What does "capped 86/50" mean?

Anything above 86 °F counts as 86 and anything below 50 °F counts as 50 before the day's average is taken, because corn stops responding outside that window. A day of 97/70 therefore earns the same as a day of 86/70. The simple method applies no cap and is what most other crops and insect models use.

Are GDD and GDU the same thing?

In the US corn belt, yes — seed companies quote GDUs and they are the capped 86/50 °F degree days this page computes. The name changes with the audience, not the arithmetic.

Can I mix Celsius data in?

Switch the temperature unit and the page computes in that scale, with a 10 °C base and a 30 °C cap, then prints the same total converted to the other scale so a figure from a seed guide in GDUs can be compared with a European table in degree days. Do not mix the two in one paste.

Why type temperatures instead of taking them from a weather service?

Because the number that matters is the one at the field, not at the airport, and because a page that depends on a weather API stops working the day the key expires. Paste from your own station, your logger or any forecast table — it works offline and nothing is sent anywhere.

Growing degree days calculator

Five days of 82/61, 79/58, 90/66, 93/70 and 88/63 °F earn 118.5 growing degree days on the capped 86/50 °F method used for corn — an average of 23.7 a day, and the same heat is 65.8 GDD on the Celsius scale with a 10 °C base. Paste your own highs and lows, one day to a line, add whatever you have already banked this season, and the running total is what seed guides mean by relative maturity — a measure of the season the crop has actually had, which a calendar date is not.

Your answer

Accumulated growing degree days97 GDD °F

Earned over these days: 97 GDD °F
Days counted: 5
Average per day: 19.4 GDD °F
The same total in the other scale: 53.9 GDD °C
Base temperature used: 50 °F

How the answer is worked out

A growing degree day is the amount of usable warmth in one day:

GDD = (high + low) ÷ 2 − base, with the day’s total never going below zero.

The capped version, which corn is tracked with, first pulls the high down to 86 °F and lifts the low up to 50 °F, then takes the average and subtracts the 50 °F base. So the warmth a crop can use is bounded in both directions: a cold night does not take degree days away, and a heatwave does not add them.

Base temperature is the crop’s floor — 50 °F for corn, and other crops sit anywhere from the low 40s to the high 50s. Leave the field empty for the method’s own default, or type the base your guide uses.
Already banked lets you pick up mid-season: enter the accumulation to date and the running total continues from it instead of starting at zero.
Both scales are printed for one total, because a Fahrenheit GDU figure and a Celsius degree-day figure describe the same days.

What the total is for

Seed is sold by the heat it needs, not by the date it is planted: a hybrid rated at 2,600 GDUs to black layer will reach it in mid-September in one season and early October in another. Accumulating from planting tells you where the crop actually is — and, after the fact, why two fields drilled a week apart finished together.

The same arithmetic runs pest and disease models, with different bases and different thresholds. This page gives you the accumulation; what a given total means for a particular crop, hybrid or insect belongs to the guide that published the threshold, and this page will not restate someone else’s table of them.

The numbers are yours

Days are parsed as you paste them — “May 1: 78 55”, “78 55” or “78,55”, one day to a line. Nothing leaves the browser, and the last thing you typed is still there when you come back.

A worked example

Every figure below is computed by the calculator above from this case — nothing here is typed in by hand:

Temperatures are in: °F
Method: Capped (corn: 86/50 °F, 30/10 °C)
Daily high and low: 82 61 · 79 58 · 90 66 · 93 70 · 88 63

Accumulated growing degree days:118.5 GDD °F

Earned over these days: 118.5 GDD °F
Days counted: 5
Average per day: 23.7 GDD °F
The same total in the other scale: 65.8 GDD °C
Base temperature used: 50 °F

Worked by Agrimetra with the same engine the calculator above runs.

What this means in the United States

Corn in the Corn Belt is tracked with the capped 86/50 °F method: anything above 86 °F counts as 86, anything below 50 °F counts as 50, because the crop stops responding outside that window. Seed companies publish relative maturity in GDUs on the same basis, which is why an accumulated total is more useful than a calendar date.

Questions growers actually ask

What does "capped 86/50" mean?: Anything above 86 °F counts as 86 and anything below 50 °F counts as 50 before the day's average is taken, because corn stops responding outside that window. A day of 97/70 therefore earns the same as a day of 86/70. The simple method applies no cap and is what most other crops and insect models use.
Are GDD and GDU the same thing?: In the US corn belt, yes — seed companies quote GDUs and they are the capped 86/50 °F degree days this page computes. The name changes with the audience, not the arithmetic.
Can I mix Celsius data in?: Switch the temperature unit and the page computes in that scale, with a 10 °C base and a 30 °C cap, then prints the same total converted to the other scale so a figure from a seed guide in GDUs can be compared with a European table in degree days. Do not mix the two in one paste.
Why type temperatures instead of taking them from a weather service?: Because the number that matters is the one at the field, not at the airport, and because a page that depends on a weather API stops working the day the key expires. Paste from your own station, your logger or any forecast table — it works offline and nothing is sent anywhere.

Last updated: September 22, 2026

Sources

Midwestern Regional Climate Center — growing degree day information (checked September 22, 2026)
Purdue University — Corn Agronomy, heat unit accumulation and crop staging (checked September 22, 2026)