It's common to want to know what a Balancer LP token has as its underlying tokens.

Directly Query The Vault and LP Tokens

The Vault can tell you exactly how many tokens are in the pool. You can query that, and then divide by the amount of the pool that is yours


(tokens, balances, lastChangeBlock) = vault.getPoolTokens(poolId);
yourPoolShare = bpt.balanceOf(yourAddress)/bpt.totalSupply();
uint256 yourUnderlyingBalances = new uint256[](balances.length);
for(i=0, i<balances.length, i++){
    yourUnderlyingBalances[i] = balances[i]*yourPoolShare;

Note about pre-minted BPT

Some pools (like bb-a-USD) have pre-minted BPT. This means all LP tokens are minted at the time of pool creation so that you can use a swap to effectively join/exit the pool. Because of this, when querying the supply, you should NOT use bpt.getSupply(), but rather use bpt.getVirtualSupply().

The above assumes you have your BPT in your wallet. If you have staked your BPT in a gauge, you'll need to calculate your BPT holdings as:
myBpt = bpt.balanceOf(yourAddress) + bptGaugeDeposit.balanceOf(yourAddress);