Free Travel Packing Volume calculator with clear step-by-step results.
Packing volume is driven by the laundry cycle, not trip length - a three-week trip with weekly laundry needs barely more clothing than a one-week trip. Outfits are sized to the shorter of trip length and laundry interval, plus one spare, then compressed and added to fixed gear.
Outfits needed
Outfits = ceil(min(trip days, laundry interval) x outfits per day) + 1
Total volume
Litres = outfits x litres per outfit x (1 - compression saving) + fixed gear
Typically 35-45 litres for a cabin bag within standard size limits, and 20-25 litres for an under-seat personal item. Compare the total against those figures.
For soft clothing, yes - compression sacks and packing cubes with zips expel trapped air. It does nothing for shoes, toiletries or electronics, which is why they are counted separately.