Should I buy a total-market fund or an S&P 500 fund?

Either is a fine core U.S. stock holding; the difference is mostly small- and mid-cap exposure. An S&P 500 fund holds about 500 large U.S. companies, roughly 80% of the U.S. stock market by value. A total-market fund adds thousands of smaller companies, so you own essentially the entire U.S. market in one fund. Historically their returns track closely because large caps dominate both. The total-market fund gives slightly broader diversification and captures small-cap growth, while the S&P 500 is the more familiar benchmark and often has equally rock-bottom fees. You don't need both — owning both just duplicates the large-cap core. Pick one as your U.S. anchor, keep the cost low, and add international separately. Build the full allocation at wealthserene.com/tools/portfolio-builder.