Is the 100 mL TSA carry-on limit really 3.4 oz?

Yes — 100 mL equals about 3.38 fl oz, which the US TSA rounds up to 3.4 oz on its '3-1-1' liquids rule.

Is fluid ounce a weight?

No. A fluid ounce measures volume (≈29.57 mL in the US). It only equals an ounce of weight when the liquid happens to have water-like density.

Convert milliliters into US fluid ounces for travel-size toiletries, drink recipes and metric cosmetic labels.

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From

1 ml = 0.033814 fl oz

fl oz = milliliters × 0.033814

Is the 100 mL TSA carry-on limit really 3.4 oz?: Yes — 100 mL equals about 3.38 fl oz, which the US TSA rounds up to 3.4 oz on its '3-1-1' liquids rule.
Is fluid ounce a weight?: No. A fluid ounce measures volume (≈29.57 mL in the US). It only equals an ounce of weight when the liquid happens to have water-like density.