Every Facebook post carries its own audience, set by the selector in the post box, and it only applies to that post. Your next post goes back to whatever default audience is configured in your settings, which is why people who carefully set one post to Friends find the following one is Public again. Getting this right means dealing with three separate things: the default for future posts, the audience on posts you already shared, and the handful of details that stay public no matter what you choose.
The audience selector
When you start a post, the audience selector sits below your name, usually showing your current default such as Friends. Click it and you get the choices for that post. The main ones are Public, Friends, Only me, Friends Except..., and Specific Friends, and if you have set up friends lists you can pick one of those as well.
Here is what each one means in practice. Public means anyone can see it, including people who are not on Facebook. Friends means your Facebook friends. If you want the tagged person's friends to see it too, choose the 'Friends (+ friends of anyone tagged)' option. Only me makes the post visible to you alone and keeps it out of your friends' feeds, which is useful for drafts and for archiving something without deleting it. Custom lets you narrow the audience further, including sharing with groups or networks you belong to.
The audience you set follows the post across devices. Change it on a computer and the change applies in the mobile app as well, because it is a property of the post rather than of the app you used.
Setting a default audience for future posts
This is the setting that saves you from deciding every time. On a computer, click your profile picture in the top right, choose Settings and privacy, then Settings, then scroll to the audience and visibility section and open Posts. Click the control next to the question about who can see your future posts, pick an audience, and confirm. The same setting exists in the iPhone, Android and Facebook Lite apps.
Setting this to Friends is the single highest value change most people can make, because it means an absent-minded post cannot accidentally be public. You can still raise an individual post to Public when you want to, and doing so does not change the default back.
Changing the audience on something you already posted
Old posts keep whatever audience they had when you published them. To change one, open the post, use its options menu and choose to edit the post audience, then pick a new one. This works post by post and is fine for a handful of items.
For a backlog, Facebook has a bulk tool called Limit past posts. Go to Settings, open Posts, find the option to limit who can see past posts, and confirm it. It takes every past post that was shared more widely than Friends, such as your Public posts, and changes it to Friends.
Two things to understand before you run it. First, it is one directional: there is no button that restores the old audiences, so putting a post back to Public means editing that post individually. Second, posts already shared with a narrower custom audience are left alone, so it tightens without loosening. It is a safe operation in the privacy direction and an annoying one to reverse.
What Limit past posts does not cover
The tool only touches posts you shared yourself. If someone else posted something and tagged you, they control that post's audience, not you. The same applies to anything you shared onto another person's profile: it belongs to their profile and their settings. For tagged content your controls are different, and live in the profile and tagging section of settings, where you can review tags before they appear and control who can see posts other people put on your profile.
Friends lists and the Restricted list
Friends lists let you split your friends into groups and then share with one group at a time, which is more workable than building a custom audience from scratch on every post. Once a list exists it appears as an option in the audience selector.
The Restricted list is the one worth knowing about even if you never build any others. Adding someone to it means they see your public information and posts you tag them in, but not your posts shared with Friends. They stay your friend and are not notified. To add someone, go to their profile, click the Friends button, choose to edit friend lists, and select Restricted. It is the standard way to handle a colleague or relative you do not want to unfriend but do not want reading everything.
Photos and albums
Photos follow the same audience logic but with a catch. You can edit the audience of an individual photo through your profile, opening Photos, then Your photos, selecting the image, opening Options and choosing to edit the post audience. But for photos that were uploaded as part of an album, in most cases you have to change the audience for the whole album instead. Only certain albums allow per-photo settings, but profile pictures and cover photos are always public and their audience cannot be changed.
What stays public whatever you set
Some information is always public and no audience setting hides it. That includes your name, gender, username and user ID, your current profile picture and your cover photo. Some additional details are public by default too, such as your age range, language and country, follower count and whether your profile is in professional mode. Anyone, including people who are not logged in, can see these.
This is the limit of what audience settings can do. If you need a particular profile photo not to be visible to strangers, the answer is not to use it as your profile picture, because setting its audience is not an option.
Profile fields have their own audiences
The audience selector governs posts, not the information in your About section. Your workplace, education, hometown, relationship status, birthday and contact details each carry a separate audience that you set by editing that field. It is entirely possible to have every post locked to Friends and still be showing your phone number and employer to the public, because those were never covered by the post setting.
Go through the About section field by field once. It takes a few minutes and it is the part of Facebook privacy that most often gets missed, partly because the fields were often filled in years ago when the defaults were different.
Checking your work
Set the default first, then run Limit past posts if your old material was public, then handle tagged content in the profile and tagging settings. When you are done, view your profile as the public sees it. That view is the only reliable check, because it shows the result of every setting at once rather than each one in isolation, and it usually turns up one old post or one profile field that is more visible than you expected.
