Short answer: Airline cabins are pressurized to the equivalent of about 6,000 to 8,000 feet, so the air trapped inside a sealed bottle expands by roughly 25% to 35% and pushes on the cap. The more air a bottle holds, the harder it pushes, which is why half-empty bottles leak more often than full ones.
By the Belle Vault Editorial Team · Sources checked September 30, 2026
You packed the foundation cap-side up. You tightened it. You land, unzip the bag, and there is a beige film on everything within four inches of the bottle.
It is tempting to blame the bottle, or the baggage handlers. The more likely cause is physics, and specifically the small pocket of air you sealed inside the bottle at home.
Aircraft cabins are pressurized, but not to sea level. Two decades of cabin measurements tell us roughly how much lower the pressure gets, and from that it is possible to work out what happens inside a bottle, and why the bottle that is half used up is the one most likely to leak.
How low the cabin pressure actually goes
U.S. certification rules require a pressurized cabin to be held at a "cabin pressure altitude" of no more than 8,000 feet under normal operation [1]. That is a ceiling, not a typical value, so it helps to look at what was measured on real flights.
Study one: 204 flights, 1988
Cottrell recorded cabin altitude on 204 scheduled commercial flights. The median was 6,214 feet, and readings ranged from sea level up to 8,915 feet [2].
Study two: 207 flights, 2013
Hampson and colleagues repeated the exercise on 207 flights across 17 aircraft types. The average peak cabin altitude was 6,341 feet, with a spread of plus or minus 1,813 feet, and peaks went above 8,000 feet on roughly 10% of flights. Longer flights, over 750 miles, ran higher [3].
Cottrell [2]; Hampson et al. [3]; 14 CFR 25.841 [1]. The two studies report different statistics (median vs mean of peak), so treat them as a range, not a trend.
The two studies are not measuring quite the same thing. One reports the median cabin altitude, the other the average of each flight's highest point, so the small gap between them should not be read as cabins getting worse. The authors of the 2013 study do say their readings were substantially higher than in 1988 [3]. The practical takeaway is the same either way: most flights sit around 6,000 feet, and some go past 8,000.
What that pressure does inside a bottle
In the standard atmosphere used by engineers, air pressure is about 101 kPa at sea level, about 81 kPa at 6,000 feet and about 75 kPa at 8,000 feet [4]. Liquids barely change volume under those conditions. Air does. A pocket of air sealed at sea level wants to grow by about 25% at 6,000 feet and about 35% at 8,000 feet.
The air cannot grow inside a rigid bottle, so it presses outward on the cap, the pump and the thread, and on the liquid between them. If the seal gives, liquid is pushed out ahead of the air. On most airliners the baggage hold is pressurized as well, so a checked bottle goes through a similar change.
Calculated from U.S. Standard Atmosphere pressures [4] using Boyle's law, at constant temperature. Rounded.
The counterintuitive part: the half-empty bottle is the risky one
Because the push comes from the air, not the liquid, how full the bottle is matters more than how big it is.
Take a 100 ml bottle. Full to the neck, it holds perhaps a few milliliters of air. At a 6,000-foot cabin, that air wants to grow by about a milliliter. Half empty, it holds about 50 ml of air, which wants to grow by roughly 12 ml at 6,000 feet and roughly 17 ml at 8,000 feet. That is the same bottle, with more than ten times the outward push.
It also explains a familiar pattern: the new bottle arrives fine and the nearly finished one leaks. And it explains why pumps and flip caps fail first. They are designed to open under a small push. That is exactly what the expanding air supplies.
Air wants about +1 ml. Little pressure on the cap.
Air wants about +12 ml. The pressure a pump or flip cap has to hold back.
Squeeze a soft tube or bottle before capping and most of the air, and the push, is gone.
Worked example from the pressures in Figure 2. Assumes a sealed, rigid container at constant temperature.
Upright helps, for a different reason
A bottle lying on its side has liquid sitting against the cap. When the air expands, the first thing it pushes through a weak seal is that liquid. Upright, the air pocket sits under the cap instead, so a small seal failure vents air rather than foundation. Upright does not stop the pressure. It changes what escapes first.
What to do with this
- Squeeze out the air before you cap soft tubes and squeezable bottles. Less air means less push at altitude, which is the whole mechanism.
- Travel with the fuller bottle, or decant the half-used one. A nearly empty container is the one with the most air in it.
- Pack liquids upright and lock pumps. Upright means the air, not the product, meets the cap. Twist-lock pumps and tape over flip caps stop the easy openings.
For which bottles may fly in your carry-on at all, see full-size bottles and travel. For bags where bottles stand upright in their own slots, see the travel makeup bag collection.
Keep reading
Frequently asked questions
Why do my toiletries leak on planes?
The cabin is pressurized to about 6,000 to 8,000 feet, not sea level. Air sealed inside a bottle at home expands by roughly 25% to 35% at that pressure and pushes on the cap or pump. If the seal gives, liquid is pushed out ahead of the air, especially from bottles lying on their side.
What pressure is an airplane cabin kept at?
U.S. rules cap cabin pressure altitude at 8,000 feet in normal operation. Measured flights run lower on average: a 1988 study of 204 flights found a median of 6,214 feet, and a 2013 study of 207 flights found an average peak of 6,341 feet, with about 10% of flights going above 8,000 feet.
Do half-empty bottles leak more on flights?
They tend to, because the push comes from trapped air, not the liquid. A half-full 100 ml bottle holds about ten times more air than a nearly full one, so at cabin pressure its air pushes out about ten times more volume against the cap.
Does the cargo hold have the same pressure as the cabin?
Checked bags travel in a pressurized hold on commercial airliners, so a sealed bottle in checked luggage goes through a similar pressure change to one in the cabin. Pack checked liquids the same way: upright, pumps locked, air squeezed out where possible.
How do I stop pump bottles leaking when I fly?
Lock the pump if it twists shut, or tape it down, and pack the bottle upright so the air pocket rather than the liquid sits against the pump. If the bottle is half empty, consider decanting into a smaller container you can fill closer to the top.
Is it the heat or the pressure that makes bottles leak?
Both can matter, but pressure is the one every flight has. The cabin-pressure drop alone is enough to make trapped air expand by about a quarter. Heat also expands air, so a bottle that sat in a hot car before the airport starts the flight with extra pressure inside.
How we sourced this
Cabin pressure figures come from the U.S. certification rule and two peer-reviewed measurement studies, reported separately because they use different statistics. Pressures at altitude come from the U.S. Standard Atmosphere. The bottle examples are our own calculations using Boyle's law at constant temperature, rounded; real bottles flex slightly, so they are illustrations, not measurements.
- Electronic Code of Federal Regulations. 14 CFR 25.841, Pressurized cabins. ecfr.gov
- Cottrell, J. J. (1988). Altitude exposures during aircraft flight: flying higher. Chest, 93(1), 81–84. pubmed.ncbi.nlm.nih.gov
- Hampson, N. B., Kregenow, D. A., Mahoney, A. M., et al. (2013). Altitude exposures during commercial flight: a reappraisal. Aviation, Space, and Environmental Medicine, 84(1), 27–31. trid.trb.org
- NOAA, NASA and U.S. Air Force (1976). U.S. Standard Atmosphere, 1976. ntrs.nasa.gov
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