A BTU calculator estimates how much heating or cooling capacity a room needs, in BTUs per hour (BTU/h). For a room air conditioner, the ENERGY STAR sizing chart is the standard starting point: a 250 to 300 sq ft room needs about 7,000 BTU/h, before adjusting for sun, people and kitchens. Contruva’s free BTU calculator applies that chart and its adjustments for cooling, and a simple area, ceiling height and climate rule of thumb for heating.
Open the free BTU calculator in your browser. No login, no email, nothing to pay.
Open the free toolIt gives the answer in BTU/h, tons and kilowatts, plus the nearest common unit size at or above the load. It works in feet or metres and needs no login. Use it for single rooms, additions, garages and quick ballpark checks. Whole-house furnaces, heat pumps and central air need a proper load calculation, which this page explains below.
How to use the BTU calculator
Cooling sizes a room air conditioner or a single-zone mini-split head. Heating estimates a room’s heat loss.
The calculator works out the floor area. Heating also asks for the ceiling height.
Heavily shaded, normal or very sunny; how many people regularly use the room; and whether the unit is cooling a kitchen.
The climate sets a heat loss factor in BTU/h per sq ft (W/m² in metric). You can edit the factor to match your own experience.
BTU/h, tons and kW, the nearest common size up, and a line-by-line breakdown of how the number was built.
BTU/h, tons and kW explained
A BTU (British thermal unit) is a quantity of heat. Equipment is rated by how much heat it moves per hour, so capacity is really BTU/h, even when the label just says “BTU”. Cooling equipment is often sold in tons, and a lot of Canadian heat pump and electric heater literature uses kilowatts.
| Unit | Equals | Example |
|---|---|---|
| 1 ton of cooling | 12,000 BTU/h | A 24,000 BTU/h unit is 2 tons |
| 1 kW | 3,412 BTU/h | A 1.5 kW baseboard heater is about 5,118 BTU/h |
| 1 BTU/h per sq ft | 3.15 W/m² | 50 BTU/h per sq ft is about 158 W/m² |
How many BTUs do I need to cool a room?
ENERGY STAR publishes a sizing chart for room air conditioners based on the floor area being cooled (energystar.gov, checked November 2026). The calculator uses these rows exactly. The chart assumes an 8 ft ceiling.
| Area to be cooled (sq ft) | Capacity needed (BTU/h) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
Rooms under 100 sq ft get the smallest row, 5,000 BTU/h. The chart stops at 2,500 sq ft, and so does the calculator: past that you are sizing a whole house, not a room. In metric mode, enter metres and the calculator converts the area to square feet for the chart (1 m² = 10.7639 sq ft). A 5 × 4 m bedroom is 20 m², or 215 sq ft, which reads off the chart at 6,000 BTU/h, about 1.76 kW.
Adjusting for sun, people and kitchens
ENERGY STAR lists four adjustments to the chart number, and the calculator applies all of them:
| Condition | Adjustment |
|---|---|
| Heavily shaded room | Reduce capacity by 10% |
| Very sunny room | Increase capacity by 10% |
| More than two people regularly use the room | Add 600 BTU/h for each extra person |
| The unit is used in a kitchen | Increase capacity by 4,000 BTU/h |
The calculator applies the 10% to the chart capacity, then adds the people and kitchen amounts on top.
Example: a sunny family room
A 20 × 14 ft family room (280 sq ft) with big south-facing windows, used by three people most evenings. These are the calculator’s defaults.
280 sq ft falls in the 250 up to 300 row.
Very sunny: 10% of 7,000.
Three people is one over two.
0.69 tons or 2.43 kW. The nearest common size up is a 9,000 BTU/h unit.
Example: a shaded open kitchen
A 500 sq ft open kitchen and eating area on the shaded side of the house, two regular occupants.
Chart 12,000 BTU/h · shaded −1,200 · kitchen +4,000 = 14,800 BTU/h. The nearest common size up is 15,000 BTU/h.
How many BTUs do I need to heat a room?
