BTU Calculator
Adjust the inputs below. Results update as you type.
How it works
A rough rule: 20 BTU per square foot for standard rooms. Adjust upward for high ceilings, poor insulation, sunny exposures, and hot climates; adjust for kitchens (add 4,000 BTU) and heavily occupied spaces. Oversized units short-cycle and fail to dehumidify; undersized units run constantly. Energy Star sizing guidelines account for climate zone.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
Heating or cooling load in BTUs scales with room area and conditions. A rough rule is about 20 BTU per square foot for cooling, adjusted for sun, ceiling height, and occupancy.
Worked example
A 300-square-foot room needs roughly 300 × 20 = 6,000 BTU of cooling, more if it is sunny or holds many people.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Find productivity tools for everyday workflows
General recommendation, not professional advice.
Frequently asked questions
What is a BTU?+
A British thermal unit — the energy to raise one pound of water by 1°F. Air conditioners and heaters are rated in BTUs per hour.
How many BTUs do I need?+
Start from about 20 BTU per square foot for cooling, then adjust up for sunlight, high ceilings, kitchens, or many occupants.
What if I oversize the unit?+
An oversized AC cools fast but cycles on and off, leaving humidity high and wearing the unit. Right-sizing is more comfortable and efficient.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.
Guided next steps
Want the full workflow? Use this calculator inside a step-by-step guide.