How degree days calculator works
Degree days translate weather into accumulated demand. Each day contributes heating degree days equal to max(0, base − daily mean) — how far the day fell short of warmth — and cooling degree days equal to max(0, daily mean − base). Sum them over a season and you get the two numbers energy managers, building modellers and farmers have used for a century to track fuel and electricity demand.
The conventional base is 65 °F in the United States and 18 °C in Europe — the outdoor temperature at which a typical building needs neither heating nor cooling. The calculator defaults to those, but the base is yours to change: use 15.5 °C for UK degree-day records, 22 °C for aggressive cooling analysis, or 10 °C and the same formula becomes growing degree days for crops and pests.
Paste daily mean temperatures (one per line or comma-separated) in °C or °F, and the tool returns total HDD and CDD in your unit, the same totals converted to the other scale, counts of heating, cooling and neutral days, the average, coldest and warmest day, and the single-day peaks. Mean temperatures should be (max + min) / 2 for the day. All math runs in your browser.