What this calculator does
Find out roughly how big a furnace or space heater you need. Enter the floor area, ceiling height, how well the space is insulated and how cold it gets outside. The calculator estimates the heat the space loses on a cold day. That's the heat your heater has to replace, in BTU per hour (BTU/h) and kilowatts.
It then suggests a rounded size to shop for. It's a quick estimate for a room or a simple one-story home. A Manual J load calculation by a heating professional is the proper way to size a furnace. It measures each wall, window and duct in your actual house.
Where it's used
Why this math matters outside the classroom:
- Choosing a space heater. A 150 ft² bedroom on a slab with one outside wall loses about 2,634 BTU/h at a 60 °F rise, so a 1,000 W heater covers it.
- Comparing insulation. The same 1,500 ft² home needs far less heat at the good insulation level than at the poor one, showing what upgrades are worth.
- Furnace replacement. A homeowner finds about 35,934 BTU/h of loss and divides by 0.95 to get a 37,825 BTU/h furnace input.
- Science classes. Students multiply area by temperature difference and divide by R-value to see how insulation slows heat flow.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.EE.A.2.c. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose feet and °F or meters and °C, then enter the floor area and ceiling height.
- Pick the insulation level, how many walls face outdoors, what's under the floor and where the heating ducts run.
- Enter the indoor temperature you want and your area's outdoor design temperature.
- Choose a furnace or electric heat. Then read the heat needed, the kilowatts and the size to look for.
What the inputs mean
- Insulation and air sealing
- Each level stands for typical construction. Poor means walls near R-4, an R-11 attic, single-pane windows, drafts and an uninsulated floor. Average means R-13 walls, an R-30 attic, double-pane windows and an R-19 floor. Good means R-21 walls, an R-49 attic, low-e windows, tight air sealing, an R-30 floor and slab-edge insulation.
- Walls that face outdoors
- Walls shared with other heated rooms lose very little heat. For a bedroom with one outside wall, choose about a quarter. For a corner room, choose about half.
- What's under the floor
- A slab loses most of its heat through its outside edge, so the calculator uses the edge length. Over an unheated crawlspace or basement, heat goes down through the floor. Over a heated basement, the floor loses almost nothing.
- Heating ducts
- Ducts running through a cold attic, crawlspace or garage leak air and lose heat through their walls before it reaches the rooms. The calculator adds 20% for that. Ducts inside the heated space add nothing, because their lost heat stays indoors.
- Outdoor design temperature
- Use a cold but typical winter low for your area, the kind heating designers plan for. It's not the coldest day on record. Local building departments and heating contractors can tell you the usual figure.
- Furnace efficiency (AFUE)
- The AFUE rating, the share of the fuel's heat that reaches your home over a year. It's on the furnace's yellow energy label or nameplate.
Formula and method
temperature rise ΔT = indoor temperature − outdoor design temperature outside wall area = 4 × √(floor area) × ceiling height × share of walls outdoors walls = 85% of wall area × ΔT ÷ R-value; windows = 15% of wall area × U-factor × ΔT ceiling = floor area × ΔT ÷ ceiling R-value air leakage = 0.018 × floor area × height × air changes per hour × ΔT slab floor = F-factor × outside slab edge × ΔT; crawlspace floor = floor area × ΔT ÷ floor R-value house loss = walls + windows + ceiling + air + floor heat needed (BTU/h) = house loss × 1.20 if ducts run through unheated space; kW = BTU/h ÷ 3,412.14 furnace input = heat needed ÷ efficiency
Heat leaks out faster the colder it is outside, so every part is multiplied by ΔT, the temperature rise you want. An R-value measures how well a material resists heat flow; higher is better. A U-factor is the reverse, used for windows; lower is better.
To keep it simple, the calculator pictures the space as a square box. It assumes 15% of the outside wall area is windows and doors. It counts heat lost up through the ceiling into an attic.
A slab's F-factor is the heat lost per hour for each foot of outside edge and each °F. The calculator uses 0.73 for a slab with no edge insulation and 0.54 for one with R-10 insulation 24 inches deep. These are the standard values in the appendix of the ASHRAE 90.1 energy standard.
Over a crawlspace or unheated basement, the full temperature rise is used. A vented crawlspace can get nearly as cold as outdoors, so this errs toward a big enough heater. The 20% for ducts is a rule of thumb; common guidance puts it at about 15% to 25%.
Air leakage uses 0.018, the heat needed to warm one cubic foot of air by 1 °F, per hour, for each air change. Air changes per hour is how many times the house's air is swapped for outside air in an hour. The levels use 1.5, 0.75 and 0.35.
