🏠 Building Size
ft²

Total heated floor area of the building

9 ft
🏗️ Construction & Insulation

BTU/hr per ft² factor

🔥 Boiler & System Type
85%

Standard: 80–85%. High-efficiency: 90–98%

⚙️ Optional Add-ons
🔲 Unfinished Basement or Crawlspace
Adds extra floor heat loss
🌬️ High Infiltration / Air Leaks
Drafty doors, windows, and unsealed gaps
🏊 Domestic Hot Water Load
Include DHW demand in the boiler sizing
—
Estimated BTU/hr (updates live)

Calculating your heat load…

📊 Your Boiler Sizing Results
—
Recommended Boiler Output
—
BTU/hr · With 25% safety factor built in
📐
—
ft²
Floor Area
🔥
—
BTU/ft²
Heat Density
⚡
—
kW
In Kilowatts
📊
—
%
AFUE Efficiency
System Efficiency Rating
Efficient Oversized —

🔥 Heat Load Breakdown

Heat Loss Source
BTU/hr
%
Relative Load

🛢️ Boiler Size Comparison

Boiler Output
In kW
AFUE Needed
Fit

💰 Estimated Annual Fuel Cost

Monthly
—
avg. per month
Heating Season
—
6-month estimate
Annual
—
full year estimate
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🛠️ Expert Recommendations

How to Size a Boiler — What You Really Need to Know

Choosing the wrong boiler size costs you money every single day. Too small and your home stays cold. Too big and it short-cycles, wastes fuel, and breaks down sooner. Here’s how to get it right.

🌡️

What Is a BTU?

BTU stands for British Thermal Unit. It measures heat energy. One BTU is enough energy to raise 1 pound of water by 1°F. Boiler output is measured in BTU per hour (BTU/hr). The higher the number, the more heat it produces.

📐

Floor Area Is Just the Start

Square footage gives you a rough number. But ceiling height, insulation quality, window area, and your local climate all matter just as much. A poorly insulated 1,500 ft² home may need twice the BTU of a well-insulated one the same size.

⚠️

Why Oversizing Is a Problem

A boiler that is too large fires up and heats the water quickly. Then it shuts off before completing a proper heating cycle. This is called short cycling. It wastes fuel, causes temperature swings, and shortens the boiler’s life.

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What Is AFUE?

AFUE (Annual Fuel Utilization Efficiency) is a percentage. An 85% AFUE boiler turns 85% of fuel into heat. The other 15% goes up the flue. A 95% condensing boiler recaptures that flue gas — saving you real money every year.

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Heat Loss Through the Building

Your boiler replaces heat that escapes. Heat escapes through walls, windows, the roof, and the floor. It also escapes with air infiltration — drafts. Better insulation means less heat loss and a smaller, cheaper boiler.

🔧

Get a Professional Check Too

This calculator gives you an excellent starting point. But for a final boiler purchase, always ask your contractor to do a Manual J heat load calculation. It uses your exact room dimensions and is the industry standard method.

📐 How We Calculate Your Boiler BTU

Heat Load (BTU/hr) = Area (ft²) × BTU Factor × Climate × Age × Window × Safety Factor Boiler Output = Heat Load / AFUE%

The BTU factor comes from your insulation level (18–60 BTU/ft²). Climate, building age, and window multipliers adjust the base number up or down. We then apply a 25% safety factor so the boiler can handle peak cold days. Finally, we divide by AFUE efficiency — because a less-efficient boiler needs a higher rated output to deliver the same heat.

Boiler Size Reference Chart

Use this table as a quick reference. Your actual number depends on insulation, climate, and building age.

Home SizeWell InsulatedAveragePoor InsulationTypical Boiler
800 ft²20,000 BTU28,000 BTU48,000 BTU60,000 BTU model
1,200 ft²30,000 BTU42,000 BTU72,000 BTU80,000 BTU model
1,500 ft²38,000 BTU53,000 BTU90,000 BTU100,000 BTU model
2,000 ft²50,000 BTU70,000 BTU120,000 BTU130,000 BTU model
2,500 ft²63,000 BTU88,000 BTU150,000 BTU175,000 BTU model
3,500 ft²88,000 BTU122,000 BTU210,000 BTU225,000 BTU model
5,000 ft²125,000 BTU175,000 BTU300,000 BTU300,000+ BTU model

Frequently Asked Questions

A quick rule: multiply your square footage by 30–50 BTU for a rough estimate. A 1,500 ft² home with average insulation needs roughly 45,000–75,000 BTU/hr. But the real answer depends on your ceiling height, insulation, climate zone, and building age. Use this calculator to get a personalised number in seconds.
Input BTU is how much fuel energy the boiler burns. Output BTU is how much of that becomes useful heat. The difference is the efficiency (AFUE). A boiler with 100,000 BTU input and 85% AFUE delivers 85,000 BTU of heat output. Always size by output — that’s the heat your home actually gets.
In most cases, yes. A 95% AFUE condensing boiler uses about 10–15% less fuel than an 80% standard boiler. On a $2,000/year gas bill that’s $200–$300 saved every year. The condensing boiler typically costs $800–$1,500 more upfront — so it pays back in 4–7 years. After that, you just save money every heating season.
Often yes. Many older homes have oversized boilers from decades ago when insulation standards were lower. If you’ve added insulation or upgraded windows since the boiler was installed, you may be able to go smaller. Run this calculator with your current building specs. If your current boiler is significantly larger than the result, downsizing is worth considering.
We use your floor area as the base. Then we apply multipliers for insulation quality, climate zone, building age, and window type. The result is your estimated heat load in BTU/hr. We add a 25% safety factor so the boiler can handle peak cold days. Finally, we adjust for AFUE — because a less efficient boiler needs a higher output rating to deliver the same heat to your rooms.
Yes, if your boiler provides both space heating and domestic hot water (combi boiler or system boiler with cylinder). We add roughly 10–20% for DHW demand. If your boiler is heating only, turn off the DHW toggle in the calculator. A dedicated water heater handles DHW separately, so you don’t need to add it to the boiler load.
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