Chill Hours Calculator
Enter a typical winter low and high to estimate the chill hours your site accumulates each dormant season.
Estimated chill accumulation
How the estimate is built
Instead of asking for hour-by-hour temperature readings, the calculator treats a typical winter day as a smooth swing between its low and its high, the same “sine method” used by growing-degree-day calculators in agriculture. From that curve it works out, using basic trigonometry, exactly how many of the day's 24 hours land at or below 45°F and above 32°F — then multiplies by the number of days in your dormant season for a season total.
This is a simplification: real winter days aren't perfect sine waves, and cloud cover, wind, and elevation all nudge the true count up or down. Use the result as a ballpark to compare against a variety's published chill-hour need, not a precise weather-station reading.
Frequently Asked Questions
What are chill hours, and why do fruit trees need them?
Many temperate fruit trees (apples, pears, peaches, cherries, and more) need a minimum amount of cold exposure over winter — measured in "chill hours" — before they can reliably break dormancy and bloom evenly in spring. Too few chill hours can mean delayed, uneven, or sparse flowering, even in an otherwise mild climate.
What is the 32–45°F chilling-hours model?
It's one of the oldest and simplest ways to count winter chill: total up the hours where the air temperature sits at or below 45°F (7.2°C) but above 32°F (0°C). Hours below freezing, and hours above 45°F, don't count toward the total in this model. Newer models (like Chill Portions) refine this further, but the classic hours model remains a widely used rule of thumb.
How does the calculator estimate hours from just a low and a high?
It models a single day's temperature as a smooth sine-wave swing between that day's low (around early morning) and high (around mid-afternoon), then works out mathematically how much of that 24-hour cycle falls inside the 32–45°F band. It's an approximation of the true hour-by-hour reading, useful when you only have a forecast or a typical daily range rather than a full weather log.
How do I compare my estimate against a variety's chill requirement?
Nursery tags and catalogs usually list a chill-hour requirement for each cultivar, e.g. a low-chill peach needing around 250–400 hours versus a high-chill apple needing 800–1,000+. Compare your season-total estimate here against that number: if your site's typical winter falls well short of a variety's requirement, that cultivar is a poor match for your climate.
Educational tool only. This is a modeled estimate from a single representative day, not a record of your site's actual winter — a local weather station log will always be more accurate.