Heating Degree Days (HDD) Calculator

What Are Heating Degree Days? Heating Degree Days (HDD) measure how much and for how long the outside air temperature falls below a baseline (typically 65 degrees F or 18 degrees C). Each degree below the baseline for one day equals one HDD. The higher the HDD total, the more energy needed for heating.

The Formula Daily HDD = max(0, Base Temperature - Average Daily Temperature). If the average temperature is 45 degrees F and the base is 65 degrees F: HDD = 65 - 45 = 20 degree days. If the average temperature is above 65 degrees F, the HDD for that day is zero. Monthly or annual HDD is the sum of all daily values.

The 65 Degree F Baseline The standard baseline of 65 degrees F (18 degrees C) was established in the early 20th century in the United States. The reasoning: at 65 degrees F outside, internal heat gains from people, lights, appliances, and solar gain are typically enough to keep a building comfortable without mechanical heating. Some modern buildings with better insulation use a lower baseline (60 degrees F or 15.5 degrees C).

Uses of HDD Energy companies use HDD to forecast natural gas and heating oil demand. Building engineers use HDD to size heating systems. Utility bills often correlate closely with HDD for the billing period. Climate comparison between cities is often done using annual HDD totals. Typical US values: Miami ~150 HDD/year, Atlanta ~3,000, Chicago ~6,500, Minneapolis ~8,000, Fairbanks ~14,000.

Relationship to Energy Cost A building’s annual heating energy is approximately proportional to its HDD total. If a building uses 100 therms of gas in a month with 800 HDD, you can estimate roughly 0.125 therms per HDD. This ratio (called the heating coefficient) helps predict future energy costs by applying it to forecasted HDD values. The coefficient depends on insulation, building size, HVAC efficiency, and thermostat settings.