Calculators
Heating sizing calculators
Furnace output and input, heat pump capacity and balance point, boilers, radiators, baseboards, space heaters, radiant floors and the cost per BTU of every fuel.
Heating load is the heat a house loses on the coldest design night, and it rises with the climate: a house in Minnesota loses two to three times as much heat per square foot as the same house in Georgia. The calculators in this group start from floor area and climate zone, adjust for insulation, ceiling height and windows, and then translate the load into the number printed on equipment, which is where most confusion starts.
A furnace is sold by input BTU, the gas it burns, while the house needs output BTU, the heat it delivers; the two differ by the AFUE efficiency. A heat pump is sold in tons that shrink as the outdoor temperature falls. A radiator is rated at a water temperature your boiler may not reach. Each calculator here does the conversion the label skips and says what standard, usually ACCA Manual J, a contractor would use for the final design.
9 calculators
Furnace Size Calculator
Heating load in BTU/hr and the furnace input to buy, from floor area, climate zone, insulation and AFUE.
Heat Pump Size Calculator
Tons for cooling and heating, capacity at your design temperature, balance point and backup heat.
Boiler Size Calculator
Boiler output and input from the radiators or the house, with the hot-water and pickup allowances shown.
Radiator BTU Output Calculator
BTU/hr and watts from a panel, column or cast-iron radiator at your water temperature, and the room it heats.
Baseboard Heater Calculator
Watts and feet of electric or hot-water baseboard a room needs, with the circuit current for information.
Space Heater Size Calculator
Watts and BTU for a space heater as supplemental or primary heat, and the nearest size sold.
Radiant Floor Heat Output Calculator
BTU/hr a heated floor delivers at a surface temperature, whether it covers the room, and the water temperature needed.
BTU, Watts, kW and Tons Converter
Heat rates in BTU/hr, MBH, watts, kW, tons, therms per hour and boiler horsepower, plus the energy over a run.
Heating Cost by Fuel Calculator
A season of heating priced for gas, propane, oil, heat pump, electric, pellets and wood, cheapest first, with cost per million BTU.
Guides for this topic
BTU per square foot by climate zone
Twenty BTU per square foot is right in St. Louis and wrong in Miami and Minneapolis. Here are the figures by climate zone, what changes them, and when to stop using them.
Comparing gas, propane, oil, electric and heat pumps per million BTU
Therms, gallons, kWh and tons do not compare directly. Turn each into dollars per million BTU of heat delivered and the cheapest fuel is obvious.
Heating and cooling degree days explained
Degree days measure how cold or hot a season was in a single number. Multiply by the house's heat loss per degree and you have the season's heating energy.
Furnace input, output and AFUE: reading the nameplate
A "100,000 BTU furnace" usually means 100,000 BTU of gas in, not heat out. Knowing which number is which is the difference between a right-sized furnace and a guess.
Heat pumps in cold climates: balance point and backup heat
A heat pump sized for summer can heat most of a northern winter. The question is what happens below the balance point, and the answer is a few lines of arithmetic.
Mini-splits: sizing each zone and the outdoor unit
A mini-split is sized twice, once for each room and once for the outdoor unit that serves them all. Most mistakes happen in the first step, with a 12k head in every bedroom.
Why an oversized AC or furnace costs you comfort and money
Bigger feels safer and works worse. Here is how an oversized system leaves a house clammy, cycles itself to an early end, and what to check before the next one is the same size.
Radiators and baseboard heaters: output, length and ΔT
A radiator rated at one temperature gives a different output at another. Once you know the ΔT rule, sizing emitters for a room and a boiler for the house is straightforward.
Thermostat setbacks: how much they save, and when they don't
Turning the heat down for eight hours saves about one percent of the bill per degree. The rule is simple; the exceptions are where people lose the saving.
Questions people ask
- What size furnace does a 2,000 square foot house need?
- With the climate-zone heating loads this site uses, a 2,000 sq ft house with average insulation needs about 76,000 BTU per hour of output in a mixed climate (zone 4), 90,000 in a cold one (zone 5) and 120,000 in a very cold one (zone 7). Divide the output by the furnace AFUE to get the input size on the label: 76,000 ÷ 0.95 is 80,000 BTU input for a condensing furnace. In a warm climate the same house needs only 40,000 to 60,000 BTU.
- What size heat pump do I need in climate zone 5?
- Size a heat pump for the cooling load first and check it against the heating load at the design temperature. A 2,000 sq ft zone 5 house has a cooling load near 35,000 BTU (3 tons) and a heating load near 90,000 BTU at 0 °F. A standard 3-ton heat pump keeps only about 45% of its capacity at 5 °F, so it covers roughly 16,000 BTU per hour, has a balance point near 40 °F and needs about 20 kW of backup heat below it. A cold-climate model keeps about 73% at 5 °F, which is why the calculator asks for the unit type and the design temperature.
- How many BTU does a radiator need to put out for a room?
- Match the room's heating load. A 150 sq ft bedroom with average insulation needs roughly 5,700 BTU per hour in zone 4 and 7,800 in zone 6. A double-panel radiator 24 in high and 40 in long delivers about 6,800 BTU per hour at the standard ΔT 50 °C rating, about 8,800 with 180 °F water, and roughly a quarter less than its rating at condensing temperatures. The radiator output calculator corrects any radiator for your water temperature.
- How many feet of baseboard heater does a room need?
- Standard electric baseboard delivers 250 watts per foot, and a room needs about 11 watts per square foot in a mixed climate, more in a cold one and less with good insulation. A 12 by 14 ft bedroom (168 sq ft) in zone 4 therefore needs roughly 1,900 watts, or 8 ft of baseboard; the same room in zone 6 needs about 2,600 watts, or 11 ft. Hydronic baseboard is rated per foot at a water temperature, typically 580 BTU per hour per foot at 180 °F and 450 at 160 °F.
- Which heating fuel is cheapest per BTU?
- It depends on local prices, but the order rarely changes. At the 2025 US average prices the site uses, natural gas in a 95% furnace costs about $16 per million BTU delivered, a heat pump at a seasonal COP of 2.8 about $18, cordwood about $21, pellets about $23, propane and heating oil about $31, and electric resistance heat about $50. The heating cost by fuel calculator uses your own prices and efficiencies and ranks the fuels for your house.
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