Calculate cooling degree days from daily temperatures. Estimate CDD for energy analysis, air conditioning cost planning, and cooling season comparisons.
Cooling Degree Days (CDD) measure how much and for how long the outdoor temperature was above a base temperature (typically 65°F). Each degree above the base for one day equals one CDD. If the average daily temperature is 85°F, that day has 20 CDD.
CDD is the cooling counterpart to HDD. Locations with high CDD have high air conditioning demand. Miami has about 4,300 CDD annually while Minneapolis has only 700. CDD data helps estimate cooling energy costs, compare seasonal AC bills, and size cooling equipment.
This calculator computes CDD from daily high and low temperatures. Use it to estimate monthly or seasonal cooling loads and compare cooling cost expectations.
Understanding this metric in precise terms allows energy managers to evaluate investment options, forecast savings, and build compelling business cases for efficiency upgrades and retrofits. Tracking this metric consistently enables energy professionals and facility managers to identify consumption trends and implement efficiency improvements before costs escalate unnecessarily.
CDD data helps you understand and predict air conditioning costs. By normalizing cooling bills against CDD, you can detect changes in system efficiency and compare cooling costs across years with different weather. Consistent measurement creates a reliable baseline for tracking energy efficiency improvements and validating the impact of conservation measures and equipment upgrades over time.
Daily CDD = max(0, Average Daily Temperature − Base Temperature) Average Daily Temperature = (High + Low) / 2 Monthly CDD = Daily CDD × Number of Days
Result: 620 CDD for July
A July with average high 95°F and low 75°F: Average temperature = 85°F. Daily CDD = 85 − 65 = 20. Monthly CDD = 20 × 31 = 620 CDD.
The southern US dominates in CDD: Phoenix 4,400, Miami 4,300, Houston 2,900. The Midwest and Northeast have much lower CDD: St. Louis 1,700, Chicago 900, Minneapolis 700, Boston 700. This explains the enormous regional variation in air conditioning costs.
CDD values underestimate cooling loads in humid climates because they don't account for the energy needed to dehumidify air. In Houston, the dehumidification load can equal the sensible cooling load. Peak latent loads often occur on muggy, overcast days that don't generate many CDD.
CDD values are increasing across the US. Many locations have seen 10–15% increases in CDD since the 1970s. This trend means cooling costs will continue rising, making AC efficiency improvements increasingly valuable. Planning for future CDD may justify investing in higher SEER equipment today.
A cooling degree day represents one degree of average temperature above the base (65°F) for one day. A day with an 80°F average has 15 CDD. A week at 80°F averages 105 CDD. More CDD means more cooling energy required.
The 65°F base is standard but may not be ideal for all buildings. Heavily insulated buildings with high internal gains may need cooling above 55°F. Many analysts use 65°F for consistency and comparability across buildings.
Annual Cooling Cost ≈ UA × CDD × 24 / (SEER × 1,000) × electricity rate. For a home with UA=600, 2,000 CDD, SEER 14, and $0.13/kWh: cost ≈ 600 × 2,000 × 24 / (14 × 1,000) × 0.13 = $268.
Degree days only measure sensible heat (temperature). Latent heat (humidity) adds 20–50% to cooling loads in humid climates. This is why CDD-based estimates underpredict cooling costs in the Southeast and Gulf Coast.
NOAA provides free historical CDD data by station and zip code. Degreedays.net offers custom base temperature calculations and longer historical periods. Your utility may also report CDD on monthly bills.
IECC climate zones consider both HDD and CDD. Zone 1 (very hot) has 5,000+ CDD. Zone 2 has 3,500–5,000 CDD. Zones 3–4 are mixed. Zones 5–7 are heating-dominated with fewer than 2,000 CDD.