Heat units from your weather, any US location

GDU Calculator

Q: What is a growing degree unit (GDU)?

A growing degree unit (GDU), also called a growing degree day (GDD), measures daily heat accumulation above a base temperature of 50°F for corn. Formula: GDU/day = ((Hi + Lo) ÷ 2) − 50, with Hi and Lo each set to 50°F if below 50 and to 86°F if above 86. GDUs accumulate daily from planting and determine crop growth stage progression.

Q: How many GDUs does corn need to reach black layer?

Corn GDU requirement to physiological maturity (black layer) is approximately (Relative Maturity × 22) + 250, an AGSIST rule set on K-State Research and Extension’s figure of about 2,670 GDUs for a 110-day hybrid, adding 22 GDUs per day of RM. Purdue puts a 112-day hybrid near 2,700, a little under this line’s 2,714. Away from 110 days it is an extrapolation. Example: a 108-day RM hybrid needs roughly 2,650 GDUs; a 110-day hybrid roughly 2,670. This is a rule of thumb. Always verify with your seed company’s published GDU requirement for your specific hybrid.

Q: What GDU accumulation does the US Corn Belt average per season?

May 1 through September 30 GDU accumulation ranges roughly 2,000 (northern Minnesota, Upper Peninsula) to 3,200+ (southern Illinois, Missouri, Kentucky). Corn hybrids should be matched to your location’s typical GDU accumulation.

Q: What is the difference between GDU and GDD?

Growing Degree Units (GDU) and Growing Degree Days (GDD) are the same calculation under two names. Both use a base temperature of 50°F for corn with a maximum cap at 86°F. GDU is the more common term in Corn Belt Extension publications; GDD is used interchangeably.

Q: Is this the same 86/50 method seed companies use?

Yes. This page uses the standard 86/50 method: each day the high and low are each held between 50°F and 86°F, averaged, and 50 is taken off. A 95°F high and 70°F low counts the high as 86, so the day is (86 + 70) ÷ 2 − 50 = 28 GDU. University of Arkansas Extension’s Corn Production Handbook (MP437) gives the same 50 and 86 limits and says most seed corn suppliers put a hybrid’s GDU rating, from planting to black layer, on the seed tag or in the hybrid description. Your total can still differ a little from another tool’s, because the weather comes from a different station or grid.

Q: How do I use this GDU calculator?

Enter your planting date and click ‘Fetch GDUs’ and the calculator pulls daily weather for your location and adds up GDUs from planting to today. Or enter your relative maturity and average daily temperatures for a manual estimate.

Q: Does a 115-day hybrid actually take 115 days?

Not exactly. Arkansas Extension says a hybrid labeled 115-day may take 110 to 120 days to mature, depending on location, planting date and the season's weather. That spread is why heat units track maturity better than a day count.

Q: Does day length affect corn the way it does soybeans?

No. Arkansas Extension says corn growth and development runs mainly on temperature and is not controlled by day length, so for corn the calendar date matters less than soil and air temperature. That is what a GDU count measures.

Heat units since planting from real daily weather at your ZIP, for corn or soybeans: crop stage, black layer date and frost risk.

Weather: daily highs and lows for your ZIP from Open-Meteo.

Setup

Relative Maturity (RM days)

Corn 80–130 RM · GDU to black layer ≈ RM × 22 + 250

Planting Date

Today / end date

First killing freeze, 28°F (optional)

Typical first 28°F freeze: Upper Midwest ~Sep 25–Oct 5, central Corn Belt ~Oct 10–20. After a fetch, this box fills with the typical date at your spot; change it to yours.

Your weather since planting

Pulls the daily high and low for every day since planting from the Open-Meteo weather archive and adds up your GDUs.

Your location (ZIP or city)

Your location appears here after fetching

Until you fetch, the results are an example from the hand-entry temperatures below (82/58°F to start, a midsummer day, so it runs ahead of a real season), not your weather.

Or enter temperatures or GDUs by hand

Manual entry

Avg daily high (°F)

Avg daily low (°F)

GDU/day = ((Hi + Lo) ÷ 2) − 50, with Hi and Lo each set to 50°F if below 50 and to 86°F if above 86. The 82/58°F filled in is a midsummer day (20 GDU), not a season average: the Wisconsin monthly averages below come to 2,269 GDU for May through September, about 15 a day. Type your own season average, or pick the Wisconsin averages.

