The carry-on limit for TSA is 3.4 ounces but unless you can survive on three products alone it’s pretty dumb. I’m sorry, I’m not Bear Grylls.
Most people have no idea how long their liquids actually last and how it compounds on space in the toiletry bag. They also don't realize 3.4 ounces (100 mL) is actually pretty darn big; if you're looking to maximize the number of liquids you can bring on a plane, it's actually not advisable to bring the full limit for each product.
Of course, us being the travel nerds that we are, we delve into the math here, and what a more realistic limit for carrying on liquids is.
First, how big can the TSA liquids bag be?
Let’s think this through one step at a time.
The Transportation Security Administration allows travelers to bring as many liquids as they want, as long as it is contained in a 1 quart bag. The dimensions of that bag can vary, as long the volume capacity is one quart.
How they measure this, I don’t know, but I assume we’re all abiding by the honor system here.
I use an “approved” Muji travel pouch—which I frankly assume is the correct size because the Japanese are crazy—which measures 7.7 inches x 6.3 inches (19.5 cm x 16 cm). FlyerTalk user stinky123 notes a legal Ziploc baggie, the poor man’s liqyuds pouch, measures 7.5 inches x 8 inches (19.1 cm x 20.3 cm) while FlyerTalk user SLC-YYZ notes TSA proper gave him a bag measured 7.0 inches by 7.5 inches (17.8 cm x 19.1 cm).
Looking at all the common measurements, I would say 7.0" by 7.5" is probably safe and the median. Let's not think too hard on this.
The liquids allowance is not what most people bring
Next, remember that the U.S. commonly uses two different sizes? There’s the actual maximum size, which is 3.4 ounces or 100 mL, or the size most people remember, which is 3 ounces or 89 mL. (People remember whole numbers easier.)
Personally, I find it easier to stick to the metric system and remember the flat 100 but yeah, fine, imperial system, whatever.
Regardless, what we have at home is a bunch of 3-ounce (89 mL) bottles, because that is the most common size on the market, so that’s what we’re going to dump into our quart-size bag.
How many travel products can fit into that quart?
Using the Muji bag, I can safely fit six bottles into the bag, seven if I tried really, really hard. So for six bottles, that’s roughly 534 to 623 mL of liquid I can squeeze into it.
If I was using 3.4-ounce bottles, which is the real maximum, you’d be able to get five bottles into it… if you manage to contort the bottles into different varying positions and shapes.
(I don’t know about you but I use well over five types of liquid products. It might be possible to squeeze more into a Ziploc bag but I doubt there’s huge give.)
Keep in mind most people actually probably don’t even need to bring along 3 ounces of shampoo unless they’re going for weeks and weeks (emphasis on the plural).
So 3.4 ounces? There's no need to bring that much product, unless you can only get by on five things for life.