Chill Hours Calculator: Winter Chill for Fruit Trees
Apples, peaches, cherries and blueberries need a certain amount of winter cold to flower and fruit well. Plant a variety that needs more chill than your winters give, and it blooms poorly.
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What this calculator does
Enter typical winter highs and lows and how many days they last. The calculator estimates the chill hours (or Utah units) and compares them with your variety's needs.
How to use the chill hours calculator
- Enter a typical winter day's high and low.
- Enter the number of winter days with similar weather.
- Choose the simple chill-hour model or the Utah model.
- Choose a fruit or enter your variety's rating.
- If you fall short, choose a lower-chill variety.
How the calculation works
hourly temperatures are estimated from the daily high and low (low near dawn, high mid-afternoon) chill hours = hours between 0 and 7.2 °C (32–45 °F) Utah units weight each hour: 1 for 2.5–9.1 °C, ½ near the edges, negative above 16 °C
Both are standard models; the Utah model also subtracts chill when it is warm.
Worked example
A winter with typical highs of 10 °C and lows of 0 °C gives about 15 chill hours a day, roughly 1,350 over 90 days: enough for standard apple varieties needing 800.
Assumptions and limitations
- Real weather varies day to day; long-term local averages are more reliable.
- Variety ratings differ between sources.
- Very cold hours below freezing do not count in either model.
Frequently asked questions
What are chill hours?
Hours between 0 and 7.2 °C (32–45 °F) during dormancy, which many fruit trees need to break dormancy properly.
How many chill hours do apples need?
Most standard apples need 500–1,000; low-chill varieties such as 'Anna' need 200–400.
What happens if a tree gets too few chill hours?
Late, sparse and uneven flowering and leafing, and a smaller crop.
What is the Utah chill model?
A model that weights temperatures by how effective they are at chilling and subtracts chill on warm days.