Like has been said by the person upstairs.
You can calculate total number of inactive Bitcoin within a timeframe
But can not calculate the total number of Lost Coin.
Many are holding, some lost could be accessed and while certain portion are really lost either through losing seedPhrase or dying with nobody been aware of the holding.

To calculate realized Cap, you can access data from Glassnodes https://glassnode.com/ about Coins that haven't moved for a long time.


https://studio.glassnode.com/metrics?a=BTC&m=supply.ActiveMore5YPercent