Apple's iPhone User Guide states the slo-mo defaults in one sentence: "Slo-mo is set to record at 1080 HD at 240 fps from the back camera and 1080 HD at 120 fps from the front camera." So the two cameras on the same phone ship with different frame rates, and neither one is something you chose.
This walks through where that setting lives, how to switch the back camera to 120 fps, what the higher rate costs you, and one shortcut that only exists on recent Pro models. You need nothing but the phone and the Settings app.
Before you start
Three things worth knowing before you touch anything.
Slo-mo is a capture setting, not a playback effect. Apple's camera page describes it plainly: "Slo-mo mode records your video as normal and then plays it back in slow motion." The frame rate you pick at record time is what the footage is actually shot at — there is no way to add those frames back later.
Your options depend on the model. The same page says: "Depending on your model, you can change the slow motion frame rate and resolution." The Tech Specs pages show how much that varies. For iPhone 17 Pro, the Slo-mo row reads "1080p up to 240 fps and 4K Dolby Vision up to 120 fps (Fusion Main)" — that (Fusion Main) qualifier means the 4K/120 path is tied to the Main camera. For iPhone 17, the same row reads "1080p at 120 fps or 240 fps", with no 4K entry.
Slo-mo and ProRes do not mix. Both Apple pages that cover ProRes say so: the support article states "You can't use ProRes with Cinematic mode, slo-mo mode, or time-lapse video", and the User Guide states "ProRes isn't supported in Cinematic, Time-lapse, or Slo-Mo mode." If ProRes is on and slo-mo looks unavailable, that is the reason.
Step 1: open the right settings screen
Go to Settings > Camera > Record Slo-mo. Apple's wording for the route is "To change slow-motion recording settings, go to Settings > Camera, then tap Record Slo-mo."
The result: you see the frame rate and resolution choices your iPhone supports. This is a different screen from Record Video, which is why so many people change the wrong one — more on that below.
Step 2: switch the back camera to 120 fps
On the Record Slo-mo screen, pick the 120 fps option. Apple's instruction is exact about which camera this affects: "To change the back camera to 120 fps, go to Settings > Camera > Record Slo-mo."
The result: the back camera now captures slo-mo at 1080 HD at 120 fps instead of 240. Note what this does not do — the guide sentence is scoped to the back camera, and the front camera default it lists is already 1080 HD at 120 fps.
Step 3: check whether your model offers 4K slo-mo
Still on the same screen, look for a 4K entry. The User Guide adds: "On supported models, Record Slo-mo also includes an option to record 4K at 120 fps from the back camera."
The result: on a model that has it, you get a 4K option at 120 fps instead of the 1080 HD option at 240 fps. The support pages we checked give no per-minute size for either mode, so treat the choice as resolution versus rate and check the file itself afterwards rather than budgeting storage in advance.
Step 4: on recent Pro models, shoot slo-mo without leaving Video mode
This is the shortcut most guides miss. Apple's camera page carries it as a tip: "On iPhone 16 Pro models, iPhone 17 Pro models, and iPhone 18 Pro models, you can record slow-motion videos in Video mode. Tap the format and resolution values on the screen, then select 120 FPS."
The result: no swiping across to the Slo-mo position, and 120 FPS chosen from the viewfinder itself. The model list is exact — 16 Pro, 17 Pro and 18 Pro models — so treat it as a Pro-line feature rather than something to hunt for on a non-Pro phone.
Step 5: adjust the slow section in Photos afterwards
High frame rate footage is not locked to one playback speed. The camera page says: "Videos you record at faster frame rates can be played back at slower speeds in the Photos app."
How much control you get depends on what you shot. Apple's editing page notes: "The playback options available depend on the format of the video you captured. The larger the format you capture, the more options are available." That is the strongest argument in the official docs for capturing at the higher setting when you are unsure — the bigger format gives you more options later, not fewer.
Deal with the file size before you share it
Here is the cost, in Apple's words: "Faster frame rates and higher resolutions result in larger video files." That sentence appears on both the recording settings page and the camera page.
