Quick answer
Cooling degree days (CDD) are a measure of how much cooling a place needs, found by subtracting 65°F from each day's average outdoor temperature and adding up the results over a month, season or year.
A day with a high of 90°F and a low of 66°F averages 78°F and counts as 13 CDD. The higher the annual total, the harder an air conditioner works and the more a high SEER2 rating is worth.
Quick facts
The key facts about Cooling Degree Days, with sources:
Key takeaways
- CDD adds up how far daily average temperatures rise above 65°F 1.
- It tracks a season's cooling need, so it scales AC electricity use.
- County normals run from about 80 CDD a year near Seattle to over 4,300 in Miami-Dade 2.
- More CDD means a higher SEER2 rating pays back faster.
- CDD ignores humidity, so humid places need more cooling than CDD alone suggests.
What cooling degree days measure
Cooling degree days are the summer mirror of heating degree days. EIA takes a day's high and low, averages them, and compares the result to 65°F. If the average is above 65°F, the difference is the day's CDD 1.
EIA describes a cooling degree day as a sign of a hot day that measures how much air conditioning is needed. A full summer's total is a fair way to compare how hard air conditioning works in two places.
The 65°F base is a convention. Many homes do not run the AC until it is warmer outside, because walls and shade buffer the heat for a while. The base still works well for comparisons.
Sources: [1]
How to calculate CDD
- Find the day's high and low outdoor temperature.
- Average them: (high + low) ÷ 2.
- Subtract 65°F from the average.
- If the answer is positive, that is the day's CDD; otherwise it is zero.
- Add the days for a month, season or year.
EIA's example: high 90°F, low 66°F, mean 78°F, so 13 CDD 1.
Kinds of CDD figures
| Figure | What it is | Best use |
|---|---|---|
| Daily CDD | One day's count | Tracking a heat wave |
| Monthly or season-to-date CDD | Sum for a period | Comparing this summer with last |
| 30-year normal | Average annual CDD, such as 1991–2020 | Comparing places; savings estimates 2 |
| Population-weighted CDD | Weighted by where people live | National electricity forecasts 1 |
Worked example: SEER2 savings scale with CDD
Annual CDD normals (base 65°F, 1991–2020) from NOAA county data 2. The kWh figures assume a home that needs 24 million BTU of cooling in a 2,000 CDD year, scaled by CDD, then divided by SEER2 (13.4 federal floor in the North vs 15.2). The scaling is a simple degree-day estimate.
| County | Annual CDD | Cooling need (million BTU, scaled) | kWh at 13.4 SEER2 | kWh at 15.2 SEER2 | kWh saved |
|---|---|---|---|---|---|
| King County, WA | 80 | 1.0 | 72 | 63 | About 8 |
| Cook County, IL | 833 | 10.0 | 746 | 658 | About 88 |
| Fulton County, GA | 1,668 | 20.0 | 1,494 | 1,317 | About 177 |
| Dallas County, TX | 2,679 | 32.1 | 2,399 | 2,115 | About 284 |
| Maricopa County, AZ | 3,724 | 44.7 | 3,335 | 2,940 | About 395 |
| Miami-Dade County, FL | 4,312 | 51.7 | 3,861 | 3,404 | About 457 |
The same efficiency upgrade saves about 50 times more electricity in Miami-Dade than in King County. Southern installs must already meet 14.3 SEER2 for most split ACs, so compare against that floor where it applies 4. Run your own numbers in the SEER2 savings calculator.
Worked example 2: a heat-wave week by hand
Take a July week with daily averages of 82, 85, 88, 90, 87, 80 and 76°F. Subtract 65 from each: 17, 20, 23, 25, 22, 15 and 11. The week totals 133 CDD.
A mild week of averages near 72°F would total about 49 CDD. So the heat wave needs close to three times the cooling of the mild week. If your AC used 200 kWh in the mild week, a similar house would use roughly 540 kWh in the heat wave, before humidity and sun are counted.
