Growing degree days
calculator

What this calculator computes

“The total amount of heat required, between the lower and upper thresholds, for an organism to develop from one point to another in its life cycle is calculated in units called degree-days (°D).”

For each day it takes the station's high and low temperature, averages them, subtracts your base temperature, and treats any negative result as zero — the average-of- max-and-min method described by UW–Madison Division of Extension. Those daily values are summed over your date range, including both boundary dates:

GDD = Σ max(0, (T_max + T_min) / 2 − base)

This is the simple-average method with no upper cutoff. Models that cap the daily high produce different numbers, especially in hot weather — the corn “86/50” system described by Ohio State University Extension stops counting heat above 86°F, while UC IPM's codling moth model caps at 88°F. Models that use the single or double sine method instead of a daily average will also produce different numbers. If a published model specifies one of those methods, this tool is not a substitute for it.

Base temperature accepts 32–86°F. Dates can run from January 1, 2022 through today, including multiple years. For example, end May 9, 2023 with no start sums January 1–May 9, 2023. A September–July season can cross January 1 without resetting the total. Your chosen interval does not by itself predict a crop’s development.

Growing fruit? Trees need winter cold before spring heat counts toward bloom; the chill hours calculator shows hours below 45°F by ZIP code for this winter and each winter since 2022–23.

Where the numbers come from

Each ZIP code is matched to its eight nearest NOAA GHCN-Daily weather stations by distance, and the nearest one with usable observations in each source year is used. Sources may change between years. When the station misses up to 5 days in a row, those days' highs and lows are filled by drawing a straight line between the observed days on either side, including across January 1; the result card shows how many days were estimated. Longer outages are left out and counted as missing, so a long gap means the total understates real accumulation.

Current-year data updates daily as NOAA publishes observations, typically with a lag of a few days. NOAA may also revise historical observations. More on sources and methods is on the About page.

Common base temperatures

Base temperature is specific to whatever you're modeling, not a universal constant. These four are common reference points; if you're following a published model, use its published base and cutoff, not this list.

Why the caps differ: an upper threshold marks where an organism's growth rate stops responding to heat, not a round number picked for convenience. UC IPM's degree-day methods page distinguishes three ways of handling temperatures above that threshold — horizontal (development holds flat), intermediate (development slows but doesn't stop), and vertical (no further development is credited) — used together with the sine and triangle calculation methods, not the simple daily average this calculator uses. This calculator implements none of them: every degree above your base counts, with no ceiling, which is why its output will run higher than a capped model's in hot weather.

Lawn and turf timing

For crabgrass pre-emergent and other lawn decisions, the ZIP trackers do the threshold interpretation for you at base 50°F from January 1 — this calculator returns the raw accumulation only.