☀️ Settings

65°F is standard. Above this temperature, AC is typically needed.

🌡️ Daily High & Low Temperatures (up to 14 days)

Enter each day’s high and low temp. We calculate the daily mean and subtract the base temperature. If it’s above the base, that’s your CDD for the day.

— CDD
Total cooling degree days (updates live)
Enter daily temperatures above
Avg: —°
0 hot days
Hottest: —°

Calculating your cooling degree days…

☀️ Your Cooling Degree Days
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Total CDD
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Cooling Degree Days
Average Temp
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Mean daily temperature
Hot Days
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above base of 65°
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—°
Hottest Mean
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—°
Coolest Mean
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CDD/day
Average Daily CDD
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65°
base
Base Temp Used
☀️ Cooling Load Severity
Mild Hot Extreme —
📊 Temperature Range Overview
🔥 Hot Days CDD
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🌤️ Mild Days
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❄️ Cool Days
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📅 Day-by-Day CDD Breakdown

Day
High
Low
Mean
CDD
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❄️ What These CDD Mean for You

Cooling Degree Days — Explained Simply

CDD is the cooling equivalent of HDD. It tells you how hard your AC had to work. Here’s everything in plain, simple language.

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What Are Cooling Degree Days?

A Cooling Degree Day (CDD) measures how much warmer the day was than the base temperature of 65°F. If the mean temperature was 80°F, you had 15 CDD that day. Add them all up over a week, month, or summer and you get your cooling load. More CDD means more AC use.

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The Formula Is Simple

Find the mean daily temperature: (high + low) ÷ 2. Then: CDD = maximum of (0, mean − base temperature). So if the mean was 78°F and the base is 65°F, that’s 13 CDD. If the mean was 60°F, you get zero CDD — no cooling needed.

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Why 65°F Is the Base

At 65°F outdoor temperature, most buildings naturally stay comfortable inside with just ventilation. Above 65°F, your AC needs to kick in. This standard base temperature makes it easy to compare CDD across locations and data sources. Some regions use 60°F or 70°F — you can change this in the calculator.

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CDD and Your AC Bill

Your cooling electricity use is roughly proportional to CDD. If you used 500 kWh in a month with 200 CDD, expect about 750 kWh in a month with 300 CDD. Divide your electricity use by CDD each month to get a “kWh per CDD” ratio. Improving this number means your home or AC system got more efficient.

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CDD vs HDD — What’s the Difference?

Heating Degree Days measure cold — how much the temperature fell below 65°F. Cooling Degree Days measure heat — how much it rose above 65°F. Miami has high CDD and almost no HDD. Minneapolis has high HDD and moderate CDD. Most US cities have both, shifting with the seasons.

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CDD Trends Are Rising

Annual CDD has been increasing in most US cities over the past few decades due to climate change. Cities like Phoenix and Houston now see significantly more CDD than they did in the 1970s. This makes cooling efficiency investments — better AC, insulation, and smart thermostats — more financially valuable every year.

📐 The CDD Calculation Formula

Mean Daily Temp = (Daily High + Daily Low) ÷ 2 Daily CDD = MAX(0, Mean Daily Temp − Base Temperature) Period CDD = Sum of all daily CDD values in the period AC kWh = (Tons × 12,000 × CDD × 24) ÷ (SEER × 1,000 × Seasonal Factor)

The base temperature is 65°F (18.3°C) in the US and most of North America. For a more accurate energy estimate, the AC kWh formula uses your SEER rating (higher = more efficient) and home size. The insulation factor adjusts for how well your home retains cool air. Actual results vary based on thermostat settings, shading, and occupant behavior.

Annual CDD by US City — Reference Table

Base 65°F. Source: NOAA 30-year Climate Normals (1991–2020).

CityAnnual CDDPeak MonthCooling SeasonClimate
San Francisco, CA~140SepJun–Oct (light)Very mild
Seattle, WA~300AugJul–AugMild
Minneapolis, MN~795JulJun–AugModerate
Chicago, IL~830JulJun–AugModerate
New York, NY~1,080JulJun–SepWarm summers
Atlanta, GA~1,590JulMay–SepHot
Los Angeles, CA~1,400AugMay–OctHot and dry
Dallas, TX~2,980JulApr–OctVery hot
Houston, TX~2,950JulApr–OctVery hot & humid
Phoenix, AZ~4,200JulApr–OctExtreme heat
Miami, FL~4,040AugYear-roundTropical

Frequently Asked Questions

It’s straightforward. For each day: add the high and low temperature, divide by 2 to get the mean. Subtract the base temperature (usually 65°F) from the mean. If the result is positive, that’s your CDD for the day. If zero or negative, you have zero CDD. Add all the daily values to get your period CDD. This calculator does all that automatically — just enter your daily highs and lows.
Cooling Degree Days (CDD) measure how much warmer a day was than the base temperature — this represents cooling demand. Heating Degree Days (HDD) measure how much colder a day was than the base — this represents heating demand. Both use the same 65°F base, but work in opposite directions. For energy planning, you need both: HDD estimates your winter fuel use, while CDD estimates your summer electricity use for cooling.
Use the AC Cost tab above. Enter your total CDD, home square footage, AC SEER rating, electricity rate, and insulation level. The calculator estimates your total electricity use (in kWh) and total cost. As a rule of thumb: a SEER 14 AC in a 1,800 sq ft average-insulation home uses roughly $0.024 per CDD. So 1,200 CDD = about $29 per month, or $348 over the cooling season.
It varies enormously. San Francisco has just ~140 CDD annually — residents almost never need AC. Chicago gets ~830 CDD — warm summers but manageable. Dallas gets ~2,980 CDD — AC runs from April through October. Miami and Phoenix top the list at 4,000+ CDD, meaning AC runs almost year-round. The national average for the continental US is roughly 1,200–1,500 CDD per year.
Directly and proportionally. A SEER 20 system uses exactly 35% less electricity than a SEER 13 system for the same number of CDD. If your cooling bill is $400 per season with a SEER 13 unit, a SEER 20 upgrade would bring that to about $260 — saving $140. The higher your annual CDD (the hotter your climate), the bigger the dollar savings from a SEER upgrade.
Yes. Select °C in the calculator and use a base temperature of 18°C (equivalent to 65°F). The formula is identical: daily CDD = max(0, mean temp − 18°C). CDD in Celsius will be smaller numbers than °F-based CDD — roughly 55% of the equivalent Fahrenheit values. Make sure you’re consistent when comparing data — don’t mix Celsius and Fahrenheit CDD values from different sources.
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