For scale, the only per-minute figure in Apple's documentation covers ordinary footage, not slo-mo: "one minute of regular 1080p 30 fps video is approximately 65 MB". That number's scope is 1080p at 30 fps — do not extrapolate it to 120 or 240 fps. For slo-mo rates specifically, check the official documentation or the size shown on the file itself.
Try it right here
Runs in your browser. Your file is not uploaded anywhere.
That runs in the browser tab — the file is read from disk, re-encoded locally, and handed back, so a personal clip never gets uploaded to anyone's server. If you want the settings exposed instead of a preset, the video compressor page covers the same job.
Common mistakes
Changing Record Video instead of Record Slo-mo. The Video screen is where Apple says "you can change the fps rates to 24, 25, 30, 60, or 120" — a different list, for a different mode. Editing it does nothing to your slo-mo capture.
Assuming both cameras match. They do not, by default: 240 fps on the back, 120 fps on the front, per the guide sentence quoted above. A selfie slo-mo will not match a rear-camera one unless you have made them match.
Expecting ProRes footage in slo-mo. See the two quotes in the Before you start section — it is documented as unsupported in that mode on both pages.
Forgetting that a saved trim is one-way. Apple's editing page warns: "A video saved as a new clip can't be reverted to the original." Save over the original instead if you might want the full-speed footage back.
Sending high-rate footage to an older device and blaming the camera. Apple's HEIF/HEVC article notes that "On some older devices, support for HEVC is affected by the resolution and frame rate (fps) of the video", and that "Resolutions of 1080p or lower and frame rates of 60 fps or lower are more broadly compatible with older devices." A 240 fps clip is outside that comfort zone by definition. On the share sheet you can also "tap Current to prevent a file format conversion, or tap Most Compatible and files may convert to JPEG or MOV" — see Current vs Most Compatible when sharing for how that differs from the camera setting with the same name.
FAQ
What frame rate does iPhone slo-mo use by default? Apple's User Guide states it as "1080 HD at 240 fps from the back camera and 1080 HD at 120 fps from the front camera." The back and front cameras differ out of the box.
How do I change iPhone slo-mo from 240 fps to 120 fps?
Go to Settings > Camera > Record Slo-mo and choose the 120 fps option. Apple's instruction for the back camera is exactly that path.
Can iPhone record 4K slow motion?
On supported models, yes. The User Guide says Record Slo-mo "also includes an option to record 4K at 120 fps from the back camera", and the iPhone 17 Pro Tech Specs list "4K Dolby Vision up to 120 fps (Fusion Main)" in the Slo-mo row. The iPhone 17 Slo-mo row lists 1080p only.
Is 120 or 240 fps better? It is a documented trade, not a quality ranking. 240 fps is the highest rate Apple lists for 1080p slo-mo, and "Faster frame rates and higher resolutions result in larger video files." If you are shooting something genuinely fast and will slow it hard in Photos, stay at 240. If storage or sharing is the constraint, 120 is the cheaper capture — and on a model that offers 4K at 120 fps, that option buys resolution rather than rate.
How many MB per minute does slo-mo use? Apple's documentation gives a per-minute figure for regular 1080p at 30 fps footage (approximately 65 MB) and for spatial video, not for slo-mo rates — check the official documentation for current figures, and see iPhone video storage per minute for what the published numbers actually cover.
Why is slo-mo missing when ProRes is on?
Because Apple documents it as unsupported in that combination: "ProRes isn't supported in Cinematic, Time-lapse, or Slo-Mo mode." Turn ProRes off in Settings > Camera > Formats to get the mode back.
Shooting at 240 fps is the easy part; moving the file afterwards is where it bites. Browse the full Clapr toolbox, or the rest of the step-by-step guides.
Settings paths and quotations taken from Apple's iPhone User Guide and support articles, as captured on 16 September 2026. Apple revises these pages between iOS versions — verify against the current official documentation for your iOS release.
