Is it common to use platforms like TransferWise to convert your SGD to USD to invest in US stock market or people generally let their banks/brokers convert at their rates + any conversion fees?

Yes, you should definitely consider the foreign exchange fees involved when you fund your investment accounts with one currency (SGD) while investing in another (USD). These exchange rates charged by the broker are usually not the best, and additional charges are incurred. You can check out my response to a similar thread here: https://seedly.sg/questions/what-s-the-cheapest...

But long story short, make sure you check first with the company/broker you are using to invest to see if they accept transfers from third-party platforms like Revolut and TW or risk getting your trasnfer refunded. Otherwise, Revolut and TW offers some of the best exchange rates out there.

Hope this was helpful! If you are keen, here's a referral link for TW (for fee free transfer of $900):

https://transferwise.com/invite/u/sebastiank815​​​

The problem with Transferwise and others (Instarem, Revolut) is that they are considered as third party transfers. Most brokers do not accept transfers from them unless it is originating from an account in your name.

For brokers, Interactive Brokers has the best foreign exchange rate (live spot rate) and commission of 0.2 pips (0.002%), min 2 USD. Other brokers with foreign currency exchange fumctionality such as FSMOne, Tiger and SCB charge a spread on top of the spot rate, which can range from 0.2-0.6%.

Do note that if you intend on getting back your original currency, you will be incurring these charges twice. The spread may vary as well. Hence it is in your best interest to minimize costs when converting currency.