Valuing
Overview
It's common to want to know what a Balancer LP token is worth. This generally comes down to its Net Asset Value (NAV), ie the value of the underlying tokens.
Directly Calculating NAV
Directly calculating NAV is the simplest way to value an LP token. In short, it is calculated by getting all underlying balances, multiplying those by their market prices, and dividing by the total supply of LP tokens.
This method is quick and easy for informational purposes, but is vulnerable to manipulation if you need to know a Balancer LP token's value on-chain. An added bonus is that this technique works for all pool types because it simply gets the value of all underlying tokens.
Equation
Pseudocode
(tokens, balances, lastChangeBlock) = vault.getPoolTokens(poolId);
prices = fetchPricesFromPriceProvider(tokens); //ex. CoinGecko
poolValueUsd = sum(balances[i]*price[i]);
bptPriceUsd = poolValueUsd/bpt.totalSupply();
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().
And if you want to calculate a pool value for a given address...
myBptValueUsd = bpt.balanceOf(myAddress) * bptPriceUsd;
Note
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);