Heat Loss Calculator

Estimate the hourly heat loss (BTU/h) for a room or home to properly size your heating system. Based on surface area, R-values, and design temperature difference.

⚠ Professional Notice This is a simplified heat loss estimate. A licensed HVAC contractor performing a Manual J calculation is required for permit-ready equipment sizing and optimal system performance.
Space Dimensions
Insulation & Construction
Climate

How Heat Loss Is Calculated

  1. : U = 1 ÷ R-value
  2. : Q (BTU/h) = U × Area (sq ft) × ΔT (°F)
  3. : Q_total = Σ (U_n × Area_n × ΔT)
  4. : Q_infiltration = Q_conductive × infiltration_factor

Pro Tips

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Frequently Asked Questions

How do I calculate heat loss for a room?

Heat loss (BTU/h) = U-value × Area (sq ft) × Temperature Difference (°F). U-value is 1/R-value (so R-19 insulation has U = 0.053). Temperature difference is your inside setpoint (68–70°F) minus your outdoor design temperature. Sum this calculation for all surfaces (walls, ceiling, floor, windows) and add 15–30% for infiltration.

What is a good R-value for walls and attics?

The DOE recommends: Attics in most US climates R-38 to R-60. Walls (2×4 framing): R-13 to R-15. Walls (2×6 framing): R-20 to R-21. Floors over unheated spaces: R-25 to R-30. These recommendations vary by climate zone — colder climates require higher R-values. Check your state energy code for specific requirements.

What temperature difference should I use for heat loss calculations?

Use your indoor design temperature (typically 68–70°F) minus your local 99% design temperature (coldest 1% of winter hours). Common design temperatures: Minneapolis: -12°F (ΔT ≈ 82°F), Chicago: -4°F (ΔT ≈ 74°F), New York: 11°F (ΔT ≈ 59°F), Atlanta: 19°F (ΔT ≈ 51°F), Dallas: 22°F (ΔT ≈ 48°F). ASHRAE Fundamentals Handbook lists design temperatures for all locations.

How accurate is an online heat loss calculator?

Online calculators provide useful estimates for initial planning and sizing, but a professional Manual J calculation is required for HVAC permit applications in most jurisdictions and is far more accurate. Manual J accounts for local climate data, exact construction details, window orientation, internal gains, and infiltration testing (blower door). Use this calculator for budgeting and understanding your home's energy profile.

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