Understanding your result
The headline is the radiator’s output at the water temperature you selected, in BTU per hour, with the watts and the room it can heat in the sub-line. The stats show the catalogue rating at the standard ΔT 50 °C, the corrected output and the room area, so the effect of your water temperature is visible at a glance: at 180 °F the second figure is larger than the first, at 140 °F it is smaller.
The table adds the correction factor as its own line and, for panel and column radiators, the output per square metre of face, which is a quick way to check a catalogue figure for plausibility: a type 22 panel is around 3,000 W/m² at ΔT 50 °C, a flat single panel about a third of that. The room area it can heat divides the output by the heating load per square foot of your climate zone from the sizing calculators, for a room with average insulation.
To add up a whole system and size the boiler, carry each radiator’s output to the boiler size calculator. For fin-tube baseboard the baseboard heater calculator works per foot of element at the same water temperatures, and for a heated floor the radiant floor output calculator does the equivalent sum.
How we calculate this
The outputs per square metre are typical catalogue values for each panel type at EN 442 conditions: about 1,000 W/m² for a flat single panel, 1,700 for a single panel with one set of convector fins, 2,500 for a double panel with one set of fins, 3,200 for a double panel with two sets, and 1,800 for a two-column steel radiator. The 1.3 exponent is the EN 442 characteristic exponent; real radiators range from 1.25 to 1.35. The water temperature options give the ΔT for a 70 °F room: 180 °F water (a traditional cast-iron system) is ΔT 61 °C, 160 °F is the standard ΔT 50, 140 °F (a condensing boiler) is ΔT 39, and 120 °F (a heat pump) is ΔT 28.
Heating loads per square foot by climate zone are on the BTU per square foot table, and the conversion between watts and BTU per hour is on the unit conversions page.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Rating condition | ΔT 50 °C (mean water − room) | EN 442, the European radiator standard used by most catalogues |
| Output per m² of face by type | 1,000 / 1,700 / 2,500 / 3,200 / 1,800 W/m² | Typical catalogue values for types 10, 11, 21, 22 and 2-column steel radiators about 600 mm high |
| Cast-iron sections | 20 BTU/hr per inch of height per section at ΔT 50 °C | Typical four-column section; about 2 to 3 sq ft EDR per section in US tables |
| Temperature exponent | 1.3 | EN 442 characteristic exponent (1.25 to 1.35 for real radiators) |
| Room temperature | 70 °F (21 °C) | Standard indoor design temperature |
| Room load | By climate zone, average insulation | See the BTU per square foot table |
Assumptions last reviewed October 7, 2026.
The calculator does not account for radiators boxed in by covers or curtains (which can cut output by 10 to 20%), for the lower output of a radiator fitted with a thermostatic valve that is throttling, for flow rates well below design, or for steam radiators, which run at 215 °F and are rated on a different basis. Painting a radiator changes little; covering it changes a lot. The guide to radiators and baseboard heaters covers how outputs are rated and how to size a system for a lower-temperature boiler.
Two worked examples
A modern double-panel radiator at the standard condition
A type 22 panel radiator, 24 in high and 40 in long, in a system running 160 °F water (ΔT 50 °C), in a zone 4 house.
- Face area: 0.610 × 1.016 = 0.619 m²
- Rated output: 0.619 × 3,200 = 1,982 W = 6,763 BTU/hr
- Correction at ΔT 50: × 1.0
- Room it can heat in zone 4: 6,763 ÷ 38 = 178 sq ft
This radiator suits a 12 × 14 ft bedroom with average insulation in a mixed climate. If the house is later fitted with a condensing boiler running 140 °F water, the same radiator gives 6,763 × 0.72 = 4,896 BTU/hr and heats about 129 sq ft, so a 12 × 14 ft room would need a second radiator or a taller one.
An old cast-iron radiator on a hot system
A cast-iron radiator with 12 sections 25 in high, on a boiler running 180 °F water (ΔT 61 °C), in a zone 6 house.
- Rated output at ΔT 50: 12 × 25 × 20 = 6,000 BTU/hr
- Correction: (61 ÷ 50)^1.3 = 1.295
- Output: 6,000 × 1.295 = 7,770 BTU/hr (2,277 W)
- Room it can heat in zone 6: 7,770 ÷ 52 = 149 sq ft
In Minneapolis this radiator heats a room of about 150 sq ft with average insulation. The note warns that 180 °F water is above the range at which a new condensing boiler would condense; at 140 °F the same radiator would give about 4,340 BTU/hr and heat only 83 sq ft, which is the arithmetic behind the common advice to insulate before switching an old radiator system to a condensing boiler.
