Growing degree days
calculator
Cumulative growing degree days for any US ZIP code, with your own base temperature and start date. Built on NOAA GHCN-Daily observations for 33,000+ ZIP codes, recomputed daily.
Accumulated growing degree days
GDD
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 from your start date through the most recent observation:
GDD = Σ max(0, (Tmax + Tmin) / 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. Start dates must fall in the year the current data covers; earlier years are not stored.
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 current observations is used. Days the station did not report are skipped rather than estimated, so the result card shows how many days in your range are missing. A long gap means the total understates real accumulation.
Data updates daily as NOAA publishes observations, typically with a lag of a few days. 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.
| Use | Base | Source |
|---|---|---|
| Turf / crabgrass (this calculator's default) | 50°F | Germination begins near 200 GDD at base 50°F, per Purdue Turfgrass Science |
| Corn maturity (“86/50” system) | 50°F, capped 86°F | Growth doesn't accelerate above 86°F, per Ohio State University Extension |
| Codling moth (a fruit-pest model) | 50°F, capped 88°F | Same floor as corn, a different cap, per UC IPM |
| Cool-season vegetables (peas, cole crops) | 40°F | Cold-tolerant crops start accumulating sooner, per UW–Madison Extension |
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.