Understanding 90% is simple...

Pre-1965 U.S. dimes, quarters, and half-dollars were struck with a composition of 90% silver and 10% copper (this 90% Ag composition is called "coin silver"). The silver content of one of these dimes when struck (uncirculated condition) was .07234 oz. of pure Ag.

Back in those days money was worth something; the metal content was what validated the worth of the coin. A 90% Ag dime contained the amount of pure silver as reported above, a 90% quarter when struck contained 2.5 times as much Ag as a dime, and a half-dollar when struck contained 5 times as much Ag as a dime and 2 times as much silver as a quarter.

But don't boggle you mind with the silver content of individual coins. Rather,

know that the mint shipped the coins in bags that contained coins that totaled $1000.00 in face-value. These $1000.00 face-value bags contained total 723.4 oz of silver.

This means that a $1000.00 face-value bag of dimes with a total of 723.4 oz. of Ag held 10,000 coins, a $1000.00 face-value bag of quarters with a total of 723.4 oz. of Ag contained 4000 coins, and a $1000.00 face-value bag of halves with a total of 723.4 oz. of Ag, contained 2000 coins. Each bag of coins regardless of denomination contained 723.4 oz. of pure Ag.

Circulation causes coins to wear = loss of metal. Someone weighed a bunch of $1000.00 face-value bags of circulated coins and determined that on "average" the bags of circulated coins contained 715 oz. of silver (an average loss due to wear, of 8.4 oz. of silver per $1000.00 face-value lot of coins. So 715 oz. of silver became the widely accepted amount of silver that is contained in an "average" $1000.00 face-value bag of 90% Ag dimes, quarters, halves, or any combination of these coins. A $1000.00 face value bag of circulated coins with less than "average" wear will contain a bit more than 715 oz. , and the same bag of more severly worn coins will contain a bit less than 715 oz. of silver. On average, a lot of half-dollars will contain more silver that the same face-value lot of dimes, because most dimes circulated more than the halves and thus sustained comparatively more wear.

A person will probably never see an actual "average" .10, .25, or .50 coin, and in fact a precisely "average" coin may not even exist. What does exist is that no two circulated coins have exactly the same weight, and since it is not practical to weigh each coin, the 715 oz. figure as reported above, has become widely accepted.

This means that a $1000.00 face-value bag of "average" circulated 90% Ag coins contains 715 oz. of pure Ag; a $100.00 face-value bag of "average" circulated 90% Ag coins contains 71.5 oz. of pure Ag; a $10.00 face-value lot of "average" circulated 90% Ag coins contains on "average" 7.15 oz. of pure Ag, and a $1.00 face-value lot of "average" circulated 90% Ag coins contains .715 oz. of pure silver.

So.........if you want to know the value of a lot of "average" circulated 90% dimes, quarters, halves, or any combination of the three, count how much is there (this is the face-value of the lot) and then multiply that amount in dollars by .715. This will give you the total amount of silver that an "average" circulated lot of this dollar amount of coins contains. To determine the silver (melt) value of the lot multiply the face-value of the lot X .715 X the current spot price of silver.

You can do the same thing to determine the melt (silver) value in a lot of "average" circulated 40%Ag 1965-1979 Kennedy Halves by substituting

.295 (there is .295 oz. of pure Ag in a one-dollar face-value lot of "average" circulated 40% halves) in place of .715.

And you can do the same thing with 80% Ag Canadian coins by substituting

.592 into the formula.

Another easy way to tell the value of a lot of coins is by knowing that a $1.40face-value lot of "average" 90% circulated coins contains 1 oz. of pure Ag....6.8 40% Ag "average" circulated halves contain 1 oz. of pure Ag, and $1.75 face value of "average" circulated 80% Ag Canadian coins contain 1 oz. of pure Ag.

Multiplying .715 times the spot price of silver will also give you the number

that multiplied times the face-value lot of coins, is the number that multiplied x face-value, gives you the amount of money that that face-value lot of coins currently trades for. As an example: If the current spot price of silver is $11.75, the value of a lot of 90% "average" circulated coins is 8.4 times the face value of the lot....(.715 x $11.75 = 8.4). Thus if a person had an $8.00 face-value lot of 90% Ag "average" circulated coins, those coins have a current value of 8.4 x $8.00 = $67.20.

I hope this helps you. :)