Heat Index Calculator

The heat index, often called the feels-like temperature, captures something a thermometer alone cannot: how hot it really feels once humidity is taken into account. This calculator works it out from the air temperature and the relative humidity. Your body keeps cool mainly by sweating and letting that sweat evaporate, but when the air is already laden with moisture, evaporation slows right down, your natural cooling falters, and the same temperature feels markedly hotter and more draining. That is why a humid 30-degree day can feel far more punishing than a dry one. Enter the temperature in degrees Celsius and the relative humidity as a percentage, and the calculator returns the heat index along with a plain-language risk level, updating as you type. It uses the United States National Weather Service regression formula, the standard method behind the feels-like figures you see in forecasts, converting to and from Fahrenheit internally so you get a Celsius answer. The figure is most meaningful in warm, humid conditions, generally above about 27 degrees, which is exactly when heat stress becomes a real concern. Knowing the heat index helps you make sensible decisions: when to slow down outdoor work or sport, when to make sure children, older people and pets stay cool and hydrated, and when conditions tip from merely uncomfortable into genuinely risky. While New Zealand is more temperate than the tropics, humid summer spells in the north can still push the feels-like temperature well above the actual reading, so it is a useful check during a heatwave. It is also a clear teaching tool for understanding humidity, evaporation and the body's cooling. The formula and a worked example are explained below.

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