I own three duffle bags and two of them were mistakes. The first, bought for the gym, is a 90-litre monster I can only describe as a fabric coffin; my kit rattles around in the bottom of it like a marble in a bin. The second, bought for weekends away, is a handsome 25-litre thing that fits a weekend’s clothes provided the weekend is four hours long. Only the third one is right, and I got it right by finally learning what those litre numbers actually mean in real objects, which nobody selling duffles ever explains.
So here’s the explanation, the one I wish had come printed on a tag: what 20, 40, 60 and 90 litres look like in actual stuff, which number fits which life, and the crucial twist at the end, which is that airlines don’t measure litres at all, so the carry-on question is a different question wearing the same trousers.
First, What a Litre of Bag Actually Holds
A litre is a litre of milk, and imagining your bag as a stack of milk cartons is genuinely the fastest way to develop an instinct for this. But real objects work better still, so here’s the rough translation table I now use, with the honest caveat that these are approximations and your rolled t-shirt is not my rolled t-shirt:
- A pair of trainers: roughly 6 to 8 litres, and the single biggest space thief in any bag.
- A rolled t-shirt: about 1 litre; jeans, 3 to 4; a jumper, 4 to 5.
- A washbag: 2 to 3 litres; a large towel, 3 to 4 rolled tight.
- A laptop in a sleeve: 2-ish litres, but flat, so it wants a bag with a flat side.
Do that arithmetic on your actual packing list before buying anything, because the right bag size is your list plus about 15 percent slack, and every mistake I’ve made came from skipping the sum and guessing off the product photo.
The Gym: 20 to 35 Litres
The gym bag question resolves fast once you list what actually travels shoes, a change of clothes, a towel, a washbag, a water bottle. Run the numbers above and that’s roughly 15 to 20 litres, which is why a 20 to 25 litre duffle is the sweet spot for a standard gym kit, big enough for everything, small enough to live in a locker without a fight.
Go up to 30 or 35 litres if your training carries bulk boxing gloves, boots, a post-work outfit on office days, or a laptop riding along. Go beyond 35 and you’ve entered my fabric-coffin problem, where the bag is heavier than its contents and everything migrates to one sad corner. A gym bag that’s too big doesn’t just look silly on the bench; loose contents shift, bottles tip, and you dig for keys through a volume you never needed.
The Wet-Kit Detail Worth Paying For
Litres aside, the one gym feature that earns its price is a separated compartment, ventilated shoe pocket or a wet pocket for swim kit, because 25 well-divided litres beat 35 litres of single-cavity soup every day of the week.
The Weekend: 40 to 60 Litres
Two or three nights away is the classic duffle job, and the range splits by season and packing personality. A summer weekend for a light packer, two outfits, one pair of spare shoes, washbag, sits comfortably in 40 to 45 litres. Add winter, and the same trip swells: a coat is 8 litres on its own, boots eat 10, jumpers stack up, and suddenly 55 to 60 litres is the honest winter-weekend number for the same three days.
The mistake to avoid here runs opposite to the gym mistake. People buy the 60 for headroom, then fill it, because an empty litre is an invitation, and arrive carrying 12 kilograms for two nights. If you’re disciplined, buy the 45 and let the bag enforce the discipline. If you’re not, at least know that you’re not, and buy the 60 with your eyes open.
Past a week’s trip, duffles run 70 to 90 litres, and my honest advice is to think hard before going there: a full 90-litre duffle weighs what a small teenager weighs, has no wheels, and hangs off one of your two shoulders. Beyond 70 litres, wheeled luggage stops being a style question and starts being a spine question.
The Carry-On Twist: Airlines Don’t Measure Litres
Here’s the part that catches everyone, and it’s the most useful section of this article. Your bag is sold in litres. Airlines measure centimetres, three of them, height, width and depth, with a metal sizer at the gate, wheels and handles included. A 40-litre duffle shaped long and soft slides into a sizer that a 40-litre cube fails, so the litre number on the label tells you almost nothing about whether you’ll board without a fee.
For UK flyers, the current allowances, per the 2026 hand-luggage breakdowns of the major carriers, sort like this:
| Airline | Free bag (under seat) | Overhead bag | Duffle sweet spot |
|---|---|---|---|
| Ryanair | 40 x 30 x 20 cm | 55 x 40 x 20 cm, paid Priority, 10kg | ~20L soft duffle free; ~35 to 40L with Priority |
| easyJet | 45 x 36 x 20 cm | 56 x 45 x 25 cm, paid add-on | ~25L free; ~45 to 50L paid |
| Jet2 | 40 x 30 x 20 cm personal item | 56 x 45 x 25 cm, free, 10kg | ~45 to 50L free in the locker |
| British Airways | 40 x 30 x 15 cm personal item | 56 x 45 x 25 cm, free | ~45 to 50L free, plus the small bag |
Three practical truths fall out of that table. First, the safest single size in Britain is a soft duffle built to 40 x 30 x 20, roughly 20 usable litres, which boards every major carrier free, and Ryanair enforces its version with gate fees of up to £70, so “roughly” 40 centimetres is not 40 centimetres. Second, BA and Jet2 quietly include what the budget airlines charge for, a full 56 x 45 x 25 locker bag, which fits a 45-to-50-litre duffle and covers an entire weekend without fees, a fact that Which? notes can flip the total price comparison against the budget carriers once bags are added. Third, soft beats hard at the gate: a duffle under-packed by 10 percent squashes into a sizer that rejects a rigid shell of identical nominal size, which is the one genuine superpower duffles hold over suitcases. One more date for the diary: an EU ruling will require airlines flying to the EU to allow a free cabin bag of at least 40 x 30 x 15 from 2027, though it won’t automatically cover UK carriers on UK routes, so don’t bin the tape measure yet.
