Zerk 40 UL Backpack

Mountainsmith Zerk 40 UL Backpack is a 40L, 0.97kg2.1lb travel backpack measuring 50.8×30.5×22.9cm20×12×9in. It fits the carry-on size limit of 30 of 36 major airlines and the personal-item limit of 0 of 26. Under the seat, its 22.9cm9in side is over the 20cm7.9in limit at 15 of the 26 airlines it misses. Specs from mountainsmith.com, checked September 2026.

Volume: 40Lexpands to 44L
Weight: 0.97kg2.1lb2.1lb0.97kg
Carry-on: 30/36airlines fit it
Personal item: 0/26under-seat limits

Dimensions: 50.8×30.5×22.9cm20×12×9in 20×12×9in50.8×30.5×22.9cm
Price: $229.95 as of September 2026
Laptop: no laptop sleeve
Opening: Roll-top, Roll top closure with V-strap compression
Warranty: Lifetime Covers material and workmanship defects, abuse and natural material aging may not be repairable.
Torso length: 40.6 to 53.3cm (16 to 21in)
Compartments: n/a
Bottle pockets: 2

Price history

$229.95 at every weekly check since Oct 5, 2026 (3 checks).

Checked weekly on the brand's US store.

Hydration sleeve
Frame sheet (removable)
Hip belt (removable)
Sternum strap

Material: 210D and 100D Robic nylon X-Grid Extreema (UHMWPE) ripstop with UTS coating, Spundura stretch mesh, 150D polyester liner

44 L with the roll top extended; 0.86 kg stripped
Co-designed with The Real Hiking Viking for ultralight thru-hiking
Removable 1.5 in waistbelt, frame sheet and modular side compression straps

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Last checked: 2026-09. Prices and specs may have changed.

Does the Mountainsmith Zerk 40 UL Backpack fit airline carry-on limits?

Draw against

Too bigToo big on Ryanair. cabin bag 55×40×20cm21.5×15.5×8in · under the seat 40×30×20cm15.5×12×8in. Bag in cminches, turned to fit.

Flag as a tight fit undercm spare

Airline	Carry-on	Personal item	Cabin-bag weight
Air Arabia	Too bigToo big	Too bigToo big	6kg to pack (of 7kg)13.2lb to pack (of 15.4lb)
Air CanadaCabin bag not on every fare	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	No published limitNo published limit
Air FranceCabin bag not on every fare	FitsFits	Too bigToo big	11kg to pack (of 12kg shared with personal item)24.3lb to pack (of 26.5lb shared with personal item)
Air India	Too bigToo big	No published limit	6kg to pack (of 7kg)13.2lb to pack (of 15.4lb)
AirAsia	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	6kg to pack (of 7kg shared with personal item)13.2lb to pack (of 15.4lb shared with personal item)
Alaska Airlines	Tight fit (0.1cm spare)Tight fit (0in spare)	No published limit	No published limitNo published limit
Allegiant AirCabin bag not on every fare	FitsFits	Too bigToo big	No published limitNo published limit
American Airlines	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	No published limitNo published limit
ANA (All Nippon Airways)	FitsFits	Too bigToo big	9kg to pack (of 10kg shared with personal item)19.8lb to pack (of 22lb shared with personal item)
AviancaCabin bag not on every fare	FitsFits	No published limit	9kg to pack (of 10kg)19.8lb to pack (of 22lb)
British Airways	FitsFits	Too bigToo big	No published limitNo published limit
Cebu Pacific	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	9kg to pack (of 10kg shared with personal item)19.8lb to pack (of 22lb shared with personal item)
Delta Air Lines	Tight fit (0.1cm spare)Tight fit (0in spare)	No published limit	No published limitNo published limit
easyJetCabin bag not on every fare	FitsFits	Too bigToo big	14kg to pack (of 15kg)30.9lb to pack (of 33.1lb)
Emirates	Too bigToo big	No published limit	6kg to pack (of 7kg)13.2lb to pack (of 15.4lb)
Frontier AirlinesCabin bag not on every fare	FitsFits	Too bigToo big	14.9kg to pack (of 15.9kg)32.8lb to pack (of 35.1lb)
Gol Linhas Aereas	FitsFits	No published limit	9kg to pack (of 10kg)19.8lb to pack (of 22lb)
IndiGo	FitsFits	No published limit	6kg to pack (of 7kg)13.2lb to pack (of 15.4lb)
JetBlue Airways	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	No published limitNo published limit
KLMCabin bag not on every fare	FitsFits	Too bigToo big	11kg to pack (of 12kg shared with personal item)24.3lb to pack (of 26.5lb shared with personal item)
LATAM Airlines	FitsFits	Too bigToo big	11kg to pack (of 12kg)24.3lb to pack (of 26.5lb)
LufthansaCabin bag not on every fare	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	7kg to pack (of 8kg)15.4lb to pack (of 17.6lb)
Norwegian Air ShuttleCabin bag not on every fare	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	9kg to pack (of 10kg shared with personal item)19.8lb to pack (of 22lb shared with personal item)
Qantas	Tight fit (0.1cm spare)Tight fit (0in spare)	No published limit	6kg to pack (of 7kg shared with personal item)13.2lb to pack (of 15.4lb shared with personal item)
Qatar Airways	Too bigToo big	No published limit	6kg to pack (of 7kg)13.2lb to pack (of 15.4lb)
RyanairCabin bag not on every fare	Too bigToo big	Too bigToo big	9kg to pack (of 10kg)19.8lb to pack (of 22lb)
SAS	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	7kg to pack (of 8kg)15.4lb to pack (of 17.6lb)
Singapore Airlines	FitsFits	Too bigToo big	6kg to pack (of 7kg)13.2lb to pack (of 15.4lb)
Southwest Airlines	FitsFits	No published limit	No published limitNo published limit
Turkish Airlines	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	7kg to pack (of 8kg)15.4lb to pack (of 17.6lb)
United AirlinesCabin bag not on every fare	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	No published limitNo published limit
Vietjet Air	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	6kg to pack (of 7kg shared with personal item)13.2lb to pack (of 15.4lb shared with personal item)
VolarisCabin bag not on every fare	FitsFits	Too bigToo big	9kg to pack (of 10kg)19.8lb to pack (of 22lb)
VuelingCabin bag not on every fare	Too bigToo big	Too bigToo big	9kg to pack (of 10kg)19.8lb to pack (of 22lb)
WestJetCabin bag not on every fare	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	No published limitNo published limit
Wizz AirCabin bag not on every fare	Tight fit (0.1cm spare)Tight fit (0in spare)	Too bigToo big	9kg to pack (of 10kg)19.8lb to pack (of 22lb)

Compared in any orientation against each airline's published size limit. A tight fit has under 2cm to spare, so a packed soft bag may not pass a sizer. Set it to 0 to turn the warning off. Weight is what the airline allows in the cabin minus this bag's empty 0.97kg2.1lb. Limits and sources. Own a different bag? Check it against every airline.