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BTU Logic

Tools / Degree Day Calculator

Degree Day Calculator

A free degree day calculator that works from the weather file for your site. Load an EPW, TMY2, TMY3, or NSRDB-style CSV and get annual and monthly heating and cooling degree days on the bases you pick. The file is read in your browser.

  • Upload, drop, or paste the weather file, or try a sample EPW, TMY2, or TMY3 year
  • Base presets: US 65/65 °F, commercial 60/70 °F, balance point 55/75 °F, SI 18.3 °C, or your own HDD and CDD bases
  • Results in °F-days or true °C-days
  • Occupied / unoccupied split on a schedule you set (default Mon–Fri 08:00–18:00 LST)
  • Monthly table, plus daily and monthly CSV
HDD/CDD values are calculated from the weather file you provide (EPW / TMY2 / TMY3 / NSRDB-style CSV) with the base temperatures and occupied / unoccupied splits you set. Different bases, periods, or file sources will change the numbers. Not a substitute for utility-billing degree days, code-required climate data, or licensed engineering judgment. Informational / preliminary use only; verify before energy models, compliance, or design decisions.
Input format

Changing format re-parses the loaded file immediately. Accepts EPW, TMY2, TMY3, and NSRDB-style CSV.

Get NSRDB / TMY weather data (NREL) · NREL SAM weather data

Occupied schedule

Default Mon–Fri 08:00–18:00 interval-beginning LST (8am through 6pm). EPW / TMY hour-ending 1–24 is converted to 0–23 on parse; NSRDB is already 0–23. Unchecked days are unoccupied. Start is inclusive, end is exclusive. If start is later than end, the window wraps overnight.

Typical-year files stitch months from different source years. Weekdays follow January 1 (from EPW DATA PERIODS when present, else Sunday) so month boundaries stay continuous. Use file Y/M/D only for a real calendar year.

Daily-mean HDD/CDD and hourly occupied slices are different methods.

Combined HDD/CDD use the daily mean of hourly dry-bulb (°F-days). Occupied / unoccupied columns are hourly degree-days (each hour contributes max(0, ΔT)/24). Those two hourly slices add to each other; they do not add to the daily-mean HDD/CDD columns. Temperature-based only — not dew-point, RH, or wet-bulb.

Utility only — not a PE stamp, certified weather product, or code-approved calculation. Outputs are estimates from your inputs/files; verify before design, bidding, or compliance use. Terms