The Clo Insulation Unit
How engineers quantify "enough clothing for this temperature".
One clo is a business suit. Everything else is a multiple — and activity level changes the required multiple.
Dressing for cold is not about stacking layers until you feel warm. The engineering discipline is quantifying how much insulation (in clo units) your body needs to maintain thermal neutrality at a given outside temperature and activity level. Runners over-dress; still hikers under-dress. The math is simple, and it scales.
Quick answer
One clo is a business suit. Everything else is a multiple — and activity level changes the required multiple.
Key points
- ▸ One clo = 0.155 m^2·K/W. Physical definition: insulation of a 1950s business suit worn at rest in 21°C, 50% humidity, no wind.
- ▸ Formula: required_clo = (33°C − air_temp) / (8 × met). Higher activity (met) lowers the required insulation.
- ▸ Typical clothing values: t-shirt ~0.1 clo, dress shirt ~0.2, sweater ~0.3, fleece ~0.5, parka ~1.5, heavy-duty snow jacket ~3.0.
- ▸ Layering rule of thumb: sum the individual clo values of your layers, multiplied by 0.8 for overlap losses.
- ▸ Wind and wetness destroy clo — effective insulation of a wet down jacket drops 50-70%.
Examples
- 40°F air · brisk hiking (4 met)required_clo = (33 − 4.4) / (8 × 4) = 0.89 clo. Base layer + fleece + light shell. Full winter jacket would cook you.
- 20°F air · standing outdoors (1.2 met)required_clo = (33 − −6.7) / (8 × 1.2) = 4.13 clo. Expedition-grade stack: base, mid, heavy insulation, shell, gloves, hat.
- 65°F air · sitting at desk (1.1 met)required_clo = (33 − 18.3) / (8 × 1.1) = 1.67 clo. Business casual stack — long pants, shirt, light sweater.
When to use which tool
Related
- Insulation Logic · Clo UnitsRequired clothing insulation in clo units for air temperature and activity level. Suggested garment stack.
- Interpreting CloTranslating a clo number into the shirts, layers, and jackets on your shelf.
- Clo Formula Edge CasesWhen the simple formula under-states required insulation — sometimes badly.
- What a MET Actually IsThe metabolic equivalent of task, and how it turns activity into calories.
Frequently asked questions
› Why do runners under-dress for cold weather? Troubleshooting
At a met level of 8-10, metabolic heat output is high. Running at 30°F requires less than 0.5 clo — a long-sleeve shirt and tights. Over-dress and sweat soaks layers, then you chill hard at the first stop.
› How does humidity affect clo? How-to
Cold-dry air feels warmer than cold-wet air at the same temperature because moisture conducts heat away faster. Clo math does not directly include humidity; add one band of insulation for very humid cold conditions.
› How should I use this guide with a Kefiw tool? How-to
Use the guide as the plan and the linked Kefiw tool as the check. Read the steps first, try the move manually, then use the tool to compare outputs, catch edge cases, and decide whether the result actually fits your task.
› What mistake do tool guides help avoid? Troubleshooting
Tool guides help avoid using a utility mechanically without understanding what you are trying to accomplish. Most word, writing, and text utilities are fast, but speed can hide context mistakes. Know whether you are solving a puzzle, cleaning copy, drafting a line, or checking a rule.
› Can a tool guide help me learn the skill? How-to
A tool guide can help you learn if you pause before accepting the output and ask why it worked. Compare your first guess with the tool result, look for the rule or pattern, and repeat that review. Passive copying solves one task; active review builds the skill.