London City Airport recently scrapped 100 ml liquid rule, which still applies in airports worldwide.

On 4 April 2023, London City Airport scrapped the 100 ml liquid rule. 1 Since 2006, airports worldwide have insisted that travellers can only bring liquids in containers of up to 100 ml in their carry-on bag. 2 These days, we take it for granted that the 100 ml limit for liquids at international airports around the world is exactly the same quantity. This is a triumph for the metric system, which replaced many different national systems long ago. Before the metric system, the same unit names were used for different quantities in different national measurement systems and there was no common definition for these units. The worldwide 100 ml liquid rule shows the benefits of a world standard measurement system for international travellers (i.e., the metric system).

Whatever country we are travelling from, we know that 100 ml of liquids is exactly the same wherever we are. We also take it for granted that liquid products (e.g., toiletries, shampoos, creams, cosmetics and bottled water) sold all over the world show quantities in millilitres, centilitres, decilitres and litres. You can easily convert the other metric units to millilitres by moving the decimal point. The containers for liquids must not be larger than 100 ml. You can easily check the label on the container, which tells you its capacity in millilitres (or a closely related decimal-based submultiple of litres).

This makes it easy to compare quantities with the 100 ml liquid volume limit in airports. This would not have been possible without the metric system. Before the metric system, different national measurement systems made conversions inevitable for trade, travel, communication and co-operation. And it makes mistakes a lot more likely. This obviously causes problems.

By contrast, there are various definitions of non-metric units such as cups, fluid ounces, tablespoons, dessert spoons and teaspoons. 3 For example, there are three different definitions of the fluid ounce in current use. 4 They are the imperial fluid ounce of 28.41 ml, the US customary fluid ounce of 29.57 ml and the US food labelling fluid ounce of exactly 30 ml. If these units were used to define the volume limits for liquids at international airports, one question that would arise is “Which one do you mean?”. This would cause confusion and potentially disputes at airports. As the metric system is used to define this limit, which contains only one definition, this problem does not exist.


  1. (“London airport officially scraps 100ml liquids rule in hand luggage from today” by Sian Elvin, Metro, 4 April 2023)
  2. (“Woman shares ‘genius tip’ for getting around 100ml liquids rule at airports” by Lucy Thackray, The Independent, 30 June 2022)

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