Calculate winter chill hours for fruit trees with the Standard and Utah models, and see which fruit crops your climate can support.
Standard counts each hour from 32–45°F equally. Utah weights every hour and is stricter in mild climates, where warm spells cancel chill.
Estimate assumes a constant average temperature. For precise tracking, use local hourly temperature data.
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Chill hours are the accumulated hours of cold temperatures (typically 32-45°F) that deciduous fruit trees and perennial plants need during winter dormancy. Proper chill accumulation is essential for healthy bud break, flowering, and fruit production in spring.
Chill hours measure the time trees spend within a cool temperature range during winter dormancy — cold enough to satisfy the dormancy requirement, but not freezing. This calculator uses the two most widely used models: the Standard model, which counts every hour between 32°F and 45°F (0–7.2°C) as one chill hour, and the Utah model (Richardson et al., 1974), which weights each hour and penalizes warm spells. A third approach, the Dynamic model (chill portions), better captures mild-climate conditions but requires continuous hourly temperature data and a kinetic computation, so it isn't estimated from a single average here.
Standard Chill Hours Formula
Most apple varieties need 800-1200 chill hours for proper fruiting.
Peaches, cherries, and plums have varying chill requirements (300-1000 hours).
Evaluate locations for new orchard establishment.
Monitor winter progress and predict spring growth timing.
Choose fruit tree varieties suited to your region's typical chill accumulation.
Estimate when trees will break dormancy and begin flowering.
Identify years with insufficient chill that may affect fruit production.
Trees may experience delayed or irregular bud break, reduced fruit set, poor fruit quality, and extended bloom periods that increase frost-damage risk.
The Standard model counts every hour between 32°F and 45°F equally as one chill hour — simple and the most widely cited. The Utah model (Richardson et al., 1974) assigns a weight to each hour: 34–48°F is worth a full unit, cooler and warmer hours less, and hours above 60°F subtract chill. Utah is more realistic in mild climates where warm afternoons erode chill, but the two agree closely in consistently cold winters.
The Dynamic model (Fishman et al., 1987) treats chilling as a two-stage process in which an intermediate product can be destroyed by heat before it becomes a permanent 'chill portion.' It models real hourly temperature cycles and is favored in warm regions, but it cannot be reduced to a per-hour lookup or estimated from a single average temperature — it needs continuous hourly data. For that reason this tool reports the Standard and Utah figures and leaves chill portions to hourly-data services.
Chill hours typically accumulate from November through February in the Northern Hemisphere, starting after the first significant cold period and ending when trees begin breaking dormancy.
In the Utah model, yes — hours above 60°F are assigned negative units that offset previously banked chill, which is why mild, fluctuating winters often fall short. The Standard model has no negation: it simply counts qualifying hours, so it tends to overestimate chill in warm climates.