1). Gather some data based on historic coins.
Used to gather approximate weight and value of gold, silver, bronze, copper and tin coins around the year 1700: http://www.coins-of-the-uk.co.uk/coins.html#size
Coin weights:
1 gold = 8g
1 silver = 6g
1 bronze = 9.4g
1 copper = 9.3g
1 tin = 4.7g
Coin conversions:
20 silver = 1 gold
12 bronze = 1 silver
2 copper = 1 bronze
4 tin = 1 copper
2). Gather worth of coins to compare with Middle Earth.
Historical Prices: In 13th century England, beer was between .14 and .28 grams of silver per liter.
The MERP Second Edition main book has beer at approximately 4 tin pieces per liter. 4 tin pieces in the above defined coin system converts to about .24 grams of silver. Close enough to guestimate MERP coin weights.
3). Adjust for MERP denominations and decimalization:
Convert all coins to equal value (1 gold) to get the overall weight.
1 gold = 8g
20 silver = 1 gold = 120g
240 bronze = 1 gold = 2256g
480 copper = 1 gold = 4464g
1920 tin = 1 gold = 9024g
Adjusted for base 10 denominations to get the weight per coin.
1 gold = 8g
1 silver = 12g
1 bronze = 22.5g
1 copper = 4g
1 tin = 0.9g
Middle Earth Coin Coversion
1 gold = 1 gold
10 silver = 1 gold
10 bronze = 1 silver
10 copper = 1 bronze
10 tin = 1 copper
Example 1:
56 gold = 1 lb
37 silver = 1 lb
19 bronze = 1 lb
112 copper = 1 lb
497 tin = 1 lb
Example 2:
10 gold = 10 gold = 0.18 lbs
100 silver = 10 gold = 2.67 lbs
1000 bronze = 10 gold = 50 lbs
10000 copper = 10 gold = 89 lbs
100000 tin = 10 gold = 201 lbs
4). The Middle Earth Rules Second Edition indicates to use 1/4 ounce per coin for weight (approx 7g).
10 gold = 10 gold = 0.16 lbs
100 silver = 10 gold = 1.56 lbs
1000 bronze = 10 gold = 15.63 lbs
10000 copper = 10 gold = 156.3 lbs
100000 tin = 10 gold = 1563 lbs