Gardening Zone Finder
Turn your coldest winter temperature into a USDA-style hardiness zone and planting calendar.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
This rough USDA-style estimate maps one entered average annual extreme minimum temperature to a 5°F half-zone. The official USDA map uses 30 years of location-based climate data; this calculator is not a location lookup. It accepts -60°F to 70°F (-51.1°C to 21.1°C). Hardiness zones do not describe summer heat, rainfall or soil.
Worked example
calculated with the default values below
Average annual extreme minimum -15 · Temperature unit Fahrenheit · Typical snow cover Light and patchy Hardiness zone 5b · Zone as a number 5.5 · Average annual low -15.0 °F · Average annual low -26.1 °C · What winter feels like Cold to moderate — a classic four-season garden · Plants that thrive Apple, rose, lavender, tomato · Last frost, typically Mid April · First frost, typically Early October · Snow cover effect Patchy snow offers limited insulation — mulch after the ground freezes to protect roots.
General background references
Disclaimer
These are general gardening estimates. Planting windows, growth, water and nutrient needs vary by place and plant. Check local extension guidance and product labels before acting on an estimate.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
t_F=t_C×9/5+32; z=clamp((t_F+60)/10+1,1,13.5); zone=floor(z)+half
- t_F
- average annual extreme minimum (°F)
- z
- mapped zone coordinate
- half
- a below midpoint, b at/above midpoint
Valid input range is −60°F to 70°F (−51.1°C to 21.1°C), matching USDA half-zone ranges 1a–13b. This single-input estimate is not the official 30-year location map.
Frequently asked questions
What does this zone estimate represent?
The calculation maps one average annual extreme minimum temperature to 5°F half-zone bands. The official USDA map uses 30-year, location-based climate data, and recommends the zone only as a general guide for perennial cold hardiness. It does not describe summer heat, rainfall, soil or microclimates.