Chill Hours Calculator
Many deciduous fruit and nut trees need a period of winter cold before they will break dormancy and flower properly in spring. Growers track this as chill hours: the number of hours the temperature sits within a chilling range, classically between 0 and 7.2 degrees Celsius (32 to 45 degrees Fahrenheit). Each fruit variety has a chill requirement, and falling short can mean poor, delayed, or uneven bloom. This tool totals chill hours from the hours you record in the chilling band and compares them against a variety's requirement so you can see whether the cold accumulated is enough. The temperature thresholds and the variety requirement are all user-editable.
Chill hours formula
chill hours = count of hours where temperature is from 0 C to 7.2 C inclusive
percent met = chill hours / requirement * 100
shortfall = max(0, requirement - chill hours)
The classic chill-hours model counts one chill hour for every hour the air temperature falls within the chilling range, typically 0 to 7.2 degrees Celsius. Hours outside that band, whether warmer or freezing, count zero in the basic model. Add the qualifying hours through the dormant season and compare the total against the published chill requirement for your variety.
Worked example
A dormant season records 600 hours within the 0 to 7.2 degree Celsius chilling range. A peach variety needs 800 chill hours. Percent met = 600 / 800 * 100 = 75.00 percent. The shortfall is 800 - 600 = 200.00 hours. With only three quarters of its requirement met, that variety may bloom unevenly, so a lower-chill cultivar might suit the site better.
Frequently asked questions
What is a chill hour?
A chill hour is one hour during which the temperature stays within the chilling range that promotes dormancy break, classically 0 to 7.2 degrees Celsius (32 to 45 degrees Fahrenheit). Summing chill hours across the dormant season gives the chill accumulation, which growers compare against each variety's published requirement.
Which temperature range should I use?
The traditional model uses 0 to 7.2 degrees Celsius. Some regions use a simpler below-7.2-degree count, and other models such as Utah chill units and the Dynamic Model (chill portions) weight temperatures differently. This calculator uses the classic range, with the thresholds editable so you can match local guidance.
What happens if a tree does not get enough chill?
Insufficient winter chill can cause delayed or erratic bud break, prolonged or weak flowering, reduced fruit set, and lower yields. In mild-winter areas, growers choose low-chill cultivars bred to need fewer hours. Knowing your site's typical accumulation helps you match varieties to climate.
Do warm spells cancel out chill hours?
In the basic chill-hours model used here, warm hours simply do not count; they neither add nor subtract. More advanced models such as the Utah model do subtract chill for high temperatures, reflecting evidence that warm spells can negate prior chilling. Use a weighted model if your extension service recommends one.
Official sources
- USDA Agricultural Research Service: tree fruit dormancy and chilling research.
- USDA: U.S. Department of Agriculture.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.