How many BTUs do I need to heat a room?

For a 12 by 14 foot bedroom, 168 square feet, the four Canadian rules of thumb on this page land between 5,040 and 10,080 BTU per hour. That is a factor of two, on one room, from four sources that all describe themselves as guidelines for Ontario. Pick a number from the middle of it and you have made a choice, not a calculation. What the range is genuinely good for is telling you whether a heater or a mini split head you are looking at is in the right order of magnitude.

Why does this page give four answers instead of one?

Because four sources publish four different rules and averaging them would invent a fifth that nobody stands behind. HRAI's guideline is 30 to 45 BTU per square foot. Dynamic Heat & Cool publishes 30 to 50 for Ontario homes. HVAC Market in Toronto publishes 30 to 60 for moderate insulation. Ontario Heat Loss quotes 40 as the common shorthand and then spends its page arguing that square-footage sizing fails. Showing you the disagreement is more useful than hiding it.

Why is there no box for ceiling height or windows?

Because none of the four sources publishes a multiplier for them. A room with a nine foot ceiling really does lose more heat than the same room with eight, and a corner room with two exterior walls and a picture window loses far more than an interior one. Adding a factor for any of that would mean making a number up and presenting it as research. The variables are real, which is exactly why a CSA F280 calculation exists to measure them.

How do I convert BTU to watts, or to tons?

One watt is 3.412142 BTU per hour, so dividing a BTU figure by 3.412142 gives watts, which is the unit an electric heater is sold in. One ton of refrigeration is 12,000 BTU per hour, roughly 3,517 watts, which is how heat pumps and air conditioners are sold. Both are unit definitions rather than estimates, and this page prints all three so you can match a room to whatever the equipment is labelled in.

Is a rule of thumb good enough to buy equipment with?

Not according to the people who publish the rules. Ontario Heat Loss reports that most Ontario furnaces are oversized by 30 to 50 percent, and points at square-footage sizing as the cause. Its own published design heat loss for a 2,000 square foot GTA house is 20,000 to 26,000 BTU per hour, while the widest rule of thumb on this page puts the same house at 120,000. Ontario Building Code 9.33.2.2 requires the real figure to come from a stamped CSA F280 calculation for anything needing a permit.

Does the same number work for cooling?

No, and only one of the four sources publishes both. Dynamic Heat & Cool puts Ontario cooling at 20 BTU per square foot against 40 for heating, so a room needs roughly half as much cooling capacity as heating capacity by that rule. The calculator prints the cooling figure beside the heating ones rather than in place of them, because a single system sized for one is rarely ideal for the other.

How Many BTUs Does One Room Need?

Last updated: August 25, 2026By HeatPumpIndex Evidence Desk

Type in the room and this applies every BTU-per-square-foot rule published by a named Canadian source, all four of them, side by side. They disagree by a factor of two, which is worth seeing before you buy anything sized by one of them.

Measure the room, then read four different answers

Room size

Length

Width

feet

That is 168 sq ft, or 15.6 m².

There is no box here for ceiling height, window count or insulation, and their absence is the honest part. Every rule below is published as a flat rate per square foot, and none of the four sources publishes a multiplier for any of those. Inventing one would make this page look more precise and be less true.

Four published rules, one room

5,040 to 10,080

BTU/h, which is 1,480 to 2,950 watts, or 0.42 to 0.84 tons.

The four sources disagree by 5,040 BTU/h about this one room. That is the finding, not a rounding error.

Rule by rule

HRAI guideline 30 to 45 BTU/sq ft5,040 to 7,560HRAI's guideline for high efficiency furnaces in colder climates like Ontario.
Dynamic Heat & Cool 30 to 50 BTU/sq ft5,040 to 8,400What Dynamic Heat & Cool publishes for Ontario homes generally.
HVAC Market, Toronto 30 to 60 BTU/sq ft5,040 to 10,080HVAC Market's general guideline for a house with moderate insulation.
The common shorthand 40 BTU/sq ft6,720The shorthand Ontario Heat Loss quotes, under a heading arguing against it.
Cooling, at the one published cooling rate 20 BTU/sq ft3,360

All four rules are written for whole houses, not single rooms. A corner bedroom with two exterior walls and a big window loses far more than an interior one of the same size, and no rate per square foot can see the difference.

What the disagreement is actually telling you

Take the four rules to a whole house rather than a room and the problem becomes impossible to miss. At 2,000 square feet, HVAC Market’s guideline gives 60,000 to 120,000 BTU per hour, and the company publishes that worked example on its own page. Ontario Heat Loss publishes real design heat loss for a well built 2,000 square foot GTA house at 20,000 to 26,000 BTU per hour. The top of the rule of thumb is more than four times the top of the measured figure.

