BTU Calculator
Estimate the cooling capacity a room needs in BTU per hour from its size, ceiling height, sun exposure, insulation, and use, then read the matching tonnage and a suggested air conditioner size.
Units
Room size
The base rule assumes a standard 8 ft ceiling. A taller room holds more air, so its cooling load scales up.
Sun exposure
Insulation
Room use
Each person beyond two adds 600 BTU, and a kitchen adds 4,000 BTU for cooking heat. Everything runs in your browser. Nothing is uploaded.
This is an estimate. Window size, climate, floor position, and the number of outside walls all move the real figure, so treat it as a starting point.
How to calculate the BTU an air conditioner needs
Choose your units
Pick feet with square feet or meters with square meters so every measurement uses the same system.
Enter the room
Type the length and width, or switch to a total area you already measured, then set the ceiling height.
Describe the room
Set the sun exposure and insulation, count the people who use the room, and mark it if it is a kitchen.
Read the result
See the recommended BTU per hour, the equivalent tonnage, and a suggested air conditioner size to match.
Why use this tool
Room size or dimensions
Enter length and width and the area is worked out for you, or type a total floor area you have already measured.
Sun, insulation, and use
Adjust for a sunny or shaded room, how well it is insulated, how many people share it, and whether it is a kitchen, so the estimate reflects the real cooling load.
Tonnage and unit size
Alongside the recommended BTU per hour you get the equivalent tonnage and the next standard air conditioner size that covers it.
See the breakdown
Every adjustment is listed on its own line, from the base area figure to the sun, insulation, occupant, and kitchen allowances, so you can see how the number was built.
Feet or meters
Switch between square feet and square meters at any time. Your measurements convert automatically so the result stays the same.
Runs entirely in your browser
Every calculation happens on your device as you type. Nothing is uploaded, stored, or logged.
About this tool
This BTU calculator estimates the cooling capacity a room needs, measured in BTU per hour, and translates it into tonnage and a suggested air conditioner size. It starts from the widely used rule of about 20 BTU per square foot of floor space, then adjusts for the things that change a room’s real cooling load: ceiling height, how much direct sun the room gets, how well it is insulated, how many people usually share it, and whether it is a kitchen, where cooking adds a steady source of heat.
Enter the room as length and width or as a total area you already measured, in feet or meters, and set the ceiling height. A sunny room adds about ten percent while a shaded one subtracts about the same, and a poorly insulated room needs more than a well sealed one. Each person beyond two adds a small fixed amount, and a kitchen adds a larger fixed allowance. For working out the running cost of a unit, try the electricity cost calculator, and the unit converter helps with odd measurements.
Use the result as a starting point rather than a final specification. Window size, climate, floor position, and the number of exterior walls all move the true figure, so a professional load calculation is worth it for a whole home. Every calculation runs in your browser as you type, and nothing you enter is uploaded, stored, or logged.
Frequently asked questions
- How does the BTU calculator work?
- It multiplies the room floor area by about 20 BTU per square foot to get a base cooling load, scales it for ceiling height above the standard 8 feet, then applies your sun and insulation choices. It adds 600 BTU for each person beyond two and 4,000 BTU for a kitchen, and rounds the total to a tidy figure.
- How many BTU do I need per square foot?
- A common starting point is 20 BTU per hour for each square foot of floor space, so a 250 square foot room needs roughly 5,000 BTU before adjustments. Sun, insulation, ceiling height, occupants, and kitchen heat can push that up or down, which is why this tool asks for them.
- What is a ton of cooling and how does it relate to BTU?
- One ton of cooling equals 12,000 BTU per hour, a figure that dates from the cooling power of a ton of ice. The tool divides the recommended BTU by 12,000 to show the tonnage, so 12,000 BTU is one ton, 24,000 BTU is two tons, and so on.
- Why do kitchens and sunny rooms need more cooling?
- Cooking appliances give off a lot of heat, so a kitchen adds a fixed 4,000 BTU allowance. A room that gets strong sun for much of the day gains heat through its windows, so it adds about ten percent, while a heavily shaded room subtracts about the same.
- Is the estimate exact?
- It is a close estimate, not a final specification. Window size and orientation, local climate, whether the room is on a top floor, and how many walls face outside all affect the true load. For sizing a whole home or a heat pump, a professional load calculation is the reliable option.
- Is my data uploaded anywhere?
- No. Every measurement and result stays on your device. All calculations run in your browser as you type and nothing is sent to a server, stored, or logged.
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