Layering System Calculator

This calculator translates ambient conditions into a required CLO value — the standard unit of clothing insulation — and then maps that number onto concrete layer recommendations. It first applies the National Weather Service's published Wind Chill Temperature Index formula (35.74 + 0.6215T − 35.75V^0.16 + 0.4275TV^0.16, with T in °F and V in mph) whenever the temperature is 50°F or below and wind exceeds 3 mph, since wind chill only meaningfully differs from air temperature in that range. It then adds back an "activity heat" offset (0° for sedentary or light activity, 8° for light hiking, 15° for moderate hiking, 22° for strenuous climbing) to reflect that your body generates real heat while moving, effectively warming the conditions you're dressing for.

The resulting effective temperature feeds into a simplified formula, required CLO = (68 − effective temperature) / 18, which estimates how much insulation you need to stay comfortable relative to a 68°F comfort baseline; a Math.max(0, ...) clamp keeps the result from dropping below zero on warm, low-wind days when no extra insulation is needed at all. From there, thresholds on that CLO value determine whether you need no base layer or a synthetic/merino one, how many mid-layer fleece or puffy pieces (each roughly 1.0-1.5 CLO) to add, whether a wind shell is warranted (triggered independently whenever wind tops 15 mph or any precipitation is selected), and whether you need a dedicated insulated jacket (CLO above 2.5) or rain layer. Treat the output as a solid starting checklist, not a substitute for real-world layering experience — individual cold tolerance, sweat rates, and gear quality vary enough that you should adjust up or down based on how you personally run hot or cold.