The 2016 Annual Report Part 02 Accounts and Operations of the Central Bank of Sri Lanka §5.2 Currency Printing and Minting
In 2016 the Currency Department has taken steps to reduce the cost of minting coins by changing the metal type of Rupee 1 and Rupees 5 coins (minted after 2005) from brass plated steel to stainless steel.
Further, arrangements were made to design a new set of circulation standard coins from stainless steel metal for the denominations of Rupee 1, Rupees 2, Rupees 5 and Rupees 10 reducing the sizes of coins with the objectives of reducing the cost of minting and easy handling.
Based on documents I found online I was able to estimate average cost per coin of minting 2016 and the new 2017 coins
The 2107 coins will add more than 50% to the value of the Rs10,356M of these coins in circulation as of 2016 December 31st.
I suspected from the cost of Minting coins that all of the coins ordered in 2017 March are smaller, and that was unofficially confirmed. As this is the case, 2016 Re 1 and Rs 5 will be 1 year types. The total cost of Minting the 970 Million coins is less than then 50% of Face value. However the Re 1 and Rs 2 coins still cost more than face value to mint. They could have switched them to Aluminium which would have also aided their identification.
After 2017 coins are issued, probably later in 2017 or early 2018, coin change in Sri Lanka will be a total mess, as many types of coin sizes of each denomination will then be in circulation.
In my opinion they should go to the size and shapes of the 1, 2, 5, 10 cents coins minted in 1963 in Aluminium and Brass and since 1978 in Aluminium. The value of the current Sri Lanka rupee is the about the same as the Sri Lanka cent during that era. Around 1970 a Gold Sovereign was worth Rs150 and now over Rs45,000. A factor of over 300.