These are the same rules the heat-loss calculator uses for each surface. That calculator lets you enter every area and R-value yourself.
Worked example
A 1,500 ft² one-story home with 8 ft ceilings sits on a concrete slab and has average insulation. Its ducts run through the attic. It's kept at 70 °F, with a design low of 10 °F, and heated by a 95% efficient gas furnace.
- Temperature rise: 70 − 10 = 60 °F.
- Outside wall area: 4 × √1,500 × 8 = 1,239.35 ft², of which 185.90 ft² is windows and 1,053.45 ft² is solid wall.
- Walls: 1,053.45 × 60 ÷ 13 = 4,862.08 BTU/h. Windows: 185.90 × 0.5 × 60 = 5,577.10 BTU/h.
- Ceiling: 1,500 × 60 ÷ 30 = 3,000 BTU/h. Air leakage: 0.018 × 12,000 × 0.75 × 60 = 9,720 BTU/h.
- Slab edge: 0.73 × 154.92 ft of edge × 60 = 6,785.47 BTU/h. The house loses 29,944.65 BTU/h in all.
- Ducts in the attic: 29,944.65 × 1.2 = 35,933.58 BTU/h, or 10.53 kW. That's about 23.96 BTU/h per square foot.
- Furnace input: 35,933.58 ÷ 0.95 = 37,824.82 BTU/h.
Result: The home needs about 35,934 BTU/h of heat. Rounded up, that points to a furnace with an input of about 40,000 BTU/h.
Understanding the result
The heat needed is what the heater must deliver on a design-cold day. On milder days it runs less. Choose the smallest heater whose output covers this figure.
Bigger isn't better. An oversized furnace heats in short bursts, which wastes fuel and leaves rooms uneven. That's one reason a professional Manual J calculation matters before buying a furnace.
The breakdown shows where heat escapes. In the example, air leakage is the largest part, followed by the slab edge and the ducts. Sealing gaps and moving or sealing ducts would cut the load the most.
Assumptions and limitations
- A square, one-story space with an attic above. Floor loss uses the slab edge, the floor R-value over a crawlspace, or nothing over heated space.
- Ducts outside the heated space add a flat 20%, a rule of thumb rather than a measurement.
- Windows and doors make up 15% of the outside wall area. Each insulation level uses the typical values listed above.
- Steady heat loss on a design-cold day. Sun, people, lights and appliances add free heat, which isn't counted.
- This is a quick estimate, not a load calculation for buying equipment. A Manual J load calculation by a professional is the proper way to size a furnace.
- Long, narrow or two-story homes, big windows, vaulted ceilings, heated basement walls and very leaky ducts all change the real load. The heat-loss calculator handles each surface in detail.
- Suggested sizes are rounded steps, not product listings. Check the output ratings of the heaters you can actually buy.
Frequently asked questions
How many BTUs do I need per square foot?
It depends on your climate and insulation, so a single figure can mislead. In the example, an average home on a slab, with attic ducts and a 60 °F rise, needs about 24 BTU/h per square foot. The same house at the poor level with an 80 °F rise needs about 65.5, nearly three times as much.
What size space heater do I need for a bedroom?
Try it here. A 150 ft² bedroom on a slab, with one outside wall, average insulation and a 60 °F rise, needs about 2,634 BTU/h, or 772 W. A heater set to 1,000 W would cover it.
Is the furnace size the input or the output?
Furnaces are often listed by input, the fuel they burn per hour. The heat they deliver is the input times the efficiency. A 95% furnace with a 30,000 BTU/h input delivers about 28,500 BTU/h.
Why does the calculator recommend a Manual J?
Manual J is the standard method professionals use to size heating and cooling. It measures each room's walls, windows, ducts and air leaks. This page uses a simple box shape, so its answer is a starting point.
How do I convert BTU/h to watts?
Divide by 3.41214. So 5,118 BTU/h is about 1,500 W, and 1 kW is 3,412.14 BTU/h.
How this calculator is checked
Method: Steady-state conduction Q = A × ΔT ÷ R (U × A × ΔT for windows) and sensible air leakage 1.08 × CFM × ΔT (0.018 × volume × ACH × ΔT), the same physics as the heat-loss calculator, applied to a square single-story model with typical R-values per insulation level; slab-edge loss with ASHRAE 90.1 Appendix A F-factors (0.73 uninsulated, 0.54 with R-10 for 24 in); crawlspace floor at full ΔT; 20% rule-of-thumb duct loss. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-26. Found a problem? Tell us.
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