There is no ENERGY STAR-style chart for heating, because heat loss depends on how cold it gets outside, the insulation, the windows and how leaky the room is. The calculator uses a simple rule of thumb that is fine for a space heater, a garage or a quick check on an addition:
| Step | Formula |
|---|---|
| Floor area | Length × width |
| Ceiling height factor | Ceiling height ÷ 8 ft (or ÷ 2.44 m) |
| Heat loss factor | BTU/h per sq ft for the climate (editable) |
| Insulation multiplier | Good × 0.8 · average × 1.0 · poor × 1.25 |
| Heating load | Area × ceiling factor × heat loss factor × insulation multiplier |
Example: an addition in a cold climate
A 20 × 20 ft (400 sq ft) family room addition with a 9 ft ceiling in a cold climate, using the calculator’s default factor of 50 BTU/h per sq ft and average insulation.
9 ÷ 8 = 1.125.
400 × 1.125 × 50 × 1.0 = 22,500 BTU/h, which is 1.88 tons or 6.59 kW.
The nearest common size up.
The same room comes out at 18,000 with good insulation (× 0.8) and 28,125 with poor insulation (× 1.25).
That spread is the point: insulation and air sealing swing the number as much as the climate does. If you’re choosing a heat pump for that addition, its capacity also drops as the outdoor temperature falls, so check the manufacturer’s capacity at your local design temperature, not just the nameplate rating.
Climate factors for Canadian jobs
Published rules of thumb for heating usually fall somewhere around 30 to 60 BTU/h per sq ft from mild to cold climates, and they vary a lot from source to source. None of them come from a standard. The calculator’s defaults sit inside that range and are meant as a starting point you adjust:
| Climate setting | Default factor | Metric | Example areas |
|---|---|---|---|
| Mild winters | 30 BTU/h per sq ft | 95 W/m² | Coastal BC |
| Moderate | 40 BTU/h per sq ft | 126 W/m² | Southern Ontario, the Maritimes |
| Cold | 50 BTU/h per sq ft | 158 W/m² | The Prairies, Ottawa, much of Quebec |
| Very cold | 60 BTU/h per sq ft | 189 W/m² | Northern Canada |
If you’ve done load calculations on similar houses in your area, use the factor those calculations support instead. A new build to current code with good air sealing can need far less than an old house with original windows. The editable field is there so the calculator matches what you see on real jobs.
Why bigger isn’t better
It’s tempting to round up a size or two “to be safe”. The sources that study this say not to:
- Clammy rooms. ENERGY STAR says an oversized room air conditioner cools the room before it has removed enough humidity, leaving it cool but damp.
- Short cycling. The U.S. Department of Energy says oversized units cycle on and off frequently, which lowers efficiency, raises maintenance costs and means they don’t dehumidify as well as a properly sized unit.
- More money up front. A bigger unit costs more to buy and install, and ENERGY STAR notes it doesn’t cool any better.
That’s why the calculator shows the nearest common size at or above the load, not the next size up from that. If the load lands just over a size break, look at the room again before jumping a size: shading, a ceiling fan or better windows can bring it back down.
When you need a load calculation instead
Rules of thumb are for single rooms and ballpark checks. For a furnace, a whole-house heat pump, a boiler or central air, size the equipment from a room-by-room load calculation:
| Where | Standard | What it covers |
|---|---|---|
| Canada | CSA F280, “Determining the required capacity of residential space heating and cooling appliances” (current edition F280-12, reaffirmed 2021) | Heating and cooling capacity for houses covered by Part 9 of the National Building Code |
| United States | ACCA Manual J (load) and Manual S (equipment selection) | The method the U.S. Department of Energy recommends contractors use for residential cooling loads |
In Canada, HRAI publishes an F280 residential worksheet, and some heat pump rebate programs ask for a copy of the load calculation, so check the program rules before you quote. Codes and rebate rules change; confirm what applies in your province (checked November 2026).
This calculator is a rule-of-thumb estimate for single rooms. It doesn’t replace a CSA F280 or Manual J calculation, the manufacturer’s data, or your local code.
From BTUs to a quote
Once you know the size, the quote is the equipment, line set or venting, electrical, labour, permit and your markup. Work out your labour rate with the hourly rate calculator, price the lot with the markup calculator, and if the job includes new walls or flooring, the square footage calculator handles the areas.
Contruva is built for HVAC contractors who quote on site. Build the quote from your price catalog while you’re still in the customer’s house, they sign on their phone and pay the deposit, and the install books into the schedule. When it’s done, the quote becomes the invoice with online payments. Everything is in one plan, Contruva Pro, at $249.99 CAD a month. Try it free for 7 days, no credit card needed.