A food flask and a thermos are not the same thing, and mixing them up is the single biggest reason people end up disappointed with either. A food flask has a wide mouth built for eating soup, pasta or stew straight from it with a spoon; a classic narrow-neck thermos is built for pouring hot drinks like tea or coffee, and it's a poor tool for solid food. If what you actually want is a hot meal at your desk or in a school bag by lunchtime, you need a food flask.
Food Flask or Thermos? The Difference That Actually Matters
The confusion is understandable, because both are vacuum-insulated steel containers that keep their contents hot for hours — the difference is the opening and what it's designed to do. A drinks thermos has a narrow neck sized for pouring liquid into a cup, which makes it almost impossible to get a spoon into, let alone clean properly afterwards. A food flask has a wide mouth, usually 6–7cm across, specifically so you can eat straight out of it and wash it by hand without fighting the shape. Good For You's 730ml food flask has a 7.1cm wide-mouth opening for exactly this reason, with a lid that doubles as a bowl for pasta, cereal or salad, and a foldable spoon that stores inside it so there's nothing extra to pack or lose.
How Long Will It Actually Keep Your Lunch Hot?
A good stainless steel food flask will keep a meal properly hot for 6 to 9 hours and cold food cold for 16 to 24 hours, which covers a full working day or school day from breakfast to lunch. That figure depends heavily on one step most people skip: pre-warming the flask. Pour in boiling water, screw the lid on, leave it five minutes, tip it out, then add your hot food straight away — a cold steel interior pulls heat out of your meal in the first few minutes, so skipping this is the most common reason a flask underperforms. Filling it right to the top also holds heat noticeably better than filling it half way, since there's less empty air space to keep warm.
What Size Do You Actually Need — For You or For a Child?
Sizing a food flask comes down to how long you'll be away from a fridge or microwave and who's eating from it. A 2026 UK buying guide to food flasks suggests roughly 400ml for a light lunch like soup or porridge, 500–600ml for most adults' main meal, and 700ml or more for a full dinner-sized portion or a long day out — see Just Average Jen's food flask guide for the full breakdown. Good For You's 730ml flask (also available in white) sits at the generous end for an adult who wants a proper dinner-sized lunch, while the 530ml kids' flask is sized for a toddler through to primary-school portion — enough pasta, soup or rice to see them through to home time without being too heavy for a school bag.
Keeping It Leak-Free and Working for Years
A food flask that leaks is almost always a sealing problem, not a design fault: a screw-on lid needs to be turned down fully until it stops, and food sitting in the thread of the lid — usually from overfilling past the neck — is what breaks the seal and lets soup or sauce escape in a bag. Hand washing rather than putting it through the dishwasher matters more than people expect, because dishwasher heat and strong detergent gradually wear down the vacuum seal that gives the flask its heat retention in the first place. Every Good For You flask carries a 2-year warranty against manufacturing defects, which is worth knowing about before you consider binning one — it's usually cheaper for everyone, and better for landfill, to get a faulty part replaced than to replace the whole flask.
Reheating Before You Fill It: What Food Safety Guidance Actually Says
If you're filling your flask with leftovers rather than something freshly cooked, how you reheat it matters as much as the flask itself. The Food Standards Agency's guidance on reheating food is that it should be heated until it is steaming hot all the way through, reheated only once, and then served promptly rather than left to cool and sit — see the FSA's reheating guidance. In practice, that means reheating leftovers properly on the hob or in the microwave until they're piping hot, then transferring straight into a pre-warmed flask rather than reheating a partial portion twice or topping up an already-cooling flask with more food later in the day.
Frequently Asked Questions
What's the actual difference between a food flask and a thermos flask?
A food flask has a wide mouth, typically 6–7cm across, designed so you can eat solid food like soup, stew or pasta straight from it with a spoon. A classic thermos has a narrow neck designed for pouring liquids such as tea or coffee into a cup, and it isn't practical for eating solid food or for cleaning afterwards.
How long does a food flask keep food hot?
A good stainless steel food flask keeps food hot for roughly 6 to 9 hours and cold food cold for 16 to 24 hours, which covers a full working day or school day. Pre-warming the flask with boiling water for five minutes before adding hot food makes a noticeable difference to how hot it stays by lunchtime.
What size food flask should I buy for an adult, and what size for a child?
Most adults are well served by a 500–730ml food flask for a full main meal, while 400ml suits a lighter lunch like soup. For toddlers through to primary-school children, around 500–530ml holds a proper portion of pasta, soup or rice without being too bulky for a school bag.
Is a stainless steel food flask dishwasher safe?
Most stainless steel food flasks, including Good For You's, are recommended for hand washing rather than the dishwasher. Dishwasher heat and strong detergent gradually wear down the vacuum seal that keeps the flask insulated, so hand washing with a soft sponge and warm soapy water keeps it performing for longer.
Will a food flask leak in a bag or rucksack?
A properly sealed food flask shouldn't leak as long as the lid is screwed down fully until it stops and the flask isn't overfilled past the neck. Leaks almost always happen when food sits in the thread of the lid, which stops the seal from closing properly, rather than because of a fault with the flask itself.
Can I reheat leftovers twice before putting them in a food flask?
No — the Food Standards Agency recommends reheating food only once, heating it until it's steaming hot all the way through, and serving it promptly rather than letting it cool and reheating it again. For a flask, that means reheating leftovers properly on the hob or microwave first, then transferring them straight into a pre-warmed flask rather than warming a partial portion a second time.