Season Progress

GDUs accumulated

Select dates to begin

Days since planting-

Average GDU/day-

Data source-

Maturity Projection

GDU Target

to maturity

GDUs Left

remaining

Days Left

at Wisconsin rates

Progress to maturity-

Est. black layer, Wisconsin rates-

Frost risk-

Growth stage milestones

Scaled to your selected RM/MG. GDU requirements still vary ±100–200 by hybrid. Verify with your seed company’s published GDU chart.

Upper Midwest GDU Reference (WI example)

Month	Avg Hi	Avg Lo	GDU/Day	Monthly
May	65°F	44°F	7.5	232
June	76°F	55°F	15.5	465
July	82°F	61°F	21.5	667
August	80°F	59°F	19.5	605
September	70°F	50°F	10.0	300

WI 30-year NOAA normals shown as one reference. Southern WI runs ~5–8% higher; central Corn Belt (IA/IL/IN) runs ~15–20% higher; Northern Plains similar to WI. For your exact location, use the Fetch GDUs button above.

Corn stage reference

Stage	GDUs	Key Event	Management
VE	~120	Emergence	Stand count; scout black cutworm V1–V3
V2	~200	Growing point below soil	Side-dress N window opens (V3–V6 optimal)
V4	~345	Ear shoots initiated	n/a
V6	~475	Growing point above soil	Post-emerge herbicide window closes on many labels: read yours; rapid growth begins
V8	~610	Kernel rows determined	S and micronutrient uptake peaks; ECB scouting
V10	~740	Ear size potential being set	Tassel forming inside stalk
VT	~1135–1350	Tasseling: pollen shed	Critical drought window. Sources differ on the GDU: Purdue NCH-40 / UKY AGR-202 list 1,135; tassels usually emerge 1–3 days before silking (UW-Madison). Time a VT/R1 fungicide by what you see in the field, not by GDU
R1	~1400	Silking	Most drought-sensitive stage; rootworm silk threshold
R2	~1660	Blister	Kernel number locked; protect from disease
R4	~1925	Dough	Grain fill accelerating; watch Gray Leaf Spot
R5	~2190	Dent	Watch for stalk rot; plan harvest order
R5.5	~2450	Full dent	Silage harvest window opens
R6	~2700	Black layer / maturity	~30–35% moisture; harvest at <25%

Soybean stage reference

Stage	GDUs	Key Event	Management
VE	~130	Emergence	Stand count; 100k plants/ac minimum
V1	~200	First trifoliate	Post-emerge herbicide window
V3	~400	Third trifoliate	Post-emerge herbicide window: cutoff stage is set by the product label
V5	~600	Fifth trifoliate	Branching begins; canopy closure
R1	~800	First flower	Fungicide window opens (frogeye, white mold)
R2	~1000	Full bloom	N fixation climbing; fungicide is usually timed at R3
R3	~1200	Beginning pod set	Primary fungicide window; most drought-sensitive stretch through R6
R4	~1400	Full pod	Irrigation highest priority; pod abortion risk
R5	~1600	Beginning seed fill	Seed size and oil/protein content determined
R6	~1750	Full seed	Harvest planning
R7	~1900	Beginning maturity	One normal pod on the main stem at mature color
R8	~2000	Full maturity	95% pods tan/brown; harvest <15% moisture; watch for shattering after R8

GDU formula and method

Corn Formula

GDU/day = ((Hi + Lo) ÷ 2) − 50, with Hi and Lo each set to 50°F if below 50 and to 86°F if above 86.

Above 86°F corn growth doesn’t increase; below 50°F there is no heat accumulation, so a day can never go negative.

RM → GDU rule of thumb

GDUs to black layer ≈ (RM × 22) + 250

Example: 108-day hybrid ≈ 2,650 GDUs (2,626 on the line; the calculator rounds to the nearest 50). AGSIST line: 22 GDUs per day of RM, set on K-State’s 110-day ≈ 2,670 (Purdue’s 112-day ≈ 2,700 sits 14 under the line); an extrapolation away from 110 days. Always verify with your seed company’s published hybrid-specific GDU requirement.

Soybean Notes

Soybeans use the same base formula but are photoperiod-sensitive: flowering is triggered by day length (short-day plant), not GDUs alone. US maturity groups run from 00 in northern North Dakota to VII and VIII on the Gulf Coast; this calculator takes MG 0.5 to 5.5. Check your local Extension for recommended MGs.

