On the calculator's own default — a $64.50 bill at 18%, split between 2 people — the tip comes to $11.61, making the total bill $76.11. Split evenly, that's $38.06 per person ($5.81 of which is each person's share of the tip). Splitting the pre-tip bill and the tip separately and then adding them, versus splitting the already-combined total, produces the same per-person number here — the only place it can differ is when the split doesn't divide evenly into cents, where someone ends up a cent or two off from an exact even share either way.
You don't need a calculator for most common tip percentages if you know one trick: 10% of any bill is just the decimal point moved one place left ($64.50 → $6.45). Once you have 10%, 20% is simply double it ($12.90), 5% is half of it ($3.23), and 15% is 10% plus that 5% ($9.68). Combining those covers the entire 10–25% range most tipping guidance falls into, without needing to do any real percentage math at the table — useful as a sanity check even when you do use a calculator.
This varies a lot by country and situation — in the US, 15–20% at sit-down restaurants is common, while many other countries have little to no tipping culture.
Enter the number of people and the total (bill plus tip) is divided evenly. It doesn't currently support splitting unevenly by what each person ordered.
Conventions differ — many people tip on the pre-tax subtotal, but this calculator uses whatever bill amount you enter, so use pre-tax or post-tax as you prefer.