A "dime" is the slang term for a US 10 cent coin.
The Royal Mint has never produced "dimes" or 10 cent coins.
Ten cents was the value of the dime in 1940.
If you have a 1940 Roosevelt dime it is fake. The Roosevelt dime was not produced by the US Mint until 1946.
They don't exist, the first Roosevelt dime was 1946. With a date of 1940 the dime is a MERCURY dime and is common with average values of $1.00-$3.00 depending on condition.
The British have never produced a "Dime" coin. The "Dime" is a US 10 cent coin.
The British didn't make a half dime.
If it is a circulated dime, then it is worth about $1.00, mostly depending on the value of silver
1940 dime were only made from 90% silver. See the related question below for more information.
Priceless. No barber dimes were minted anywhere near 1940.
A 1940 Mercury dime (not Liberty) is very common. If it shows any wear, the value is just for the silver, about $1.90.
C$0.75 to C$10 depending on its condition.
The dime you have is a mercury dime and the value depends on the condition of the coin. They can range from 99 cents to 200 dollars and they can also go up to 800 dollars if they are a proof.
Chances are, if it is the same size or looks like a dime, it may be counterfeit.