30 liters is plenty for a 3-day trip.

For a carry-on-only 3-day trip, a 30-liter backpack is the sweet spot. It forces you to be honest about what you actually need, and it still fits under the seat or in the overhead bin on basically every airline.

Three days is short enough that you don’t need a week’s worth of clothes. Pack three outfits, wear the bulkiest stuff on the plane, and use packing cubes to keep it organized. You’re not doing laundry on a three-day trip, so the old “pack for one less week” advice gets even simpler.

Yes, most carry-on allowances are around 40–45 liters, and if you bring a big camera or work laptop, you might need 35–40. But 30 liters keeps you light, makes public transit easier, and stops you from packing “just in case” stuff you won’t touch.

If you’re buying new, go with a 28–32L bag. You’ll never regret the smaller one on a short trip. You’ll only regret the bigger one when you’re hauling it up stairwells.

Pack less than you think you need.