Use this worksheet to add one day’s growing degree days (GDD) to a running total. Supply the day’s minimum and maximum temperatures in °C, a base temperature in °C from a named crop or project reference, and the previous cumulative GDD.
Method and versioned assumptions
The supported method is the simple daily average:
mean temperature = (daily minimum + daily maximum) / 2
daily GDD = max(0, mean temperature - base temperature)
new cumulative GDD = previous cumulative GDD + daily GDD
Record the temperature method, base-method source and version, and the uncertainty or range for the temperatures and base. The daily minimum cannot exceed the daily maximum, previous GDD cannot be negative, and every required value and source field must be present and finite. Results are displayed to one decimal place.
This method applies no upper-temperature cutoff, hourly integration, Fahrenheit conversion, crop threshold, or date projection. A base from one crop, cultivar, location, or method should not be silently reused for another.
Example: estimating one day of plant growth
With a minimum of 10°C, maximum of 24°C, base of 10°C, and previous total of 0:
mean = (10 + 24) / 2 = 17°C
daily GDD = max(0, 17 - 10) = 7
The outputs are 7.0 daily GDD and 7.0 cumulative GDD.
Build a daily accumulation log
The following arithmetic illustrates how the prior total carries forward while the base remains 10°C:
| Day | Minimum / maximum | Mean | Daily GDD | Cumulative GDD |
|---|---|---|---|---|
| 1 | 10°C / 24°C | 17°C | 7.0 | 7.0 |
| 2 | 8°C / 22°C | 15°C | 5.0 | 12.0 |
| 3 | 5°C / 14°C | 9.5°C | 0.0 | 12.0 |
For each new day, enter the prior row’s cumulative value. Keep the same observation timing, temperature source, base definition, and method version throughout the log. If any of those changes, start a separately labelled series or document the break.
Interpretation, limits, and next step
GDD is an accumulated temperature index under the entered method. It does not diagnose plant condition or establish growth stage, harvest date, watering, light, nutrient, pest, or safety guidance. Measurement error near the base can change whether a day contributes zero or a positive amount; an entered range is a sensitivity range, not a confidence interval.
Next, preserve the daily temperatures and source notes with the running total. Compare that total with a crop-specific threshold only when the threshold and this exact temperature method are supported by the same applicable reference.
For related plant-care arithmetic, pair growing degree days with the plant light requirements calculator, plan the season’s planting windows with the gardening frost date calculator, and track watering with the plant watering schedule calculator.
Keep the base-method record through accumulation
Save each day’s minimum, maximum, base temperature, and source/method description with the accumulated GDD. If you change the base method midway, keep that scenario separate instead of silently combining it with an older accumulation. The arithmetic does not establish a species’ growth threshold.
A daily log that survives a method change
Keep one row per day with minimum temperature, maximum temperature, chosen base, daily GDD, and running total. Retain the base-temperature source and method version alongside that log.
If you revise a temperature, recalculate that day’s contribution and the later running totals under the same method. If you change the base or method, start a separately labeled series so a cumulative number does not silently combine incompatible assumptions.
Sources
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National Weather Service, “Glossary: Growing Degree Day” — current glossary edition; complete definition, accessed 2026-07-09.
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Ddconcepts — Degree-day concept and daily minimum/maximum estimation-method documentation (accumulated heat between thresholds), matching the worksheet’s daily-average accumulation approach.