Understanding your result
The headline is the boiler input to look for, rounded up to the next 10,000 BTU per hour, with the gross output needed and the net load in the sub-line. Those three figures are the three lines on a boiler label. The net load is the heat the radiators deliver to the rooms, which should match the net I=B=R rating of a cast-iron boiler. The gross output adds your piping and pickup allowance and should match the DOE heating capacity. The input is the gas or oil burned and is the number in the model name.
The margin over the net load row shows how much bigger the stock boiler is than the house really needs. Because of the pickup allowance and the rounding, a conventional boiler lands 20 to 30% above the net load, which is normal for a cast-iron boiler and too much for a modulating condensing one; the notes say when to drop the allowance.
If you sized from the radiators, run the calculation again from the house and compare: a connected radiation far above the heat loss is a sign the system can run at lower water temperatures, which the radiator output calculator works out radiator by radiator and the baseboard heater calculator does for fin-tube. The heating cost by fuel calculator takes the same load and prices a season of gas, oil or propane.
How we calculate this
The house method uses the heating loads by climate zone from the BTU per square foot table, with the same insulation and ceiling factors as the furnace calculator. The radiator method adds up what is connected; the average output per radiator should be taken at the water temperature the system will actually run, which is what the radiator output calculator provides. The 15% pickup factor is the Hydronics Institute (I=B=R) convention that the net rating on cast-iron boilers is built from; the 20,000 BTU/hr indirect-tank allowance is a common rule for systems without priority control.
AFUE tiers and the venting they imply are on the efficiency ratings table: 80 to 85% for cast-iron boilers vented into a chimney, 90 to 96% for condensing boilers vented in plastic.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Pickup and piping allowance | 15% | Hydronics Institute I=B=R net rating convention (gross ÷ 1.15) |
| Indirect tank allowance | 20,000 BTU/hr (optional) | Common installer rule for systems without domestic hot water priority |
| Heating load per sq ft | By climate zone | Rule-of-thumb climate-zone charts; see the BTU per square foot table |
| Average radiator output | 6,000 BTU/hr | A double-panel radiator about 24 × 40 in at the standard ΔT 50 °C rating; replace with your radiators’ figures |
| AFUE | 85% | Mid-range cast-iron boiler; condensing boilers are 90 to 96% |
| Stock sizes | 10,000 BTU/hr steps | Residential boilers are sold in roughly 10,000 to 25,000 BTU steps |
Assumptions last reviewed October 7, 2026.
The calculator does not model the boiler’s turndown ratio, outdoor reset, buffer tanks for low-mass boilers on small zones, or the altitude de-rating of combustion equipment. Steam boilers are sized differently, by the connected square feet of equivalent direct radiation, and are outside its scope. The guide to radiators and baseboard heaters explains how outputs change with water temperature and why that matters for a new boiler.
Two worked examples
Sizing from the radiators
Eight radiators averaging 6,000 BTU/hr each, no indirect tank, 15% pickup, an 85% AFUE cast-iron boiler.
- Connected radiation: 8 × 6,000 = 48,000 BTU/hr (net load)
- Gross output: 48,000 × 1.15 = 55,200 BTU/hr
- Input: 55,200 ÷ 0.85 = 64,941 BTU/hr
- Boiler: 70,000 BTU/hr input, gross output 59,500; margin 24% over the net load
A 70,000 BTU/hr cast-iron boiler with a net I=B=R rating near 52,000 matches this system. The note asks the right question: do the radiators represent the house? If this is a 1,200 sq ft house in zone 4, its heat loss is about 46,000 BTU/hr and the radiators are about right; if it is an 800 sq ft house, they are generous and a 50,000 BTU/hr boiler running cooler water would serve it better.
Sizing from the house with a condensing boiler
2,400 sq ft in zone 6 (Minneapolis), good insulation, 8 ft ceilings, an indirect tank without priority, 10% pickup, a 95% AFUE condensing boiler.
- Space heating load: 2,400 × 52 × 0.85 = 106,080 BTU/hr
- Indirect tank: 20,000 BTU/hr; net load 126,080 BTU/hr
- Gross output: 126,080 × 1.10 = 138,688 BTU/hr
- Input: 138,688 ÷ 0.95 = 145,987 BTU/hr
- Boiler: 150,000 BTU/hr input, gross output 142,500; margin 13%
With priority control on the indirect tank the 20,000 BTU/hr comes off, the net load drops to 106,080 and a 130,000 BTU/hr boiler covers it with the same margin. For a modulating boiler that turns down to 25%, a 130,000 unit can run as low as 32,500 BTU/hr on mild days, which is why the smaller size is the better choice here.
