Following on from the previous MV article about the 100 ml liquid rule, we look at other airport regulations that involve measurements. Where measurements are involved, they tend to be expressed in the metric system, which is used worldwide. Metric units are used in airport regulations around the world. Here, we focus on rules at British airports.
The information here is based on government guidance when this article was published but could change in the future so check the latest official government guidelines before you travel.
If you take liquids in your hand luggage, containers must be in a single, transparent, resealable plastic bag, which holds no more than a litre and measures approximately 20 cm x 20 cm.
Liquids in containers larger than 100ml generally cannot go through security even if the container is only part full. There are some exemptions (e.g., essential medicines).
Individual containers of breast milk must hold no more than 2000 ml in your hand luggage.
There is a 6 cm limit for blades in your hand luggage. This applies to items such as knives and scissors.
Airlines impose weight limits in kilograms for checked-in luggage and hand luggage. These limits vary between airlines. The maximum size of carry-on bags is expressed in centimetres. These bags must fit into a specific dimension, expressed as width in cm x length in cm x height in cm. These dimensions vary between airlines.
The Civil Aviation Authority (CAA) explains the British civil aviation regulations. Here are some of these regulations that involve measurements.
For aerosols, the total net quantity of each single article must not exceed 0.5 kg or 0.5 L. Also, the total net quantity of medicines, toiletry articles and aerosols for sporting or home use must not exceed 2 kg or 2 L (e.g., four aerosol cans of 500 mL each) each (only allowed for hold baggage).
Alcohol limits are no more than 5 L per individual container; and no more than a total net quantity of 5 L per person.
Ammunition (cartridges for weapons) securely packed in quantities not exceeding 5 kg gross mass per person for that person’s own use (only allowed for hold baggage with airline approval).
CAA rules for batteries use watt-hour (Wh) ratings and grams. You can arrive at the number of watt-hours your battery provides if you know the battery’s nominal voltage (V) and capacity in ampere-hours (Ah) using this calculation – Ah x V = Wh. If only the milliampere hours (mAh) are marked on the battery, then divide that number by 1000 to get ampere-hours (Ah). For example, 4400 mAh / 1000 = 4.4 Ah.
Spare non-spillable batteries must not have a voltage greater than 12 volts and a watt-hour rating of not greater than 100 Wh.
Rules for medical and mobility aids containing batteries define limits in grams and watt-hours (see above for explanation of watt-hours).
Medical aids not containing batteries express mass limits in kg for cylinders; non-radioactive medicines (including aerosols) are in kg and litres.
Limits for diving cylinders are expressed in kilopascals.
Unsurprisingly, all measurements in airport regulations are expressed in metric units, the global measurement language. It helps us, no matter where we are flying from or where we are flying to. We can easily see whether any products we have with us meet the regulations using familiar metric units that are used and understood worldwide.
Sources:
Metric Views 
However, if the captain informs the passengers of the flying height, this will be given in feet. Is there any part of the world where this is given in metric, and if so, where?
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Feet is used in aviation because the early standards were determined by British and Americans who dominated the aviation industry at the time. So other countries fell in line with that. Russia switched over in 2017, leaving only China and North Korea using metres. The ICAO wanted to switch over to metric units but the USA resisted.
https://aerosavvy.com/metric-imperial/
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