Radiator Size Calculator
Use the Radiator Size Calculator to measure heat output and radiator area requirements based on your project's specifications.
Calculate radiator size in seconds
Enter BTU or kW requirements for precise sizing
Use metric or imperial units
Download a professional PDF report with your results
What Is Radiator Size & Why Does It Matter?
Radiator sizing determines how much heat output is required to maintain comfortable temperatures in a space. Engineers and installers rely on accurate radiator kW / BTU calculations to ensure rooms stay warm and energy-efficient.
When you calculate radiator size for a room correctly, you avoid underheating or wasting energy on oversized radiators. Therefore, this is especially important in residential and commercial hydronic / heating systems where efficiency, comfort, and compliance all depend on exact design.
Accurate radiator output calculations also help you specify the right models and dimensions early in the project. As a result, you can reduce costs, avoid delays, and deliver a better experience for clients.
Radiator Sizing Variables
Heat Output
The required heat output, expressed in BTUs or kW, drives the radiator's size and surface area needed for proper heating.
Mean Water Temperature
The average water temperature affects how efficiently the radiator transfers heat to the entire room over time.
Room Temperature
The target room temperature influences the delta T, which is the temperature difference between the radiator and the space.
Radiator Type
Different radiator types have varying outputs and surface areas, so choosing the right model is essential.
Width
The radiator's width impacts both output and available wall space, helping you balance performance and design.
Height
The radiator's height plays a key role in its heat output and how well it fits beneath windows or along shorter walls.
Radiator Sizing Table
Radiator output is determined using standard performance tables based on the radiator type and the temperature difference (ΔT).
Furthermore, a radiator size chart provides more accurate results than a general formula by reflecting tested performance data.
Imperial Radiator Output Table
The following radiator size table shows radiator output in BTU/hr per square foot based on the ΔT in Fahrenheit for different radiator types.
| Delta T (°F) | Type 11 (K1) | Type 21 | Type 22 (K2) | Type 33 (K3) |
|---|---|---|---|---|
| 18 | 120 | 174 | 231 | 329 |
| 27 | 144 | 209 | 277 | 395 |
| 36 | 173 | 251 | 333 | 474 |
| 45 | 208 | 301 | 399 | 569 |
| 54 | 249 | 361 | 479 | 683 |
| 63 | 299 | 433 | 577 | 823 |
| 72 | 359 | 518 | 695 | 994 |
| 81 | 430 | 619 | 836 | 1190 |
| 90 | 514 | 737 | 1003 | 1420 |
| 99 | 613 | 874 | 1202 | 1702 |
| 108 | 746 | 1061 | 1430 | 2043 |
Metric Radiator Output Table
The following radiator size table shows radiator output in Watts per square metre based on the ΔT in Celsius for different radiator types.
| Delta T (°C) | Type 11 (K1) | Type 21 | Type 22 (K2) | Type 33 (K3) |
|---|---|---|---|---|
| 10 | 380 | 550 | 730 | 1040 |
| 15 | 456 | 660 | 876 | 1248 |
| 20 | 547 | 792 | 1051 | 1498 |
| 25 | 657 | 950 | 1261 | 1797 |
| 30 | 788 | 1140 | 1514 | 2157 |
| 35 | 946 | 1369 | 1816 | 2588 |
| 40 | 1135 | 1642 | 2180 | 3105 |
| 45 | 1362 | 1971 | 2616 | 3727 |
| 50 | 1634 | 2365 | 3139 | 4472 |
| 55 | 1961 | 2838 | 3767 | 5366 |
| 60 | 2353 | 3405 | 4520 | 6439 |
Radiator Sizing In h2x
h2x makes radiator sizing faster, more accurate, and easier to share with clients and colleagues.
See why thousands of engineers trust h2x for their heating / hydronic design:
Smart Layouts: Draw and adjust systems quickly with auto-connected components.
Verified Sizing: Create accurate, CIBSE-verified calculations that update instantly.
Easy Exports: Download branded PDFs or AutoCAD files in one click.
Detailed Reports: Get radiator output summaries and surface area requirements for review.
