What this tool is, and who it is for
Layering is more efficient than single thick garments because it lets you regulate temperature as conditions and effort change. This advisor does the metabolic balance work for you. It estimates the kilocalories your body produces at the chosen activity level, accounts for wind chill and rain, and recommends a stack that matches — typically a synthetic or merino base, a fleece or active insulation mid, and a wind or waterproof shell. The math comes from the same ISO standards that ergonomic consultants use in cold-room testing, simplified into something you can apply at the kitchen table.
Metabolic output matters more than temperature for what you wear while moving.
Approximate clothing insulation needed for the conditions and output entered.
Recommended system total. Wind discount applied: ×1.12.
- 1.Merino or synthetic base layer, 150–170 g/m² — Manages moisture against the skin.
- 2.Light grid fleece or active insulation midlayer — Breathable warmth you can wear while moving.
- 3.Thicker fleece or insulated midlayer — For cold stops and slow sections.
- 4.Synthetic or down belay jacket — Static warmth only. Comes off before hard effort.
- 5.Hardshell or windproof outer — Wind and precipitation protection.
Step-by-step user guide
How to operate the calculator in your browser, what to enter, and what to do with the result.
- 01
Enter the forecast low temperature and the wind speed you expect on the trip.
- 02
Choose the highest-output activity you will do, typically climbing with a load — the model assumes you will be out for most of the day.
- 03
Read the recommended stack under 'Suggested layers'. The CLO total appears next to the requirement so you can see whether you are over- or under-dressed.
- 04
If a layer is marked 'optional', drop it first when weight matters. If it is marked 'essential', do not leave it behind.
- 05
Re-run the advisor for the camp scenario (low activity, lower temperature) to confirm you have a separate camp insulation piece.
What the calculator actually does with the numbers you enter
- 01
Metabolic heat output is estimated from activity level using standard MET values, from 2.5 MET for slow walking to 8 or more for steep ascent with a pack.
- 02
Wind chill reduces the effective insulation of outer layers, so exposed garments are discounted by an amount that grows with wind speed.
- 03
Required insulation is expressed in approximate CLO units, the same measure used in ISO 9920 and ISO 11079 clothing insulation work.
- 04
Available insulation is summed across the recommended layers, and the difference between requirement and provision determines the recommendation.
Published sources behind the formulas
- [1]ISO 9920 — ergonomics of the thermal environment: clothing insulationInternational Organization for Standardization · 2024
- [2]ISO 11079 — required clothing insulation for cold environmentsInternational Organization for Standardization · 2024
- [3]Compendium of physical activities and MET valuesArizona State University / ACSM · 2024
Frequently asked questions
Short, sourced answers to the questions readers send us most often about this calculator.
Why does humidity change the math?
Moisture trapped inside a garment reduces its insulation, often dramatically. ISO 11079 includes humidity as a separate variable because the same temperature in dry cold is far easier to dress for than the same temperature in damp cold. This advisor does not yet differentiate humidity; treat its output as a dry-cold baseline and add insulation for damp conditions.
Can I substitute a fleece for a synthetic mid?
Yes for low-output use; fleece retains insulation when damp, dries slowly and is heavier per unit insulation. Synthetic puffy insulation has a better warmth-to-weight ratio but loses loft when soaked. Pick by trip profile: fleece for damp climates and short trips, synthetic puffy for cold dry days and longer trips.
What if I run cold?
Add 0.5 to 1.0 CLO to the result, either through a heavier mid layer or by adding a camp insulation piece. The advisor is calibrated to the median hiker; individual metabolic rates vary by plus or minus 20 percent, so the result is a baseline, not a prescription.
Do these numbers work in alpine summer?
Yes, but the wind adjustment is the dominant term above 4,000 metres. A 50 km/h wind at -5 degrees will strip more heat than the thermometer suggests, which is why the advisor shows both numbers and asks for the wind speed rather than letting you skip it.