For temperature:

Water freezes at 0, boils at 100. Room temperature 20 degrees Celsius. Normal body temp 37 degrees.

Trivia of minus 40 Fahrenheit being the same as minus 40 Celsius.

Height and weight are usually still thought of in imperial (canadian here), so I think of myself as 6'2", instead of 188 cm.

Volumes and lengths and weights are related. One cubic cm is one mL of liquid. One cubic centimeter of water weighs one gram. One thousand mL of water makes one Liter, which weighs one kilogram.

2.205 ponds makes one kilogram.

Shifting between miles and km is a pain in the ass, given the 1.6 km per mile.

A person who buys some material, Thinks to themselves managerial, I could use grams or litres, Maybe even amps or square meters, At least it isn't Imperial.

Fellow American convert to the metric system. Converting, in my opinion, won't get you very far in actually understanding the measurements. To this day, the conversion rate is something I have to dig through my memory for.

For me what helped with the temperature scale was breaking it into chunks based on what I would wear, 10°-15° would be a pullover sweatshirt, 15°-20° a track jacket, etc, which got me to stop focusing so much on the conversion. Eventually you just get a sense of these things, I think that most people can only really feel a difference in air temperature of about 1°C. 0° being the freezing point cutoff is super helpful for judging things like potential road conditions if it's wet.

For distances I first got the sense of how far things were in kilometers by being a runner and knowing distances around my neighborhood as to how they lined up with running a 5k, 10k, etc. For meters, at my height and gait, my stride length is about a meter long. A little bit on the shorter side of things, but it still helped me get an idea as to what a meter looked like in physical space, even if it's off a bit. Centimeters and millimeters are a different story. Hard to find perfect analogs in the world, but you'll find something eventually. I think for example long grain rice can be ~1 cm in length for example.

The biggest lesson in my own journey and seeing a lot of people online talk about trying to do the conversion is that people get overly concerned with precision when first making the switch. If you actually think about most of our daily interactions with measurements, they're much more approximate. For example, the difference between whether it's 71°F or 73°F is rarely pointed out. The temperature is just "in the low 70s". We say that something is "about 20 miles away" which is almost an implicit 7-8 mile range. I would guess 80% of the time, this is how we interact with the units we use, so focus on that. No one is going to get upset if they ask the temperature and you're off by a few degrees C.

In terms of mnemonics like US kids get in school for some of these things, everything in the metric system is a multiple of 10 from everything else, which is what makes it great. Also remember that at room temperature, water's density is 1 g/mL, so if one of capacity or weight is easier to visualize for you, it's a shortcut to the other. Standard disposable water bottle in the US is 500 mL or half a kilogram of water.

If only metric time had caught on too....

A small trick is to measure your own hand. How big is your fingers spread all the way? That will always be a good quick measure. Like this: 🤙and 🤘.

And for the hierarchy:
Kilo means 1000 of something
Centi means 1/100.
Mili means 1/1000.

kilo + meter = 1000 meter. centi + liter = is a cube of water that measures ~~1 cm all around, that actually 1/100 of a liter. And 1/100 of a kilograms if it is water.~~
Edit: 1cm cube is a mililiter, because 10x10x10 its 3dimensional as Moody pointed out.

~~We don't really need any of those mnemonics because it's a perfect system~~

More seriously there is the "King Henry Died, Drinking Chocolate Milk" for the Kilo (1000) Hecto (100) Deca (10), Deci (0.1) Centi (0.01) Milli (0.001), but that doesn't really help with measuring on the spot, aside from being able to get the prefix right.

There's an average step being 1 meter, but thats less useful for people with shorter legs unless they want to join the ministry of silly walks.

One that I use often is converting meters per second to kilometers per hour. Because 1 meter per second is 3600 meters per hour or 3.6 kilometers per hour, you can actually skip the multiply by 3600 and then divide by 1000 and just multiply by 3.6.

But aside from time conversions, there isn't really anything else that can help because it's just moving the decimal.

Slightly related, you can tell how far away lightning is by listening for the thunder and counting the seconds. Sound travels at 346 m/s so every 3 seconds is roughly 1 kilometer away. But I suppose you can do the same for miles and count to 5.

That thing about the queen and the princes etc. is silly and just gets in the way. Don't those people have anything better to do?

Get ruler. Hold your arm out 90degrees, Measure from the tip of your finger 1 metreacross your body, and rember where that Metre ends on your body. Then you always have a reference for 1metre

Celsius:

0 is the phase transitiom temperature for water between solid and liquid under normal atmospheric pressure. 100 is the phase transition temperature between liquid and gas under normal atmospheric pressure.

Man you got some giant feet and sausage fingers lol

I'm in Canada, and learning French in school actually helped me with fractional measurements since French is based on Latin.

Cent is 100 in French, so 1/100 meters is a centimeter

Mille is 1000 in French, so 1/1000 meters is a millimeter

Dix is 10 in French, so 1/10 meters is a decimeter (this is last because it's not super helpful since you never see deci- units in the wild outside of niche applications)

And for the powers of 10, we only really talked about kilo (1000) in school, but I was interested in computers since I was a child so I figured out mega, giga, terra, etc fairly early on.

It's weird i know my measurement in both metric and imperial because when I was a child i learnt to play lawn bowls and all the old people measured everything in inches feet and yards, then when I became a mechanic there's the three spanner sets so I can do all those.

As for tips, I worked out my own pace count for 100 meters, and at my old workshop we had meter increments on the floor so you could work out what kinda goofy arse step you need to take for 1 meter.

Temperature obviously 0 is frozen water 100 boiling anything over 40 is damn hot outside but that one varies for person to person.

For length, for an average male one meter is about one large step with extended legs (useful for distances), or the distance between e.g. the left side of your torso to the end of the extended right hand (useful for estimating the length of rope or smth).

For weight, it might be useful that 1 liter (that's 1 dm3 but noone uses that except sometimes in scientific literature) is almost exactly 1 kg, and a typical cup fits 0.25 liter. A shot of alcohol is either 20 or 40 milliliters (0.02 or 0.04 liter) depending on where you are and what you order.

For conversions you just need to remember the base unit (e.g. meter and grams/kilograms) and the decimal prefixes. But you really only need milli (1/1000), centi (1/100) and kilo (1000) in day to day life. Then you simply shift the decimal.