Where to find your inputs
Type. Look at the radiator from the top. One flat panel with no corrugated fins behind it is type 10; one panel with fins is type 11; two panels with fins between them is type 21 or 22 depending on whether there are one or two fin sets (type 22 is the common modern choice). Vertical tubes joined at top and bottom are column radiators; thick cast sections with a rough surface are cast iron.
Height and length. Overall height and length of the radiator body, not the brackets or valves. For cast iron, count the sections and measure the height of a section.
Water temperature. The boiler’s supply temperature minus about 10 °F for the mean; a traditional boiler set to 180 °F supply has a mean near 170 to 175 °F, which rounds to the 180 °F option. Condensing boilers on outdoor reset vary; use 140 °F for the design day and lower for mild weather.
Common mistakes
- Using the catalogue rating at a lower water temperature. A radiator rated 6,800 BTU/hr delivers about 4,900 with a condensing boiler.
- Assuming ΔT 60 catalogues match ΔT 50 ones. Older ratings at ΔT 60 are about 27% higher for the same radiator; check which the catalogue uses.
- Counting the boxed-in radiator at full output. Covers and heavy curtains cut output by 10 to 20%.
- Forgetting the flow. A radiator starved of flow by a throttled valve or air lock runs cooler at the bottom and gives less; bleed and balance before concluding it is too small.
- Treating watts as BTU. Multiply watts by 3.412 to get BTU per hour.
- Sizing the boiler from radiators without checking the house. Oversized radiators are an asset for low-temperature operation, not a reason for a bigger boiler.
Questions people ask
- How many BTU does a radiator put out?
- It depends on type, size and water temperature. A double-panel double-convector radiator (type 22) 24 in high and 40 in long delivers about 6,800 BTU per hour at the standard rating condition, a 50 °C difference between the mean water temperature and the room, which is 160 °F water in a 70 °F room. A single panel with fins of the same size gives about 3,600; a flat single panel without fins about 2,100. At 180 °F water the figures rise by about 30%; at 140 °F they fall by about 28%.
- What does ΔT 50 mean on a radiator?
- The European standard EN 442 rates radiators at a 50 °C (90 °F) difference between the mean water temperature and the room air, for example 75 °C mean water in a 20 °C room. Older catalogues used ΔT 60. The output at any other difference is the rated output multiplied by (your ΔT ÷ 50) to the power 1.3, so halving the temperature difference cuts the output by about 60%, not 50%. US cast-iron ratings use a different convention, square feet of equivalent direct radiation at 240 BTU per hour per square foot with steam or 170 with 180 °F water.
- How much does radiator output fall with a condensing boiler or heat pump?
- A condensing boiler returns water below about 130 °F to condense, so the mean water temperature is around 140 °F and the ΔT about 39 °C, which cuts output to about 72% of the catalogue rating. A heat pump supplying 120 °F water gives a ΔT of about 28 °C and about 47% of the rating. That is why a switch to either usually means larger radiators, more of them, or fan-assisted ones, unless the house has been insulated enough to need less heat.
- How do I work out the output of a cast-iron radiator?
- Count the sections and measure their height. A typical four-column section about 25 in high delivers around 500 BTU per hour at the standard ΔT 50 °C and about 650 with 180 °F water; shorter or thinner sections give less, deep six-column sections more. US tables rate them in square feet of EDR per section (often 1.6 to 3), to be multiplied by 170 BTU per hour per square foot for hot water at 180 °F. The calculator uses 20 BTU per hour per inch of height per section at ΔT 50 °C as a typical figure.
- What size radiator do I need for a room?
- Match the room's heat loss. A 150 sq ft bedroom with average insulation needs about 5,700 BTU per hour in a mixed climate (zone 4) and 7,800 in a cold one (zone 6); a 300 sq ft living room needs double that. Pick a radiator whose output at your actual water temperature is at or a little above the load, then check it fits the wall. The calculator reports the room area each radiator can heat in your zone, so you can work the question either way.
- Are radiator outputs in watts or BTU?
- Both appear. European catalogues quote watts; US tables quote BTU per hour or square feet of EDR. One watt is 3.412 BTU per hour, so 2,000 W is 6,824 BTU per hour. The calculator shows both, and the BTU, watts and kW converter handles any other heat rate.