What are growing degree days (GDD)?

Growing degree days measure accumulated heat above a base temperature that a crop needs to develop. One GDD is earned for every degree the daily mean temperature sits above the crop's base temperature. Summed over a season, GDD predicts development stages far better than calendar dates, because plants develop in response to heat, not time.

How do I calculate growing degree days?

For each day, average the maximum and minimum temperature, subtract the crop's base temperature, and count the result if it is positive: GDD = max(0, (Tmax + Tmin) / 2 - Tbase). Common base temperatures are 10 C for corn, soybeans, and grapevines, and about 4.4 C for wheat, oats, and cool-season forages. Sum the daily values across the growing season.

What base temperature should I use for GDD?

Use the temperature below which your crop stops developing: 10 C for corn, soybeans, sorghum, tomatoes, and grapevines; 4.4 C (40 F) for wheat, barley, oats, and canola; around 7 C for potatoes. Pest and disease models each use their own base, for example about 10 C for codling moth. Always match the base to the published model you are following.

How many growing degree days does corn need?

Depending on the hybrid's maturity rating, corn needs roughly 1,200 to 1,700 GDD at base 10 C (about 2,200 to 3,100 in Fahrenheit units) from planting to physiological maturity. Short-season hybrids sit at the low end, full-season hybrids at the high end. Compare that requirement with the GDD your location actually accumulates to pick a maturity group that finishes reliably.

What is the Winkler index for vineyards?

The Winkler index sums GDD at base 10 C from April 1 to October 31 (Northern Hemisphere) and sorts wine regions into five climate classes, from Region I (below 1,389 GDD, cool, suited to Pinot Noir and Riesling) to Region V (above 2,222 GDD, hot, bulk and fortified wines). It remains the standard first check on whether a site suits a grape variety.

Growing Degree Days: Calculate GDD for Any Location

Published August 31, 2026 · 9 min read

Plants do not read calendars. A corn hybrid does not silk on July 20, it silks after accumulating roughly 700 GDD from planting. A grapevine does not ripen in September, it ripens when the season has delivered the heat its variety needs. Growing degree days (GDD) are the standard way to measure that heat, and they turn "when should I plant, spray, and harvest" from guesswork into arithmetic.

This guide covers the GDD formula, the base temperatures for common crops, two worked examples computed from real 10-year climate data, and how to estimate GDD for any field or vineyard on Earth without a weather station.

Skip the math: open the PixelGust dashboard, click your field, and switch the Weather panel to Historical. The monthly mean temperatures for 2015–2025 are everything you need for the estimates in this article.

The GDD Formula

For each day of the season:

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

Average the day's maximum and minimum temperature, subtract the crop's base temperature, and keep the result if it is positive. A day averaging 22 °C contributes 12 GDD for a base-10 crop; a day averaging 8 °C contributes nothing. Sum the daily values from planting (or from a fixed date like April 1) and you have the season's accumulated heat.

Two common refinements: many corn models cap T_max at 30 °C because development does not speed up further in extreme heat, and Fahrenheit-based sources report values roughly 1.8× larger, so always check the unit before comparing numbers.

Base Temperatures for Common Crops

Crop or model	Base temperature	Typical requirement (base 10 °C)
Corn (maize)	10 °C	1,200–1,700 GDD to maturity
Soybeans	10 °C	1,300–1,700 GDD
Grapevines (Winkler index)	10 °C	1,389–2,222 GDD Apr–Oct by region class
Wheat, barley, oats	4.4 °C (40 °F)	1,300–1,700 GDD at base 4.4
Potatoes	7 °C	1,000–1,400 GDD at base 7
Codling moth (pest model)	10 °C	first spray ~140–250 GDD after biofix

Requirements vary by variety, so treat the table as orientation and use the figure your seed supplier or extension service publishes for the specific variety.

