Layer for a 40-degree swing
Namche sits around 3,440 metres and Kala Patthar is above 5,300. You will move through spring-jacket weather and well below freezing on the same trek, sometimes the same day. Three light-to-mid layers beat one heavy one: a base layer that manages sweat, an insulating mid layer, and a wind and waterproof shell. Add a down jacket for evenings and summit mornings.
Skip cotton entirely. It holds moisture against your skin at altitude, which is exactly what you do not want on a cold morning at 5,000 metres.
Footwear is not the place to save weight
Broken-in, waterproof trekking boots with ankle support are non-negotiable. Buy them at least a month out and put real miles on them before you fly. Bring one pair of camp shoes or sandals for teahouse evenings, both to rest your feet and to keep boots dry overnight.
Pack two pairs of proper trekking socks plus a thin liner sock for each. Blisters, not altitude, end more treks in the first three days than anything else.
Sleep system
Teahouses provide a bed and a blanket, not always enough for the altitude. A sleeping bag rated to at least minus 15 Celsius covers the Everest Base Camp route in any season we run it. If you would rather not fly with one, we arrange rental in Kathmandu for a small daily fee, quality-checked before it leaves our office.
The short list of things people forget
A headlamp with spare batteries, since batteries drain fast in the cold. Sunglasses rated for glacier glare, not ordinary sunglasses. Water purification tablets or a filter, since bottled water gets both expensive and wasteful above Namche. A basic first-aid kit with blister plasters and any personal medication, doubled up in case a bag goes missing on the Lukla flight.
Your guide carries a full trek first-aid kit and a pulse oximeter, but the small personal items above are yours to bring.