Radiator Size Calculator
How It Works
Choosing the right radiator size ensures your room stays comfortable without wasting energy. The calculation starts with room volume and adjusts for insulation quality, external walls, and other factors. A poorly insulated room needs up to 30% more heating capacity than a well-insulated one.
The base calculation uses approximately 4 BTU per cubic foot, which is suitable for moderate climates. Colder regions may need 5-6 BTU per cubic foot. Each external wall increases heat loss, so corner rooms with two external walls need larger radiators than interior rooms.
When selecting a radiator, choose one with a BTU rating at or above the calculated value. It is better to slightly oversize than undersize — an oversized radiator can be throttled with a thermostatic valve, but an undersized radiator will never adequately heat the space.
Frequently Asked Questions
How many BTU do I need per square foot?
As a rough guide, 20-30 BTU per square foot works for average insulation and 8-foot ceilings. Poorly insulated rooms need 30-40 BTU/sq ft, while well-insulated rooms need 15-20 BTU/sq ft.
What size radiator for a 12x12 room?
A 12x12 room (144 sq ft) with 8-foot ceilings and average insulation needs approximately 4,600-5,500 BTU. A standard single-panel radiator about 3-4 feet wide typically provides this output.
Does radiator placement matter?
Yes. Place radiators under windows where possible — the rising warm air counteracts cold downdrafts from the glass. Avoid blocking radiators with furniture, which can reduce heat output by 10-20%.
Panel vs column radiators — does it matter for sizing?
Both types are rated by BTU output, so choose based on the BTU number regardless of style. Column radiators have a traditional look, while panel radiators are slimmer and more common in modern homes.