Radiator Sizing Calculator
Enter your room dimensions, then select your system and radiator type.
About This Radiator Sizing Calculator
This free radiator sizing calculator helps homeowners, contractors, and heating installers estimate the right amount of cast iron baseboard radiator or slenderized cast iron radiator sections needed to heat a room. It works for both hot water (hydronic) and steam heating systems.
Enter your room’s length and width, select your system type and radiator style, and the calculator does the rest — giving you a BTU estimate and a direct link to the right product on our site.
How the Calculation Works
The calculator uses a standard rule of 40 BTU per square foot — a widely accepted benchmark for rooms with average ceiling heights (8–9 ft) and typical insulation. This is Consumers Supply Company’s recommended starting point for cast iron radiator sizing in Chicago-area homes and buildings.
For cast iron baseboard radiators, the BTU requirement is divided by the output per linear foot (510 BTU/ft for hot water, 816 BTU/ft for steam) to determine total footage needed. The calculator then recommends the closest available standard Burnham Baseray size.
For slenderized cast iron radiators, the BTU requirement is divided by the per-section output of each model, then rounded up to the nearest even section count to ensure your room is fully covered.
Have questions or want a second set of eyes on your project? Our team has decades of experience helping Chicago homeowners and contractors find the right radiator. Call us at 312-666-6080 — we’re happy to help.
Frequently Asked Questions
How many BTUs do I need per square foot for a radiator?
A standard rule of thumb is 40 BTU per square foot for rooms with 8–9 ft ceilings and average insulation. Higher ceilings, large windows, or poor insulation may require more.
What’s the difference between a hot water and steam radiator system?
Hot water (hydronic) systems circulate heated water through the radiator. Steam systems use pressurized steam. Steam systems generally deliver more BTUs per section or foot of radiator, so the sizing math is different — this calculator handles both automatically.
What is a cast iron baseboard radiator?
A cast iron baseboard radiator runs along the base of a wall, similar in footprint to a fin-tube baseboard but made from cast iron. It delivers more radiant heat and lasts significantly longer. Consumers Supply carries Burnham Baseray cast iron baseboard in sizes from 18 inches up to 12 feet.
What is a slenderized radiator?
A slenderized radiator is a freestanding cast iron radiator with a slim, upright profile — narrower than a traditional column radiator. They are sold in sections and work with both hot water and steam systems. We carry Burnham slenderized radiators in 4-tube and 6-tube configurations.
What are the different sizes of slenderized radiators?
Burnham slenderized radiators come in three models, each sold by the section: the 4-tube 19″ high, the 4-tube 25″ high, and the 6-tube 25″ high. All three work with both hot water and steam systems. The 6-tube 25″ high delivers the most BTUs per section, making it the most efficient choice for larger rooms — you’ll typically need fewer sections to hit your heating target, which can mean lower cost and a more compact installation.
Can I use this calculator for a whole house?
Yes — use the “Add Another Room” button to size multiple rooms at once. The calculator will produce a separate recommendation for each room.
Where can I buy Burnham cast iron radiators in Chicago?
Consumers Supply Company at 1110 W. Lake St., Chicago, IL 60607 stocks Burnham Baseray cast iron baseboard and Burnham slenderized radiators. We serve homeowners, plumbers, and contractors throughout the Chicago area — and we ship nationwide, so you can order from anywhere in the country.
