Growing degree days and Winkler index calculator

Add up a season’s growing degree days from monthly mean temperatures and see which Winkler region your vineyard falls in — or work out a single day’s degree days. Free, no sign-up.

Mean temperature of each month (°C)

The monthly mean of daily mean temperatures, from your weather station or the nearest one. Leave a month empty if you do not have it.

Degree days
Season growing degree days1,960 °C·d
Winkler regionRegion IV
April90 °C·d
May217 °C·d
June330 °C·d
July434 °C·d
August434 °C·d
September300 °C·d
October155 °C·d
  • Monthly means approximate the sum of daily degree days. They miss the days whose minimum is below the base and maximum above it, so cool months come out a little low.
  • The classic Winkler index sums degree days above 10 °C from April to October (October to April in the south). Bands follow Jones et al. (2010).
  • As a rough guide, cooler regions (Ia–Ib) suit early-ripening varieties and sparkling base wines, while warmer ones (III–V) suit late-ripening varieties and fortified styles.

An estimate from the temperatures you enter. Site, slope and canopy all change how a vineyard actually ripens.

Printed from grapeflow.app/tools/growing-degree-days

How growing degree days work

A degree day is one degree above the base temperature held for one day. Each month adds (mean temperature − base) × days in the month, or nothing when the mean is below the base. The season total is the heat the vines had to grow and ripen.

The Winkler index adds these up from April to October in the northern hemisphere (October to April in the southern) at a base of 10 °C, and sorts regions from Ia (coolest) to V (warmest). Totals in °F·d are converted to °C·d (× 5/9) to classify.

Example

Monthly means of 13, 17, 21, 24, 24, 20 and 15 °C from April to October give 90 + 217 + 330 + 434 + 434 + 300 + 155 = 1 960 °C·d — Region IV.

Frequently asked questions

Where do I get monthly mean temperatures?

From your own weather station, the national weather service’s climate normals, or a gridded weather archive for your coordinates. Use the mean of daily mean temperatures, not the mean of the maximums.

Why use an upper cutoff?

Vine development does not keep accelerating above about 30 °C (86 °F); capping the maximum keeps heatwave days from inflating the total.

Is the Winkler region enough to choose varieties?

It is a first filter. Frost risk, rainfall, soil, the length of the season and the market for the wine matter just as much.

Keep going

Related calculators

Harvest readiness calculatorFrom two ripening samples: when sugar reaches your target °Brix, and where pH and acidity stand for the wine style.Vineyard disease riskThe 3-10 rule for downy mildew, the Gubler-Thomas index for powdery mildew and botrytis infection conditions — from your own readings.

GrapeFlow

Degree days that count themselves

GrapeFlow adds up degree days from your vineyard’s daily weather and compares the season with past years — early or late, you see it as it happens.

Explore the GrapeFlow demoStart your free trial

Free for 90 days · no credit card