Divide your own cooling kWh by CDD week to week. A steady ratio means the system is behaving. A climbing ratio in similar weather points to a problem worth a service call.
CDD is not a sizing number
CDD measures a season. AC size is set by the design day, the hot afternoon near the top of the local range, and by how much heat the house gains on that day.
That is the job of a Manual J load calculation, which ACCA calls the ANSI-recognized standard for equipment sizing loads 5.
A county with modest CDD can still have hot design days, and a county with high CDD can have a house that gains little heat thanks to shade and insulation. Use CDD to judge efficiency, not size.
Sources: [5]
Where you see CDD
- On some electric bills, comparing this month's CDD with last year's.
- In AC and heat pump savings estimates.
- In energy audit weather normalization.
- In EIA summer electricity forecasts 1.
- In our county degree-day lookup (linked below).
What CDD tells you, and what it misses
What it does well
- Fair comparison of cooling need across places.
- Explains summer bill swings that come from weather.
- Helps decide how much a higher SEER2 rating is worth.
Where it falls short
- Ignores humidity, which drives latent load in the Southeast.
- Ignores sun on windows and roofs.
- Says nothing about the peak design day used for sizing.
- Night lows pull the daily average down, hiding hot afternoons.
Why CDD understates cooling in humid places
An air conditioner removes heat and moisture. CDD only counts temperature. Two places with similar CDD can have very different cooling loads if one is humid, because drying the air takes energy. That moisture part is the latent load.
Hot, dry places have the opposite twist. Big day-night swings lower the daily average, so CDD can look modest while afternoons are brutal. That is one reason DOE adds an EER2 floor, measured at 95°F, for split ACs installed in the Southwest 4.
How CDD drives cooling cost
Cooling cost is roughly CDD times the home's heat gain per degree day, divided by SEER2, times your electric rate. CDD is the weather part. The house and equipment set the rest.
Because savings scale with CDD, the payback on a higher-efficiency unit is shortest in the hottest counties. HyreHVAC does not print equipment prices here; the energy savings calculator turns kWh into dollars with your rate.
How to check your summer bills with CDD
- Collect a summer of electric bills.
- Subtract your spring or fall baseline use to estimate cooling kWh.
- Look up CDD for each billing period.
- Divide cooling kWh by CDD.
- Compare year to year; a rising ratio can point to a dirty coil, low refrigerant or leaky ducts.
Keeping use per CDD low
- Keep the outdoor coil clean and clear.
- Change filters often in heavy-use months.
- Shade west-facing windows; afternoon sun on glass adds heat that CDD does not count.
- Seal and insulate attic ducts, which sit in the hottest part of the house on high-CDD days.
- Book a pre-summer check; see HVAC maintenance.
Warning signs
- Use per CDD rises compared with last summer.
- The AC runs nonstop on average days, not just record ones.
- Indoor humidity stays high even when temperature is met.
- Ice on the refrigerant lines; refrigerant work needs an EPA 608 certified tech.
Rules tied to cooling climate
DOE does not use CDD in its rules, but its regional cooling standards follow the same climate logic. Split central ACs under 45,000 BTU/h installed on or after 1 January 2023 must meet 14.3 SEER2 in the Southeast and Southwest, versus 13.4 SEER2 elsewhere, and the Southwest adds EER2 floors 4.
NOAA publishes the official degree-day normals 2. EIA uses population-weighted CDD to forecast electricity demand 1.
CDD vs related terms
| Term | Measures | Used for |
|---|---|---|
| Cooling degree days | Season-long cooling need | AC energy use |
| Heating degree days | Season-long heating need | Heating energy use |
| SEER2 | Seasonal cooling efficiency | Turning cooling need into kWh |
| EER2 rating | Efficiency at 95°F | Peak-hour performance |
| Relative humidity | Moisture in the air | The part CDD leaves out |
Common misconceptions
- Myth Equal CDD means equal AC bills.
