Heating Degree Days Calculator
Heating and cooling degree days from temps.
What it does
Degree days turn weather into fuel math: each degree below 65 F banks heating need while each above banks cooling. Utilities price seasons from these sums.
The tool above takes average temperature with day count in Fahrenheit or Celsius. It returns heating and cooling degree days with the season verdict.
The default 45 F over 30 days banks 600 HDD with zero CDD: heating season. The hot case of 80 over 31 banks 465 CDD instead.
Climate normals and energy explainers define the same base 65 race with 18 C as the metric twin. Fuel models sum daily differences across months to forecast demand.
Buildings answer with insulation, shade, and thermostats, not thermometers alone. Track degree days against bills to separate weather from waste in any audit.
How to use
- Pick Fahrenheit or Celsius.
- Type the average temperature.
- Type the day count.
- Read HDD first.
- Read CDD beside it.
- Read the season last.
Formula
- avg
- Mean temperature in F or C.
- days
- Day count, 1 to 366.
- base
- 65 F or 18 C balance point.
NOAA climate normals and EIA energy explainers define heating and cooling degree days against base 65 F with 18 C as the metric twin. Monthly sums of the same daily differences drive utility models. Agricultural growing degree days use base 50 F and belong on the separate biology page.
Worked example
Example: cool month
Imperial defaults: 45 F, 30 days.
- Month: 45 F average over 30 days.
- HDD: (65 minus 45) x 30 = 600.
- CDD: 45 sits under 65, so 0.
- Season: heating wins 600 to 0.
- Metric: 7 C over 30 days gives 330 HDD.
- Tool shows 600 HDD, 0 CDD, Heating season.
Reference table
| Stretch | HDD | CDD |
|---|---|---|
| 45 F, 30 days | 600 | 0 |
| 80 F, 31 days | 0 | 465 |
| 7 C, 30 days | 330 | 0 |
| 65 F, 30 days | 0 | 0 |
Limits
- Temperature sums, not bills.
- Base 65 assumption fixed.
- Averages hide extremes.
- Buildings differ widely.
Common mistakes
- Using crop base 50 F.
- Averaging whole mild years.
- Reading bills off HDD alone.
- Ignoring humidity loads.
- Mixing F and C bases.
Try it yourself
Quick checks. Answers sit tucked under each question.
80 F over 31 days?
0 HDD with 465 CDD: cooling season.
7 C over 30 days?
330 HDD with 0 CDD: heating season.
Minus 5 C over 30 days?
690 HDD: deep cold banks hard.
Questions
Base temperatures?
65 F in imperial, 18 C in metric. Averages below bank heating, above bank cooling.
Used where?
Utilities and fuel budgets price seasons from degree day normals.
Climate normals?
Yes, normals average 30 years of the same math. This tool runs any single stretch.
Exact bills?
No, degree days track temperature sums. Insulation, wind, and sun move real bills.
Growing degree days?
Growing degree days use base 50 F for crops. This page uses base 65 F for buildings.
Whole years?
Enter 1 to 366 days; split years into months for better averages.