AC BTU Calculator

Calculate required BTU (British Thermal Units) for air conditioning based on room size, ceiling height, and other factors. Estimate cooling power needed.

Standard ceiling height is 8 feet
Add 600 BTU per person

How This Tool Works

This calculator determines the required BTU (British Thermal Units) for air conditioning based on room size, ceiling height, sun exposure, and number of occupants. The base calculation uses 20 BTU per square foot, which is adjusted for various factors that affect cooling needs.

The tool accounts for ceiling height (higher ceilings need more cooling), sun exposure (sunny rooms need more cooling), and occupants (each person generates heat). The result is rounded to the nearest 1000 BTU to match standard AC unit sizes.

Formula

The calculation is based on:

Base BTU = Room Area (sq ft) × 20
Height Adjustment = (Ceiling Height - 8) × Room Area × 1.5
Sun Adjustment = Room Area × (10 for high, -5 for low, 0 for normal)
Occupant Adjustment = Number of People × 600
Total BTU = Base + Height + Sun + Occupant (rounded to nearest 1000)

Example: 180 sq ft room, 8 ft ceiling, normal sun, 2 people:

Base = 180 × 20 = 3,600 BTU
Height = (8 - 8) × 180 × 1.5 = 0
Sun = 0 (normal)
Occupant = 2 × 600 = 1,200 BTU
Total = 4,800 BTU (rounded to 5,000 BTU)

BTU Sizing Guidelines

  • 100-150 sq ft: 5,000 BTU (small room, window unit)
  • 150-250 sq ft: 6,000 BTU (medium room)
  • 250-350 sq ft: 8,000-10,000 BTU (large room)
  • 350-450 sq ft: 12,000 BTU (very large room, 1 ton)
  • 450-550 sq ft: 14,000 BTU
  • 550-700 sq ft: 18,000 BTU (1.5 tons)
  • 700-1000 sq ft: 24,000 BTU (2 tons)

Additional Considerations

  • Insulation: Well-insulated rooms need less BTU, poorly insulated need more
  • Windows: Large windows or many windows increase cooling needs
  • Climate: Hotter climates need more BTU per square foot
  • Appliances: Heat-generating appliances (ovens, computers) add to cooling load
  • Room Location: Top floors and rooms with exterior walls need more cooling
  • Whole House: For whole-house systems, consult an HVAC professional for proper sizing

FAQ

  • What is BTU?
    BTU stands for British Thermal Unit, a measurement of cooling (or heating) power. One BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. For air conditioners, higher BTU means more cooling power.
  • How many BTU do I need for my room?
    As a general rule, you need about 20 BTU per square foot of space. However, this varies based on ceiling height, sun exposure, number of occupants, insulation, windows, and climate. This calculator adjusts for these factors.
  • What if my room has high ceilings?
    Higher ceilings require more BTU because there's more air to cool. For every foot above 8 feet, add approximately 1.5 BTU per square foot. For example, a 10-foot ceiling adds about 3 BTU per square foot to the base calculation.
  • How does sun exposure affect BTU needs?
    Rooms with high sun exposure (south-facing, lots of windows) need 10-15% more BTU. Rooms with low sun exposure (north-facing, shaded) need about 5% less. This calculator adjusts for sun exposure automatically.
  • Should I round up or down for BTU?
    It's generally better to round up slightly rather than down. An AC unit that's slightly oversized will cool more efficiently and can handle temperature swings better. However, significantly oversized units can cause humidity issues. Aim for the calculated BTU or the next standard size up.
  • What other factors affect BTU requirements?
    Additional factors include: insulation quality, number and size of windows, climate zone, heat-generating appliances, room location (top floor vs. ground floor), and whether the room is well-sealed. This calculator provides a good starting point, but consult an HVAC professional for whole-house systems.

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