Estimating Seasonal GDD from Monthly Climate Normals

You do not need daily records to answer strategic questions like "can this site ripen this crop at all." Monthly mean temperatures approximate it well: for each month, take max(0, monthly mean − base) × days in month and sum. The approximation slightly underestimates GDD in shoulder months (spring and autumn days above the base get averaged with days below it), so treat the result as mildly conservative.

Worked Example 1: A Corn Field in Central Iowa

Using PixelGust's 2015–2025 monthly climatology for a field near Ames, Iowa (42.03, −93.62), the months above base 10 °C contribute:

Month	Mean temp (°C)	GDD (base 10)
May	16.4	198
June	22.7	380
July	23.9	430
August	22.9	400
September	20.2	307
October	12.2	67
Season total	–	~1,780

Around 1,780 GDD comfortably matures a full-season hybrid needing 1,600–1,700 GDD, with a margin for a late planting or a cool year. A grower further north, where the same calculation might return 1,400, would pick a shorter-season hybrid instead. That is the whole decision, made from one table.

Worked Example 2: A Douro Valley Vineyard (Winkler Classification)

The same method classifies wine regions. For a vineyard near Pinhão in Portugal's Douro Valley (41.196, −7.557), summing April through October at base 10 °C gives roughly 1,820 GDD, which lands the site in Winkler Region III (1,671–1,944). Region III suits full-bodied reds, exactly the Touriga Nacional and port grapes the Douro is famous for. The classification you would guess from a wine atlas falls straight out of the climate data.

We used this exact vineyard for our sample Property Climate Report, which shows the full monthly climatology behind these numbers alongside terrain, hazard, and vegetation data.

What GDD Decisions Look Like in Practice

Variety selection: compare your site's seasonal GDD against the requirement of the variety or hybrid. If the numbers barely match, a cool year fails the crop.
Site comparison: two parcels 20 km apart can differ by hundreds of GDD due to elevation and aspect. Compute both before buying or leasing; combine with slope and aspect data for the full picture.
Pest and disease timing: insect phenology models are published in GDD after a biofix date. Tracking accumulation against the local climatology tells you whether this season is running early or late.
Harvest logistics: a season running 150 GDD ahead of normal by July means harvest arrives days earlier. Contractors, storage, and labor can be booked accordingly.
Climate trend checks: a warming site gains GDD over time. The 10-year monthly temperature time series shows whether varieties that were marginal a decade ago are now viable.

How to Get GDD Data for Your Location

Open the dashboard at pixelgust.com/app and click your field, or draw a polygon around it.
Switch the Weather panel to Historical to see 2015–2025 monthly means, minimums, and maximums from ERA5 reanalysis.
Apply the monthly method above with your crop's base temperature. Six multiplications and a sum give the seasonal total.
Check the extremes too: monthly minimums flag frost risk during flowering, and maximums flag heat stress during grain fill, both of which GDD alone does not capture.
Automate it if you like: the REST API and the MCP server return the same monthly climatology as JSON for any coordinates.

Get the Full Climate Picture for Your Land

A Property Climate Report bundles the 10-year monthly climatology behind GDD with frost minimums, hazards, terrain, and vegetation history into one dated PDF. One-time purchase, EUR 19.

See a Sample Report

Limitations Worth Knowing

GDD is a heat clock, not a growth guarantee. It says nothing about water (pair it with evapotranspiration and precipitation), nothing about late frosts (check monthly minimums), and the monthly approximation smooths out day-to-day variability. For within-season operational tracking, daily station or forecast data is the right tool; for siting, variety choice, and comparing locations, the 10-year climatology approach in this article is the standard and it is free to run.

Data Sources

Monthly climatologies are computed from ERA5 reanalysis (ECMWF, 2015–2025) at 0.25° resolution, with current conditions and 7-day forecasts from NOAA GFS. Terrain context comes from the Copernicus DEM at 30 m. GDD thresholds cited follow standard extension-service and Winkler index conventions.