Choose a backpack by measuring your torso length first, then match liter capacity to the trip type — not your height.
The right size backpack does two separate jobs: it fits your body and it fits your trip. Most people size by height or looks and end up with a pack that carries poorly no matter how the straps feel. The fix is simple: measure your torso, then pick a liter range that matches what you’ll carry.
Why Torso Length Matters More Than Height
Backpack fit starts at your back, not your head. Torso length — the distance between your C7 vertebra at the base of the neck and the top of your hipbones (iliac crest) — determines how the pack transfers weight. REI’s fit guide uses exactly this measurement, and MEC’s sizing guide does the same.
Height alone misleads because two people of the same height can have very different torso lengths. A pack that’s too long rides on your shoulders; one that’s too short sits high and unstable. When you try a pack on, adjust the hipbelt, shoulder straps, stabilizer straps, and sternum strap until the hipbelt bears most of the weight and shoulder straps lie flat without touching your neck.
Backpack Size in Liters by Use Case
Capacity is measured in liters, and the right number depends entirely on what you’re doing. Here’s the practical range for common scenarios:
| Use Case | Capacity Range | Notes |
|---|---|---|
| Everyday / school / commuting | 15–30 L | 10–19 L suits light loads and 13-inch laptops; 20–29 L fits students and 15-inch laptops. |
| Weekend trips / short travel | 25–35 L | 30–45 L is the largest carry-on zone, but budget airlines often cap it around 30–35 L. |
| Day hikes | 18–40 L | Lowe Alpine’s guide puts day hiking at this range depending on gear. |
| 1–2 nights hiking | 20–50 L | REI’s sizing reference uses this span for short overnight trips. |
| Multi-day hiking / trekking | 35–60 L | Multi-day hiking runs 35–50 L; trekking pushes to 40–60 L. |
| Remote multi-day trekking | 60–120 L | Expedition-length trips with full gear need the big-format packs. |
A “medium” pack lands around 25–40 L, usually 18–20 inches tall.
How to Measure Your Torso and Check the Fit
You can measure torso length at home with a tape measure. Feel for the bony bump at the base of your neck — that’s your C7 vertebra. Then find the top of your hipbones on both sides; the imaginary line between them is your iliac crest. Measure the distance between those two points. REI’s expert advice uses this exact method.
Since torso ranges vary by brand and model, check each pack’s size chart. Many packs come in split sizes like small/medium and medium/large, so one manufacturer’s medium may fit where another’s doesn’t.
When you try a pack on, MEC recommends loading it with a realistic weight of about 8–10 kg before judging the fit. Loosen all straps, set the hipbelt so it fully covers your hipbones, then tighten shoulder straps, stabilizers, and the sternum strap. The pack should sit snugly against your back without restricting your breathing, and the weight should ride on your hips — not your shoulders.
Choosing the Right Size Without Overpacking
The biggest mistake buyers make is buying extra volume “just in case.” Empty space invites overpacking, and an oversized pack shifts weight poorly on the trail. Pick the smallest capacity that fits your actual gear list for the trip you’re planning.
Before you buy, verify external dimensions if you’ll fly with the pack. Liters are an approximation; airline carry-on rules depend on the carrier and route, and a 35 L pack can still exceed a budget airline’s limits.
For cabin-compatible travel packs, check the airline’s dimension policy first.
If you’re ready to narrow down a pack for work, our tested roundup of business backpacks for men breaks down the best options for commuting and travel.
References & Sources
- REI. “Backpacks: Fit & Torso Size Guide.” Details the C7-to-iliac-crest torso measurement and split sizing.
- REI. “How to Choose a Backpack: Sizing & Fit Guide.” Provides capacity ranges by trip type and fit principles.
- MEC. “How to fit a backpack: sizing guide.” Recommends loading the pack with 8–10 kg before final fitting.