If you don't have a passcode enabled on your phone, you're missing out on a virtually impenetrable way to prevent strangers from accessing your phone if it gets lost or stolen. And it keeps your phone safe from run-of-the-mill prying eyes as well.
In addition to enabling the passcode, you may want to adjust the time it takes the iPhone to auto-lock. You can choose a time from 30 seconds to 5 minutes.
How to lock your iPhone using a passcode
1. Start the Settings app.
2. Depending upon which model iPhone you have, tap "Face ID & Passcode" (for iPhone X or later), "Touch ID & Passcode" (for iPhones 5s and later) or "Passcode" (for the iPhone 5 or older).
3. Tap "Turn Passcode On."
You should enable the passcode for your iPhone, since it provides almost effortless security.Dave Johnson/Business Insider
4. Enter a six-digit passcode that you'll enter whenever you want to unlock your phone. If you don't want to use the default 6-digit number, first tap "Passcode Options" and choose the kind of passcode you want to use: an alphanumeric password, a 4-digit number, or a custom number.
You have a lot of choices for how long and complicated to make your passcode. Most people use either a four- or six-digit numeric code.Dave Johnson/Business Insider
5. Confirm the passcode by entering it a second time on the verification screen.
6. Your iPhone may ask you to enter your Apple ID password. This allows you to use your new passcode instead of the Apple ID password for some tasks.
Your phone now has a passcode assigned to it, which means when it turns off automatically after a couple of minutes, or when you turn it off immediately with the Power button, it will be locked and require a passcode to wake.
Change the time to auto-lock
1. Start the Settings app.
2. Tap "Display & Brightness."
3. Tap "Auto-Lock" and then choose a time from 30 seconds to 5 minutes.
How to lock your phone by putting it in Lost Mode
No one looks forward to losing a phone, but it sometimes happens. The good news is that Apple provides a tool for locking and helping you recover it. When put into Lost Mode, your phone will automatically lock, even if you had never set up a passcode before — though it can only do that if it still has battery life remaining and has access to a phone signal. The lock screen also displays a message of your choice, including as a phone number to call if it's found.
1. In any web browser, open the Find My iPhone web page.
2. Log in with your Apple ID password.
3. When the map appears, click the device menu at the top of the screen and choose your lost iPhone.
Lost Mode locks your phone's screen remotely.Dave Johnson/Business Insider
4. In the box that appears on the right, click "Lost Mode."
5. Enter a phone number which will appear on the phone's lock screen, so if found, you can be notified. Then click "Next."
6. Enter a message that you want to appear on the lock screen — or accept the default message. Click "Done."
You can create a custom message for your phone's lock screen. You can offer a reward if it's returned, for example.Dave Johnson/Business Insider
7. Your phone is now locked remotely (assuming it's in range of a wireless signal).
If your phone is later recovered, you can return to this web page and click "Lost Mode" again. You'll need to confirm by clicking "Stop Lost Mode" several more times.
After your phone is found, you can turn off Lost Mode, which returns your phone to normal.Dave Johnson/Business Insider
Related coverage from How To Do Everything: Tech:
How to change your Apple ID on an iPhone
How to turn off the Find My iPhone feature on your iPhone
How to find your Apple Watch if it's been lost or stolen
How to backup your iPhone to iCloud, your computer, or an external hard drive
Dave Johnson
Freelance Writer
Dave Johnson is a technology journalist who writes about consumer tech and how the industry is transforming the speculative world of science fiction into modern-day real life. Dave grew up in New Jersey before entering the Air Force to operate satellites, teach space operations, and do space launch planning. He then spent eight years as a content lead on the Windows team at Microsoft. As a photographer, Dave has photographed wolves in their natural environment; he's also a scuba instructor and co-host of several podcasts. Dave is the author of more than two dozen books and has contributed to many sites and publications including CNET, Forbes, PC World, How To Geek, and Insider.