- Reality Humidity, sun and house design change the load a lot.
- Myth High CDD means a bigger AC.
- Reality CDD is seasonal energy; size comes from a Manual J design-day load.
- Myth Cool nights cancel hot days.
- Reality Days below a 65°F average count as zero CDD, not negative.
CDD in the states we cover
NOAA county normals for 1991–2020: Miami-Dade, Florida, about 4,312 CDD; Maricopa, Arizona, about 3,724; Dallas, Texas, about 2,679; San Diego, California, about 1,413; Fulton, Georgia, about 1,668; Wake, North Carolina, about 1,572; Franklin, Ohio, about 955; and King, Washington, about 80 2.
Florida and Georgia add heavy humidity on top, which CDD does not show.
When CDD matters to you
Check your county's CDD before you pay extra for efficiency. Above roughly 2,000 CDD, higher SEER2 tiers tend to earn their keep; below about 1,000, the savings are small and comfort features may matter more.
Next step: compare models on our best air conditioners page once you know your number.
Questions about Cooling Degree Days
What is a cooling degree day?
It measures how much warmer a day was than 65°F. Average the high and low and subtract 65. A day with a 90°F high and a 66°F low averages 78°F, which is 13 cooling degree days. Cooler days count as zero.
What is a high number of cooling degree days?
Among US counties, Miami-Dade, Florida, averages about 4,312 a year and Maricopa, Arizona, about 3,724, per NOAA's 1991–2020 normals. Northern coastal counties can be under 100, such as King County, Washington, at about 80.
How do cooling degree days affect my electric bill?
Cooling electricity rises roughly in step with CDD. A summer with 20% more CDD than normal will usually mean about 20% more AC use, other things equal. Humidity and sun can push it higher.
Do cooling degree days account for humidity?
No. CDD uses only temperature. Humid places need extra energy to remove moisture, so their cooling load is higher than CDD alone suggests.
Should I buy a higher SEER2 unit if I have low CDD?
The energy savings are small where CDD is low, so the extra cost may not pay back. Comfort, noise and humidity control may still justify a variable-speed unit. In high-CDD areas, higher SEER2 usually pays back faster.
How are CDD and HDD related?
Both compare daily average temperature with 65°F. Above 65 counts toward CDD, below counts toward HDD. Together they describe a place's cooling and heating needs over a year.
Where can I find cooling degree days for my area?
NOAA's National Centers for Environmental Information publish county and station normals. Our degree-day lookup lists annual HDD and CDD for every county in the lower 48 states.
Do cooling degree days tell me what size AC I need?
No. CDD measures seasonal cooling need. AC size depends on the design-day heat gain of your home, which comes from a Manual J load calculation.
Why do cooling degree days use 65°F?
It is the standard base EIA and NOAA use, chosen because a typical home needs little cooling below that daily average. Real homes may not run the AC until it is warmer, but the fixed base keeps comparisons fair between places and years.
Sources
- U.S. EIA, Energy Explained: Degree days, retrieved .
- NOAA NCEI, nClimDiv county degree-day normals, 1991–2020 (files dated 6 August 2026), retrieved .
- U.S. EIA, Today in Energy: Nearly 90% of U.S. households used air conditioning in 2020 (31 May 2022), retrieved .
- eCFR, 10 CFR 430.32, Energy and water conservation standards (version of 1 September 2026), retrieved .
- ACCA, Manual J Residential Load Calculation (8th Edition), retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Definition and example from EIA's degree-day page; county normals from NOAA nClimDiv 1991–2020 as held in the site's degree-day table; regional SEER2 floors from 10 CFR 430.32, read 10 October 2026. The kWh table scales an assumed 24 million BTU, 2,000 CDD load by CDD and divides by SEER2.
Suggest a correction. We fix errors and say what changed.