Remember the old fractions? 1/2, 1/4, 1/8, 1/16 - each one smaller than the previous one? 0.5, 0.4, 0.2, 0.1 etc is the same thing - each one smaller than the previous one.
For future reference, if you can remember that 300 mm is approx 1 foot, you should be able to work out other sizes.
Yes Rox, quite right, and that’s another thing inflicted upon us, as well. 
Thank you Judsy, I think I might remember what you said about the 0.4, etc , but the other one about how many mm’s in feet, will be long gone by the time I wake up in the morning. 
Naw, I’ll stick with my own way of coping. 
When i first came to Australia the country was just changing to metric units and with hindsight the way they did it was excellent. I say “with hindsight” because I too hated the change. I was at school in the UK when they started the first of their many bungles toward metric, first it was CGS units, then MKS units and, I presume, they finally settled on SI units, nobody stuffs up change better than the British.
What they did here was ban all references to the old system, no stating the temperature in the weather forecast in both Celsius and Fahrenheit, no lbs or ounces in the shops, 8ft by 4ft wall panels were still the same size but referred to in mm - it was metric only everywhere. Importing rulers with inches on them was totally banned, the road signs were changed virtually over night. Imperial measurements were never mentioned again.
Because of this even objectors like me learned the metric system because we had no choice and now I am so glad they did it this way rather than muddling along with both systems as has happened in the UK.
I still lapse, I might say to my kids something like, “Move it a couple of inches toward me.” The object in question could then move anything from a centimetre to foot! My daughter once asked me, “What are lumbs and ozzies?”. On the whole I much prefer metric.
OK, here are my comments on the metric system
For years Britain and its Empire used The Imperial System, hence the name
From time to time there were discussions about changing to the Metric System, but nothing much came of it, though it was increasingly adopted by the scientific community
Then after the Second World War the idea of changing became more serious because -
The British Empire was breaking up, so Britain was increasingly looking to trade with other countries, especially in Europe
Computerisation was just around the corner
Thus it was fairly obvious that Britain would need to change both the Imperial System to Metric and the currency from pounds, shillings and pence - £sd to decimal currency
So Britain officially adopted the Metric System in 1965, but the government decided to make it a gradual change as this would be less disruptive, and cheaper
The idea was that as new generations were taught Metric at school, machines were renewed, shops & manufacturers changed packaging, and so on, the change to Metric would happen naturally over just a few years, certainly within a new generation of school kids
Well that was the plan, but as we know it didn’t happen like that, and we now have a silly and confusing mixture of Imperial and Metric
Compare this with places like Canada, India, & Australia where the change was done properly, completely, and fairly quickly
My opinion is that the sooner we get rid of the old fashioned units and adopt the Metric System completely, the better
I voted Leave, and I have no wish to return to the EU, but its not about that
I think the Metric System is better, and also it is used all over the world
Although the USA is now the only country that hasn’t officially adopted the Metric system, I have American friends who tell me that it is becoming more common
I use grams & kilos for weighing, metres, cm, & mm for decorating, DIY, carpentry, litres for milk & petrol and so on
I must admit that I’m not yet completely familiar with some units, such as kilometres for distances & driving, but that’s because we hardly use them; I’m sure that will improve as I get more familiar
I was taught and have used both systems, but I find that I prefer Metric.
It was obvious that we couldn’t run £sd & decimal at the same time, so that was done completely & quickly
I have to say that was stuffed up too, as soon as they issued half and quarter pence you knew someone had failed (and guaranteed built in inflation).
Instead of New Pence it should have been New Pounds, a new pound being worth the old 10/-
I’ve just realised another bit of confusion -
I weigh myself in kg
The medical centre measures my height in cm, and I’ve got used to that - I live on my own, and if you think about it, it’s difficult to measure how tall you are by yourself
But the waist of my trousers is quoted in inches
My trousers are 92cm so that is fine.
The only anomaly I than think of is TV screen size, they used to be in centimetres but when they started to change from 4:3 to 16:9, to avoid confusion they started to quote 16:9 TVs in inches and it has stuck. My TVs are 40 and 55 I suppose it doesn’t matter that much as they are just quoted as a number, in the TV specs everything is metric.
I didn’t mention the one exception to the metric only rule was the police “Wanted” or Persons Missing notices they were allowed to quote a person’s height in both metric and imperial. I am pretty sure it was the only exception, I notice that heights are now given purely in cm.
Hi
We have a weird system here in the UK.
In Hospital you are weighed every Sunday in Kilos.
That is the minimum, for some it is twice a day.
Many cannot stand up, so you are measured in your bed, which is in feet and inches,
All good fun.
A cricket pitch wicket to wicket is a chain and ever more shall be so.
I think you will find that nations that actually know how to play the game of cricket measure it as 20.12 metres stump to stump and 17.68 metres crease to crease. 
Completely agree it is another language to me , I have to take my tape measure with me . It’s the same when I bake I don’t know the liquids or weights I have to convert. It drives me absolutely mad
Steer well away from DIY stores and head off to a builder’s merchant’s trade counter.
Is there a Toolstation anywhere near your location? Ask for 50mm wire nails and they will have a flat head and not those lost-heads B&Q offered you.
Round Wire Nail Pack 50mm | Toolstation
When I was a Lass, hardware shops used to sell nails by weight - they priced them by the pound then, not by the kilogram, but the principle is still the same - they weighed them out and charged by weight.
The difficulties with buying DIY stuff from hardware shops began when they started putting them into little packets and opening out of town DIY shops - most of the old fashioned shops, owned by experienced hardware retailers, who had a bit of everything and could give you a bit of advice too have gradually disappeared from our High Streets.
Even if you have a nearby hardware shop, it’s rare you can go in and ask for advice about the product you want and ask for the exact number of screws or nails you want for a small job - you have to buy them in set pack sizes, unless you go to a builders merchant.
The 0.4 mm and 0.5 mm measurement thing may be easier if you think of the numbers as fractions of 10 - whether you are talking about millimetres, inches or even slices of cake, 4/10ths of something is smaller than 5/10ths.
I agree, Susan, then just when I think I have the hang of ounces/grams/pints/litres etc - some idiot starts using cups!!
A few of you here know that my wife is an American. She likes to cook and bake - and I am the lucky recipient! Naturally she usually uses pounds and ounces and can convert metric units. Just now and again she will ask the odd unusual unit.
I use cups occasionally when baking or making Yorkshire puds, it’s a lot easier because they are units of volume rather than weight. 1 x cup is around 8fl oz, or 5oz dry weight flour, 7.5oz sugar etc.
Thank you LD. There is a Toolstation and a Screwfix not too far from me, I never thought of going there before.
I have bought some now though although they are not quite what I really wanted, but I’ll see how I get on. If I don’t like them, I will try one of those other places.
