Find the total before tax and tip (subtotal).
Choose the tip percentage (e.g., 15%, 18%, 20%).
Convert the percentage to a decimal (15% → 0.15).
Multiply: tip amount = subtotal × tip decimal.
If you want the total bill: total = subtotal + tip amount (and add tax if not included in the subtotal).
If you know the tip amount and want the percentage: tip percentage = (tip amount ÷ subtotal) × 100.