BTU / Room Volume Calculator
Results update as you type, there is no submit button.
Enter a room's length, width and ceiling height, and this calculator returns both the room's volume in cubic feet and an estimated cooling load in BTU, starting from the standard 20 BTU per square foot baseline and adjusting it for ceilings taller or shorter than the 8 ft the baseline assumes.
The Three-Step Method
Step 1, Room volume (ft³) = length × width × ceiling height
Step 2, Baseline load ≈ floor area (sq ft) × 20 BTU
Step 3, Adjust the baseline for ceiling height above or below 8 ft
The common rule of thumb starts from floor area at roughly 20 BTU per square foot, which puts a 300 sq ft room at about 6,000 BTU, but that figure quietly assumes a standard 8 ft ceiling. Where the ceiling is taller, the volume of air that actually needs conditioning rises proportionally, so a 12 ft ceiling in that same room means half again as much air, and the load needs scaling up to match. This calculator applies that height adjustment automatically rather than leaving you to remember it.
Air Changes, CFM and Why Volume Matters Beyond Cooling
Airflow for ventilation systems is specified in CFM, cubic feet per minute, and the number of air changes a system delivers per hour is calculated by multiplying CFM by 60 and dividing by the room's volume in cubic feet. That makes room volume the starting input for ventilation sizing just as much as for heating and cooling, which is why this calculator surfaces the plain cubic-feet figure alongside the BTU estimate rather than hiding it. Air purifiers, dehumidifiers and extractor fans are all rated against a room volume in the same way, so the number calculated here has uses well beyond a single HVAC purchase.
If you're sizing several rooms in a house, running each one's floor area through the square-feet-to-cubic-feet calculator first can be a faster way to get each room's volume before bringing the figures back here for the BTU step.
Worked Example: A Bedroom With a Vaulted Ceiling
A 14 ft × 12 ft bedroom with a 10 ft ceiling
- Floor area: 14 × 12 = 168 sq ft
- Baseline load: 168 × 20 = 3,360 BTU
- Height adjustment: 3,360 × (10 ÷ 8) = 4,200 BTU
Plan for roughly 4,200 BTU of cooling capacity for this room.
Without the height adjustment, the same room would look like it only needed 3,360 BTU, a meaningful undersizing for a unit that then runs constantly on hot days without ever quite catching up.
This is a planning estimate, not a substitute for a full load calculation. A proper assessment accounts for insulation values, air infiltration, window orientation and local climate, it's worth commissioning one before specifying equipment for a whole property.
Factors This Baseline Doesn't Capture
The 20 BTU per square foot baseline, even after the ceiling-height adjustment this calculator applies, is still a starting point rather than a finished answer, because it doesn't know anything about a room's insulation, windows or orientation. A well-insulated room with small, shaded windows on a north-facing wall needs meaningfully less cooling capacity than the baseline suggests; a poorly-insulated room with large south- or west-facing windows in direct sun needs meaningfully more. Both rooms could have the identical volume this calculator computes and still need noticeably different equipment.
Occupancy and equipment inside the room matter too, a home office with two computers running and a kitchen with an oven in regular use both add real heat load beyond what the room's bare volume accounts for. None of this means the volume-based baseline is wrong to start from; it means the figure this calculator produces is the number to walk into a conversation with an HVAC contractor holding, not the number to buy equipment against without any further adjustment.
Whole-House Sizing vs. Room-by-Room
A central air system is sized against a whole house's total load, not room by room, but adding up each room's individual figure from this calculator is still a useful way to sanity-check a contractor's proposed system size, and it's the right approach entirely for room-specific equipment like a window unit, a portable air conditioner, or a ductless mini-split serving a single room or zone. If you're sizing several rooms for a whole-house estimate, running each one through this calculator and keeping a running total gives you a number worth comparing against whatever a contractor's own load calculation software produces, the two won't match exactly, since a full Manual J load calculation accounts for far more than volume alone, but a wildly different total is worth asking about before signing off on equipment.
Heating Load vs. Cooling Load
This calculator's baseline is generally framed around cooling, but the same room-volume figure is the starting point for a heating load estimate too, the two loads respond to different factors even though they share the same underlying air volume. Cooling load is driven heavily by solar heat gain through windows and by outdoor temperature and humidity; heating load is driven more by insulation quality and how much heat the building envelope loses to the outside per degree of temperature difference. A room that needs relatively little cooling because it's shaded and well-insulated can still need substantial heating capacity in a cold climate, and vice versa for a sunny, poorly-insulated room in a mild climate that barely needs heat but overheats badly in summer.
Because of this asymmetry, don't assume a single BTU figure covers both directions, a heat pump or furnace sized purely off this calculator's cooling-oriented baseline may be undersized for heating in a cold climate, or oversized for cooling in a mild one. Use this page's volume figure as a shared starting point for both calculations, but apply climate-appropriate adjustments separately for each.
Open-Plan Spaces and Adjoining Rooms
Open-plan layouts, a kitchen, dining and living area sharing one continuous space, should be calculated as a single combined volume rather than as separate rooms added together after the fact, since air moves freely between them and a single system typically serves the whole space. Measure the combined footprint's length and width as one figure, run it through this calculator once, and size equipment against that combined total. Rooms separated by a door that's normally kept closed are better calculated separately, since each one is effectively its own thermal zone regardless of how the overall home is laid out.
Frequently Asked Questions
Why does BTU sizing depend on room volume instead of just floor area?
Heating and cooling equipment conditions the air in a space, and a room with a vaulted or unusually high ceiling has more air in it than a same-footprint room with a standard ceiling, floor area alone misses that extra volume entirely.
What does the 20 BTU per square foot baseline actually assume?
It assumes a standard 8-foot ceiling, the moment your ceiling is taller or shorter than that, the baseline needs the height adjustment this calculator applies automatically rather than being used as-is. Sunrooms and rooms with heavy appliance heat often need a separate multiplier layered on top of the same volume figure.
Is this calculator accurate enough to buy an air conditioner from?
It's a solid starting estimate, but a proper sizing also factors in insulation quality, window area and orientation, and local climate, treat this figure as a ballpark to compare units against, not a final spec for a purchase. Manufacturers publish their own sizing charts that weigh those factors more precisely once you have a model in mind.
How does room volume relate to CFM and air changes per hour?
Air changes per hour are calculated by multiplying a ventilation system's CFM (cubic feet per minute) rating by 60, then dividing by the room's volume in cubic feet, so the same volume figure this calculator produces is also the starting input for ventilation sizing.