There are 86,400 seconds in a day. Here's how it's calculated:

• 1 minute = 60 seconds
• 1 hour = 60 minutes = 60 × 60 = 3,600 seconds
• 1 day = 24 hours = 24 × 3,600 = 86,400 seconds

So, 24 hours × 60 minutes × 60 seconds = 86,400 seconds in a day.