What size backpack do I need: liters and torso fit

How to work out backpack size

Backpack size in liters depends first on where you sleep: in a hut you leave the tent, sleeping bag and stove at home, in a tent you carry them all. The calculator starts from the ranges in the outdoor-renner.de guide: 20-30 L for a day hike, 35-40 L for one or two nights in a hut, 40-50 L for a longer hut-to-hut trip, 45-50 L for one or two nights in a tent, 50-70 L for a backpacking trip and 60-80 L for a week in a tent. Alpkit and Mountain Warehouse give similar bands (45-65 L for self-sufficient trips, 70 or 80 L for a week or more).

Then it adds what changes the volume. In winter it adds 15 L, the middle of the 10-20 L the same guide gives for bulkier clothing. In a hut without meals, with your food in the pack, it adds 5 L, the gap Toporando shows between hut trips with meals and trips with your own food. Water counts above 2 L, because the ranges already allow for a couple of bottles (Toporando puts 2-3 L of water in a day pack), and every extra liter of water takes a liter of pack.

The pack to buy is the 10 L step inside the range that sits closest to its middle. On a tie the smaller one wins: more volume means more weight and less stability, as outdoor-renner points out. You can change the additions in the advanced options. Packs are sold in liters in the US too, so the calculator keeps liters for packs and water.

Pack size depends on torso length, not height. Measure from the C7 vertebra, the bump at the base of your neck when you tilt your head forward, down your spine to the point level with your iliac crest, the top of your hip bones. That is how Gregory and Deuter measure. The calculator gives the Gregory size (S 16-18", M 18-20", L 20-22") and the Osprey sizes and Deuter fixed backs that fit the same torso.

Worked example

In inches (US). The calculator opens on two nights in a tent, summer, 2 L of water and a 19" torso.

• Range from the guide: 45-50 L, nothing added
• 10 L step inside the range: 50 L
• 19" torso (48.3 cm): Gregory size M (18-20"), Osprey S or M, Deuter men's or EL

The same two nights in winter move the range to 60-65 L and the pack to 60 L.

In centimetres (UK). Two nights in a hut with meals, 2 L of water, a 50 cm torso: range 35-40 L, a 40 L pack, Gregory M (45.7-50.8 cm), Osprey M. With your own food in the pack the range becomes 40-45 L and the pack stays at 40 L.

Backpack size chart by trip

Pack to buy, with the range in liters in brackets, carrying 2 L of water.

Trip	Summer	Winter
Day hike	20 L (20-30)	40 L (35-45)
Hut, 1-2 nights	40 L (35-40)	50 L (50-55)
Hut, 3+ nights	40 L (40-50)	60 L (55-65)
Hut, own food, 1-2 nights	40 L (40-45)	60 L (55-60)
Tent, 1-2 nights	50 L (45-50)	60 L (60-65)
Tent, 3-5 nights	60 L (50-70)	70 L (65-85)
Tent, 6+ nights	70 L (60-80)	80 L (75-95)

Sizes by torso length:

Torso	Gregory	Osprey	Deuter fixed
16" (40.6 cm)	S	S	SL
18" (45.7 cm)	M	S	SL, men's
19" (48.3 cm)	M	S, M	men's, EL
20" (50.8 cm)	L	M	men's, EL
21" (53.3 cm)	L	M, L	men's, EL

Packs for sale: liters and torso sizes

Liters on the label don't compare exactly between brands. Alpkit counts only the main compartment; outdoor-renner notes there is no common standard and that some brands include side pockets, the lid and outer pockets. A "+10" or "+15" in a model name means extra liters: on the Deuter Aircontact Core, the 10 L you gain by raising the lid.

Deuter fits an adjustable back from 35 L up; below that, back length comes from the SL (women's), men's and EL (extra long) versions. Size ranges overlap and change from model to model: Gregory says the M of some adjustable packs fits torsos from 38.1 to 48.3 cm (15-19"). So at 18" or 21" two sizes can work. Check the chart for the model you want and try it loaded in a store. With the right size, Gregory says, the hipbelt pad sits about 1" above your iliac crest.

Common mistakes

• Picking the size from your height. Two people of the same height can have different torsos: Gregory and Deuter measure from C7 to the iliac crest.
• Measuring down to the waist. The lower point is the top of the hip bone. Deuter suggests placing a belt along it and measuring to the belt's top edge.
• Buying big just in case. People often take the larger pack when unsure, outdoor-renner says, but extra volume adds weight and costs stability.
• Counting only the days. Where you sleep matters more than one extra day: outdoor-renner puts a week in huts at 40-50 L and three days in a tent at 45-50 L.
• Forgetting winter. A puffy jacket, warm layers and a winter sleeping bag take room: the guide adds 10-20 L.