GDU to R8 here is 1750 + 280 × MG, a straight line through the NDSU soybean GDD model (planting to R8: 1,694 for 00.8, 1,890 for 0.5, 2,058 for 1.1 RM). It is used only for MG 00.8–1.1. For later groups there is no published GDD-to-maturity figure, so the target, R8 date and frost check are withheld. Use the seed company’s rating.

Data Source

Real GDU fetch uses the Open-Meteo weather archive API: no API key required. Historical data is ERA5-based, validated against NOAA station data. Accuracy: typically ±2 GDU/day vs. on-farm weather stations.

Method: daily 86/50 GDUs from the Open-Meteo archive; stage GDUs scaled to your hybrid’s target; after a fetch, the rest of the season is projected from that spot’s average daily GDU for 1991–2020 (same archive), through Oct 31; without one, at Wisconsin monthly rates and withheld outside the Upper Midwest; VT shown as 1,135–1,350 because sources differ · Sources: Purdue NCH-40 and UKY AGR-202 (Neild & Newman corn stages), UW-Madison (tassel to silk 1–3 days), NDSU soybean GDD model, K-State and Purdue maturity anchors, Wisconsin 30-year monthly GDU normals · Reviewed by Sigurd Lindquist, CCA

Common questions

What is a growing degree unit (GDU)?

A growing degree unit (GDU), also called a growing degree day (GDD), measures daily heat accumulation above a base temperature of 50°F for corn. Formula: GDU/day = ((Hi + Lo) ÷ 2) − 50, with Hi and Lo each set to 50°F if below 50 and to 86°F if above 86. GDUs accumulate daily from planting and determine crop growth stage progression.

How many GDUs does corn need to reach black layer?

Corn GDU requirement to physiological maturity (black layer) is approximately (Relative Maturity × 22) + 250, an AGSIST rule set on K-State Research and Extension’s figure of about 2,670 GDUs for a 110-day hybrid, adding 22 GDUs per day of RM. Purdue puts a 112-day hybrid near 2,700, a little under this line’s 2,714. Away from 110 days it is an extrapolation. Example: a 108-day RM hybrid needs roughly 2,650 GDUs; a 110-day hybrid roughly 2,670. This is a rule of thumb. Always verify with your seed company’s published GDU requirement for your specific hybrid.

What GDU accumulation does the US Corn Belt average per season?

May 1 through September 30 GDU accumulation ranges roughly 2,000 (northern Minnesota, Upper Peninsula) to 3,200+ (southern Illinois, Missouri, Kentucky). Corn hybrids should be matched to your location’s typical GDU accumulation.

What is the difference between GDU and GDD?

Growing Degree Units (GDU) and Growing Degree Days (GDD) are the same calculation under two names. Both use a base temperature of 50°F for corn with a maximum cap at 86°F. GDU is the more common term in Corn Belt Extension publications; GDD is used interchangeably.

Is this the same 86/50 method seed companies use?

Yes. This page uses the standard 86/50 method: each day the high and low are each held between 50°F and 86°F, averaged, and 50 is taken off. A 95°F high and 70°F low counts the high as 86, so the day is (86 + 70) ÷ 2 − 50 = 28 GDU. University of Arkansas Extension’s Corn Production Handbook (MP437) gives the same 50 and 86 limits and says most seed corn suppliers put a hybrid’s GDU rating, from planting to black layer, on the seed tag or in the hybrid description. Your total can still differ a little from another tool’s, because the weather comes from a different station or grid.

How do I use this GDU calculator?

Enter your planting date and click ‘Fetch GDUs’ and the calculator pulls daily weather for your location and adds up GDUs from planting to today. Or enter your relative maturity and average daily temperatures for a manual estimate.

Does a 115-day hybrid actually take 115 days?

Not exactly. Arkansas Extension says a hybrid labeled 115-day may take 110 to 120 days to mature, depending on location, planting date and the season's weather. That spread is why heat units track maturity better than a day count.

Does day length affect corn the way it does soybeans?

No. Arkansas Extension says corn growth and development runs mainly on temperature and is not controlled by day length, so for corn the calendar date matters less than soil and air temperature. That is what a GDU count measures.

Formula: USDA / Purdue / UW-Extension / Arkansas MP437 · Weather data: Open-Meteo (ERA5 archive) · Stage data: Iowa State, Purdue, UW-Extension · ← AGSIST Dashboard · Built by Sigurd Lindquist · 715-797-2428