Where to find your inputs
Radiator outputs. From the maker’s catalogue at your water temperature, or from the radiator output calculator using each radiator’s dimensions. For fin-tube baseboard, multiply the feet of element by its rating per foot (about 580 BTU/hr per ft at 180 °F).
Heated area and zone. As for the furnace calculator: conditioned area only, zone by county.
AFUE. On the yellow label or in the model specifications of the boiler you are considering. The existing boiler’s nameplate shows input and output; output ÷ input is close to its efficiency when new.
Pickup allowance. 15% for a cast-iron boiler on a system with long pipe runs or deep setbacks; 0 to 10% for a modulating condensing boiler, following the manufacturer’s sizing instructions.
Common mistakes
- Matching the gross output to the net I=B=R rating. The net rating already deducts 15%; matching your gross figure to it adds the allowance twice.
- Replacing like for like. The old boiler was sized to the radiators or to a colder, leakier house. Size to today’s heat loss.
- Keeping the 15% pickup on a condensing boiler. It pushes the boiler up a size and out of its condensing range more of the time.
- Adding the indirect tank on a priority system. Priority control means the tank borrows the heating output; the allowance is for systems that run both at once.
- Ignoring the turndown. A boiler that cannot modulate below the mild-day load will short-cycle whatever its size; check the minimum firing rate against the smallest zone.
- Sizing a steam boiler this way. Steam boilers are sized to the connected radiation in square feet of EDR, not to the heat loss.
Questions people ask
- How do I size a boiler for my house?
- Work out the house's heat loss on the design night, from a Manual J or from the climate-zone rule of thumb this site uses, add any domestic hot water load, add a piping and pickup allowance of up to 15% for a conventional boiler, and that is the gross output the boiler must deliver. Divide by the AFUE for the input on the label. A 2,000 sq ft house in zone 5 loses about 90,000 BTU per hour, so a conventional 85% boiler needs about 122,000 BTU per hour of input and a condensing boiler with no pickup allowance about 100,000.
- What do input, DOE heating capacity and net I=B=R rating mean?
- Input is the fuel burned per hour. DOE heating capacity (gross output) is the heat delivered to the water, which is the input multiplied by the efficiency. Net I=B=R rating, printed on cast-iron boilers, is the gross output divided by 1.15, a deduction for piping losses and warm-up that the Hydronics Institute standardised decades ago. Match the net rating to the connected radiation or the house's heat loss, and match the gross output to the load with your own pickup allowance added.
- Should I size a boiler to the radiators or to the heat loss?
- To the heat loss. Radiators were often installed generously, and houses have been insulated since, so the connected radiation can be 50% or more above what the house needs; a boiler sized to it will short-cycle. Sizing to the heat loss and running the oversized radiators at a lower water temperature gives even heat, longer cycles and, with a condensing boiler, higher efficiency. Use the radiator method only when you cannot estimate the heat loss, and compare the two.
- Do I need to add for an indirect water heater?
- Usually not. Most systems use priority control, which switches the whole boiler output to the indirect tank for a few minutes while the house coasts, and a boiler sized for the heating load recovers a 40-gallon tank in about half an hour. Add the 20,000 BTU per hour allowance only if the system runs heating and hot water at the same time without priority, or if the household draws a lot of hot water on cold mornings.
- Why does a condensing boiler need a smaller pickup allowance?
- The 15% pickup factor dates from cast-iron boilers that fired at one rate and had to bring cold pipes and radiators up to temperature after a setback. A modulating condensing boiler turns down to 20 or 25% of its rating and runs for long periods, so warm-up is gradual and pipe losses are small; manufacturers typically recommend sizing it to the heat loss with little or no pickup. Oversizing a condensing boiler also keeps it out of its condensing range more often.
- What is a typical boiler size for a house?
- Residential boilers run from about 50,000 to 200,000 BTU per hour of input. A well-insulated 1,500 sq ft house in a mixed climate needs around 60,000; an average 2,500 sq ft house in a cold climate around 150,000. Boilers are sold in roughly 10,000 to 25,000 BTU steps, so the calculator rounds the input up to the next 10,000. Anything above 200,000 is usually two boilers or a commercial unit.