Interactive tool
Layering Calculator
Tell the calculator what the weather is doing and how hard you'll be working, and it returns a base / mid / shell stack, an insulation weight, and the accessories that actually change your comfort.
- Base layer
- 200–250gsm merino crew plus a merino bottom.
- Midlayer
- 200-weight fleece or a light active-insulation jacket.
- Shell
- Optional wind shirt; skip a shell if it is calm and dry.
- Insulation target
- ~76g synthetic fill (or 700+ fill-power down equivalent) across your mid and puffy.
- Warm hat — most of your adjustable heat loss is here.
- Liner gloves plus an insulated overmitt.
- Carry the puffy; put it on the moment you stop moving.
The method
- Wind chill. Air temperature is converted with the National Weather Service wind-chill formula:
35.74 + 0.6215T − 35.75V^0.16 + 0.4275T·V^0.16where T is °F and V is wind speed in mph. It applies at or below 50°F and above 3 mph. - Wet penalty. Rain, wet snow, or spray subtracts a further 8°F. Water conducts heat away roughly 25 times faster than still air, and a soaked layer stops insulating almost entirely.
- Activity offset. Metabolic heat is added back as an effective temperature bump: 0°F static, +10°F light, +20°F moderate, +32°F hard output.
- Stack mapping. The effective temperature selects base, mid, and shell weights, and sets an insulation target of roughly 3.6g of synthetic fill per degree below 60°F, capped at 300g.
Rules that matter more than the numbers
- Be bold, start cold. Sweat is the enemy of every insulation system.
- Vent before you shed. Pit zips and a half-zip beat stopping to strip a layer.
- Always carry one layer more than the calculator says if you are far from a road, or in terrain where an injury means sitting still.
- Wind is temperature. A 20 mph wind at 30°F is a 17°F day to your skin.
Sources
- NOAA / National Weather Service wind chill index (2001 revision).
- Wilderness Medical Society practice guidelines on the prevention and treatment of hypothermia and frostbite.
- US Army Research Institute of Environmental Medicine cold-weather clothing guidance.
Last reviewed: 2026-08-27.
Frequently asked questions
How does a layering calculator work?
It converts the conditions you enter into an effective temperature — air temperature adjusted for wind chill and precipitation — then subtracts the heat your activity level produces. The result maps to a base, mid, and shell recommendation with an insulation weight in grams of synthetic fill or fill-power-equivalent down.
Why does activity level change the recommendation so much?
A hiker climbing with a loaded pack produces several hundred watts of metabolic heat and needs far less insulation than the same person glassing from a ridge. Dressing for the exertion, not the thermometer, is the single biggest mistake in cold-weather clothing.
Should I ever wear cotton as a base layer?
Not in cold or wet conditions. Cotton holds water against skin and keeps conducting heat away long after you stop sweating. Merino wool or a synthetic base layer moves moisture and keeps insulating when damp.
What is the 'be bold, start cold' rule?
Start a climb slightly underdressed so you warm into your layers instead of soaking them. If you are comfortable in the first five minutes of a hard uphill, you are overdressed and will sweat out your insulation.
Does this replace a manufacturer's temperature rating?
No. Use it to plan a stack, then check the actual garment ratings. Body size, metabolism, hydration, food, and wind exposure all shift real-world comfort by ten degrees or more in either direction.