Temperature-Humidity Index (THI) Calculator for Dairy & Beef Cattle
Calculate the temperature-humidity index (THI) from air temperature and relative humidity and see the cattle heat-stress level — from the 68 stress threshold for dairy cows to severe — with cooling advice.
How it works
THI = T°F − (0.55 − 0.0055 × RH) × (T°F − 58), with T°F = 1.8 × T°C + 32 (NRC 1971) · Equivalent metric form: THI = (1.8T + 32) − (0.55 − 0.0055 RH)(1.8T − 26)
The temperature-humidity index combines air temperature and humidity into one number for the heat load on cattle, because humid air stops cows from cooling by evaporation. Research on modern high-yielding dairy cows (Zimbelman and Collier) lowered the stress threshold from 72 to 68: below 68 cows are comfortable, 68–71 is the threshold where milk yield and conception start to fall, 72–79 is mild-to-moderate, 80–89 moderate-to-severe and 90 or more severe heat stress. Beef cattle tolerate somewhat more, but the same categories guide cooling and handling decisions.
Worked example
At 30 °C (86 °F) and 60% relative humidity: THI = 86 − (0.55 − 0.0055 × 60) × (86 − 58) = 86 − 0.22 × 28 = 79.8 — "Mild to moderate heat stress". Cows need fans, sprinklers and shade, and feeding in the cooler hours.
Sources: Collier, Zimbelman et al. — Managing heat stress and its impact on cow behavior (Western Dairy Management Conference 2013) · Effect of the temperature–humidity index on the productivity of dairy cows (Animals, 2024)
Frequently asked questions
At what THI do dairy cows get heat stress?
From a THI of about 68 for high-producing dairy cows. Older references used 72, but cows producing more milk generate more metabolic heat, so the threshold has been lowered.
What are the signs of heat stress in cattle?
Breathing faster than about 60 breaths per minute, panting with the mouth open, drooling, standing instead of lying down, crowding around water, eating less and a drop in milk yield. Conception rates also fall during and after hot spells.
How can I reduce heat stress in cows?
Provide shade, plenty of clean water at several points, fans plus sprinklers or soakers in the holding pen and barn, feed in the cooler hours and avoid handling cattle in the heat of the day.
Does THI apply to beef cattle?
Yes, as a guide. Feedlot cattle, black-hided animals and heavy finishing cattle are most at risk; extension services issue heat-stress alerts when THI stays above about 75–79 day and night.
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This calculator is for educational purposes. Double-check important results before acting on them.