How we measure
Tested, not estimated
How we measure temperature retention
The retention time shown on this product is a measured result. It comes from a controlled test we run the same way every time, so you can compare our flasks against each other and check our claims against anyone else's.
How we test it
Every test starts by priming the flask. We rinse it through with boiling water first, so the steel is already warm and is not drawing heat out of the drink in the first few minutes. It is the same thing we recommend you do at home, and it is why step one below matters more than most people expect.
- Hot drinks The flask is primed with boiling water, then filled to the very top with water heated to exactly 95 °C and sealed immediately. It stands in a room held at 20 °C, sealed and untouched for the whole test. When the water cools to 60 °C, we stop the clock. That elapsed time is the figure you see. 95 °C → 60 °C
- Cold drinks The same test in reverse. Primed with ice water, filled to the top with water at 4 °C, which is fridge temperature, sealed, same 20 °C room, clock stopped when it reaches 15 °C. 4 °C → 15 °C
- Food flasks Primed with boiling water, then tested with real soup rather than water, because food and holds heat differently, so water results would not tell you anything useful. Soup, 95 °C → 60 °C
What you need to know before you buy
Our test is a best case: primed, full, sealed, standing still, mild room. Your flask will do less than the stated time if any of the following apply, and this is normal.
- Fill level is the big oneThe liquid itself is what stores the heat. Half fill a flask and you get much less than half the time, because you have also left a pocket of air working against you. Most disappointment with any insulated flask traces back to this.
- Skipping the primeA cold flask absorbs heat from your drink before it starts protecting it. Filling an unprimed flask costs you real time off the clock, and it is the easiest one to fix.
- Opening the lidEvery open swaps hot air for cold. A flask opened through the morning will not track a sealed one.
- Where it sitsHeat moves faster when the gap is bigger. A flask in a cold car loses heat quicker than one on your desk, and a cold drink in the sun warms quicker than one in the shade.
- What you put inIf your coffee cooled to 80 °C while you got ready, the clock did not start at 95 °C. A flask slows heat loss. It cannot add heat back. Density works the same way: a thick soup, stew or curry holds heat far longer than something light and loose like pasta, which traps pockets of air between every piece.
- Which lid you choseA screw stopper seals tighter than a flip lid, and a flip lid tighter than a straw lid. That is why retention figures differ across our ranges even where the flask body is similar.
Getting closest to the stated time
- Prime it first. Rinse it through with boiling water for two minutes, or ice water for cold drinks, then tip it out. We do this before every test, so do it before every fill.
- Fill it to the top. Air is the enemy.
- Fill it straight after the kettle boils, not twenty minutes later.
- Keep the lid shut and pour into a cup rather than sipping from it all morning.
Most brands never publish their test conditions. We think you should be able to check ours.
