How much should you tip at a restaurant?

The US standard is 15–20% of the pre-tax bill. 15% is acceptable for standard service, 18% is good, 20% is great, and 25%+ is for exceptional service. For takeout, 10–15% is common. For delivery, $3–5 minimum or 15–20% of the order total is standard.

Should you tip on the pre-tax or post-tax amount?

Tipping on the pre-tax subtotal is technically correct, since the server didn't provide the tax. However, tipping on the total is also common and acceptable — the difference is usually small (e.g., 8.5% tax on an $80 bill = $6.80, so the difference between tipping on $80 vs. $86.80 at 20% is only $1.36).