Onion GDD (Heat Units) Calculator
Calculate growing degree days for onion (base 6°C, ~2000 GDD) and estimate days to maturity.
Onion bulbing responds to heat units and daylength; GDD helps predict bulb maturity. GDD = (Tmax + Tmin)/2 − 6°C; accumulate from sowing to predict stages and harvest.
Indicative planning figures based on published research averages. Local soil tests, varieties and weather change actual requirements — confirm with your agronomist or extension officer.
Growers use the free Onion GDD Calculator to schedule the crop by accumulated heat units rather than calendar days, improving harvest and stage prediction.
About Onion GDD (Heat Units) Calculator
Onion develops by accumulated heat, not calendar days. With a base temperature of 6°C and roughly 2000 growing degree days to maturity, daily GDD = (Tmax + Tmin)/2 − 6°C. Onion bulbing responds to heat units and daylength; GDD helps predict bulb maturity. Enter today's temperatures and your accumulated GDD to see today's heat units, the estimated days to maturity at current temperatures, and the progress to harvest.
How to use Onion GDD (Heat Units) Calculator
- 1Enter today's max and min temperature.
- 2Optionally enter accumulated GDD since sowing.
- 3Read today's GDD, days to maturity and progress.
Why use Onion GDD (Heat Units) Calculator?
- ✓Uses onion's base temperature and GDD-to-maturity
- ✓Predicts maturity from heat units, not the calendar
- ✓Tracks progress to harvest
- ✓Free, instant and fully in-browser
Frequently asked questions
What base temperature does onion use for GDD?+
About 6°C. Daily growing degree days are the mean temperature minus 6°C, so little development is credited on days below that threshold.
How many GDD does onion need?+
Roughly 2000 growing degree days to maturity. Onion bulbing responds to heat units and daylength; GDD helps predict bulb maturity.
Why track heat units instead of days?+
Because temperature drives development: the same GDD total marks the same growth stage whether the season is warm or cool, making GDD a better scheduling tool than days after sowing.
Is this tool free and private?+
Yes — free, no sign-up, and all calculation runs in your browser, so it works offline in the field and your data never leaves the device.
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