Retention hours only compare when the stated conditions match
Every vacuum flask on the shelf claims the same physics — a double-wall vacuum between two skins of stainless steel, usually published as 18/8 — so the useful differences are elsewhere. Makers publish hot and cold retention times in hours, and each figure comes from conditions the maker sets: a starting temperature, an ambient temperature, and a threshold that still counts as hot. Two flasks quoting the same duration have not necessarily been measured the same way, which is why the conditions matter more than the hours do. The mouth diameter is the specification that decides daily use. A narrow mouth pours cleanly and loses less heat; a wide mouth takes ice, a bottle brush and soup. The lid is both the part that leaks and the part that fails. A plain screw stopper has nothing to break; a push-button or flip lid is easier one-handed and hides a gasket that has to come out and dry. It is worth checking whether the maker sells replacement seals, because a flask usually outlives its first gasket by years. Then the outside: a powder coat resists chipping and adds grip, bare steel shows every knock, and the base diameter decides whether it stands in a car cup holder at all. We compare stated capacity, wall construction and steel grade, published hot and cold retention times with the conditions given, mouth diameter, lid type and seal availability, base diameter, finish, and empty weight.
500 ml · double-walled stated · leak-proof lid · no retention hours published · no steel grade named
A 500 ml double-walled stainless flask that publishes its capacity and nothing else the category is compared on: no retention hours, no steel grade, no mouth, no weight.
Best for: A day bag where 500 ml of double-walled stainless steel is the whole requirement and the hours are not being compared.
≈ $8 / £7 / 9 €
500 ml · vacuum insulated · SUS 201 outside and SUS 304 inside · 24 h cold and 12 h hot stated · brushed finish · top handle
The only flask here that names a different steel for the inside and the outside, alongside a stated vacuum, both retention figures and a brushed finish.
Best for: A day out where a named inner steel, a stated vacuum and a dishwasher-safe body matter more than the hours quoted.
≈ 13 €
18/8 stainless · double wall · 24 h cold and 12 h hot stated · three lids supplied · powder coat · capacity not published
The flask with three lids in the box and both retention figures on the page — and no capacity anywhere in the listing, which is an unusual thing to leave out.
Best for: A walker who wants a straw lid on the trail and a plain lid at work, from one flask.
≈ £10 / 11 €
1 litre · five-layer vacuum insulation stated · 12 h hot and 24 h cold stated · leak-proof with carbonated drinks · scratch-resistant surface
The only flask here that publishes a layer count for its insulation — five — alongside both retention figures, and that never describes the closure holding it all in.
Best for: A litre where the construction claim is what is being bought, and the closure is being taken on trust.
≈ 28 €
1 litre · double-wall vacuum · 18/8 stainless · 24 h cold and 12 h hot stated · wide mouth stated to take ice · screw lid with vacuum seal
A 1 litre vacuum flask with a wide mouth stated to take ice, a screw lid with a vacuum seal, a carry handle and a non-slip silicone base — and no dimension behind any of it.
Best for: A litre of iced drink where a wide mouth and a dishwasher-safe lid matter more than packing small.
≈ $22 / £22 / 24 €
1 litre · double-walled stated · 18/08 stainless · tea infuser supplied · "several hours" and no figure · 60-day money-back offer
A 1 litre flask with a removable tea infuser and a named steel, whose retention claim is the words "several hours" and no number at all.
Best for: A litre of tea brewed in the flask, where the infuser matters more than a published number of hours.
≈ 19 €
0.75 litre · double-walled stated · 24 h cold stated, no hot figure · rubberised screw stopper · leak-proof with carbonated drinks
A 0.75 litre flask that publishes a cold figure and no hot one, and whose rubberised screw stopper is stated to hold carbonated drinks.
Best for: Cold drinks, including carbonated ones, in a slim 0.75 litre bottle where the closure is the deciding part.
≈ 19 €
500 ml · double-wall vacuum · 18/8 stainless · 24 h cold and 12 h hot stated · screw cap · cotton sleeve supplied
A 500 ml vacuum flask that names 18/8 steel and both retention figures, ships with a cotton sleeve, and calls itself lightweight without publishing a weight.
Best for: A commute or a day walk where a sleeve and a plain screw cap suit better than a straw or a flip lid.
≈ £12 / 10 €
1 litre · 18/08 stainless · 13 h temperature stability stated, hot and cold not separated · bottle brush, jute bag and carabiner supplied
A 1 litre flask supplied with a natural-bristle brush and a carabiner, whose single retention figure of 13 hours is never split into hot and cold.
Best for: A litre bottle bought with its brush and its carry loop, where cleaning matters more than a comparable hours figure.
≈ $33 / £24 / 29 €
This family belongs to the cooking and water kit — see everything that has to work with it.
Prices in these summaries were read on the dates each review gives. The live figure sits beside every product.
