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Minnesota Heating Degree Days Calculator

Calculate annual or monthly Heating Degree Days (HDD) for Minnesota cities. HDD measures how much and how long outside air temperature was below a base temperature, used to estimate heating energy consumption.

Standard base is 65°F. Range: 40–75°F.
Results will appear here.

Formula

Heating Degree Days (HDD) — Mean Temperature Method (NOAA / ASHRAE):

Daily HDD = max(0, Tbase − Tmean)
Monthly HDD = max(0, Tbase − Tmonthly avg) × Daysin month
Annual HDD = Σ Monthly HDD (all 12 months)

Where Tbase = base temperature (default 65°F) and Tmean = average daily temperature = (Tmax + Tmin) / 2. Monthly normals approximate the daily mean. If Tmean ≥ Tbase, HDD = 0 (no heating needed).

Energy Estimate: Estimated therms = Annual HDD × 0.10 therms/HDD (based on EIA data for average Minnesota home, ~2,000 sq ft, natural gas heat).

Assumptions & References

  • Monthly average temperatures are from NOAA 1991–2020 Climate Normals for each Minnesota station.
  • The standard base temperature of 65°F is used per ASHRAE and U.S. EIA convention; below this temperature, most buildings require heating.
  • February is assumed to have 28 days (non-leap year average).
  • The mean temperature method (monthly normal ≈ daily mean) is an approximation; NOAA's official method integrates hourly data for higher precision.
  • Energy cost estimate uses 0.10 therms per HDD for a ~2,000 sq ft Minnesota home with natural gas heat (EIA Residential Energy Consumption Survey).
  • Natural gas price of $1.10/therm reflects approximate Minnesota average (EIA, 2023); actual prices vary by utility and season.
  • International Falls and Bemidji are among the coldest locations in the contiguous U.S., with annual HDD often exceeding 10,000 at base 65°F.
  • References: NOAA Climate Normals (ncei.noaa.gov), ASHRAE Handbook of Fundamentals, U.S. EIA.

Formula

Heating Degree Days (HDD) — Mean Temperature Method (NOAA / ASHRAE):

Daily HDD = max(0, Tbase − Tmean) Monthly HDD = max(0, Tbase − Tmonthly avg) × Daysin month Annual HDD = Σ Monthly HDD (all 12 months)

Where Tbase = base temperature (default 65°F) and Tmean = average daily temperature = (Tmax + Tmin) / 2. Monthly normals approximate the daily mean. If Tmean ≥ Tbase, HDD = 0 (no heating needed).

Energy Estimate: Estimated therms = Annual HDD × 0.10 therms/HDD (based on EIA data for average Minnesota home, ~2,000 sq ft, natural gas heat).

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