Both numbers are honestly published and neither is a mistake. They are answers to different questions. A rate per square foot is a way of picking equipment off a shelf without measuring anything, and it errs upward because an undersized furnace generates a complaint while an oversized one only generates a slightly worse life. A design heat loss is what the building actually loses on the coldest day the code sizes for.

So the useful way to read this calculator is as a sanity check with a wide tolerance. If a heater or a mini split head you are considering sits somewhere inside the span, nothing is obviously wrong. If it sits well outside it in either direction, something needs explaining before you buy.

Why a room is harder than a house

All four rules are written for whole houses, and a house averages out. Its interior rooms lose nothing to the outside, its corner rooms lose through two walls, its south-facing rooms gain sun for part of the day and its basement sits against ground at a steady temperature. Applied to the whole building those cancel into something a flat rate can approximate.

One room does not average anything. A 168 square foot bedroom in the middle of a second storey, with one small window, shares five of its six surfaces with heated space. The same 168 square feet on a corner, above an unheated garage, with a window wall, is a different heating problem entirely, and the two can differ by more than the whole span this page prints.

That is not an argument against the calculator, it is an argument for what to do with the answer. Use it to check the order of magnitude, then let whoever is installing the equipment measure the room properly. If you are sizing one ductless head for one room, the person quoting it should be doing a load calculation for that room, not multiplying its floor by forty.

Frequently asked questions

How many BTUs do I need to heat a room?: For a 12 by 14 foot bedroom, 168 square feet, the four Canadian rules of thumb on this page land between 5,040 and 10,080 BTU per hour. That is a factor of two, on one room, from four sources that all describe themselves as guidelines for Ontario. Pick a number from the middle of it and you have made a choice, not a calculation. What the range is genuinely good for is telling you whether a heater or a mini split head you are looking at is in the right order of magnitude.
Why does this page give four answers instead of one?: Because four sources publish four different rules and averaging them would invent a fifth that nobody stands behind. HRAI's guideline is 30 to 45 BTU per square foot. Dynamic Heat & Cool publishes 30 to 50 for Ontario homes. HVAC Market in Toronto publishes 30 to 60 for moderate insulation. Ontario Heat Loss quotes 40 as the common shorthand and then spends its page arguing that square-footage sizing fails. Showing you the disagreement is more useful than hiding it.
Why is there no box for ceiling height or windows?: Because none of the four sources publishes a multiplier for them. A room with a nine foot ceiling really does lose more heat than the same room with eight, and a corner room with two exterior walls and a picture window loses far more than an interior one. Adding a factor for any of that would mean making a number up and presenting it as research. The variables are real, which is exactly why a CSA F280 calculation exists to measure them.
How do I convert BTU to watts, or to tons?: One watt is 3.412142 BTU per hour, so dividing a BTU figure by 3.412142 gives watts, which is the unit an electric heater is sold in. One ton of refrigeration is 12,000 BTU per hour, roughly 3,517 watts, which is how heat pumps and air conditioners are sold. Both are unit definitions rather than estimates, and this page prints all three so you can match a room to whatever the equipment is labelled in.
Is a rule of thumb good enough to buy equipment with?: Not according to the people who publish the rules. Ontario Heat Loss reports that most Ontario furnaces are oversized by 30 to 50 percent, and points at square-footage sizing as the cause. Its own published design heat loss for a 2,000 square foot GTA house is 20,000 to 26,000 BTU per hour, while the widest rule of thumb on this page puts the same house at 120,000. Ontario Building Code 9.33.2.2 requires the real figure to come from a stamped CSA F280 calculation for anything needing a permit.
Does the same number work for cooling?: No, and only one of the four sources publishes both. Dynamic Heat & Cool puts Ontario cooling at 20 BTU per square foot against 40 for heating, so a room needs roughly half as much cooling capacity as heating capacity by that rule. The calculator prints the cooling figure beside the heating ones rather than in place of them, because a single system sized for one is rarely ideal for the other.

Where each rule comes from

Four Canadian sources, each publishing its own rate and its own wording about what the rate is for. Nothing here is averaged, blended or converted from a United States guide, and the scope sentence beside each rule in the calculator is the source’s own description rather than HeatPumpIndex’s summary of it.

Where to take this next

A whole house is a different question from a room, and it has a better-founded answer: published design heat loss by region rather than a rate per square foot. If what you are sizing is one ductless head for one room, the cost guide below is what those actually go for in this market.

Size a whole house from design heat loss Turn BTU into tons What one ductless head costs