common mistakes
- Two heaters, one strip.
- Sizing by feel not math.
- Heating whole houses singly.
- Ignoring 80% circuit caps.
- Draping cords under rugs.
Try it yourself
Quick checks. Answers sit tucked under each question.
300 sq ft?
3000 W, 2 heaters on own circuits, 25 A on 120 V.
13.9 m2?
1496 W, 1 heater, 12.5 A: mirrors the imperial default.
0 sq ft?
Zero area heats nothing: enter room area above 0.
Questions
Sizing rule?
10 watts per sq ft of room. A 150 ft2 room needs 1500 W.
How many heaters?
One 1500 W unit per dedicated 15 A circuit. Two heaters need two circuits.
Share circuits?
No, 1500 W draws 12.5 A, past the 12 A continuous cap on 15 A breakers with anything else.
Whole house heat?
Supplement, not replace: close doors, draft proof, and keep 3 ft clear around units.
Fire risk?
Yes, tip overs, pets, and curtains all find heaters. Clearance and shutoffs save houses.
Heater types?
Ceramic, oil, and micathermic all deliver the same watts; fans spread it faster.