How to merge video clips online (and what happens when they do not match)

How do I merge video clips online for free?

Use the Koldflux merge videos tool. It runs ffmpeg inside the browser tab, so nothing is uploaded. The first run downloads the converter engine and takes longer than later runs. The recording below joins two of our test clips.

1. Open the merge videos tool and choose two or more video files (up to 100 MB each, as stated on the tool page). They are joined in the order shown under the picker.
2. Check the clips first (next section). Put the clip whose frame rate you want first, because the result takes the first clip's rate.
3. Under "Output shape" pick vertical 9:16 (1080x1920, the default), horizontal 16:9 (1920x1080) or square (1080x1080). Every clip is scaled to fit and padded with black, never stretched.
4. Press "Merge N videos" and wait for the percentage to finish.
5. Press "Download merged MP4".

Worked result: our 3 second 1080p clip joined with our 3 second 720p clip came out as one 1920x1080 file, 30 fps, 6.000 s of picture, H.264 and AAC, 220,623 bytes. The result file was made on the tool before its update. We re-ran the same pair on the updated tool and got the same 1920x1080 frame, 30 fps, 6.000 s of picture and 48 kHz stereo AAC (run 13 in the test log). Download the two sample clips and the result below to repeat it.

Free tools:Merge videos (free, in your browser)Sample clip A, 1920x1080 30 fps (our own, 3 s)Sample clip B, 1280x720 30 fps (our own, 3 s)Result: A then B, horizontal frame (merged by the tool)

What happens when my clips are different sizes, formats or have no sound?

We ran 22 cases through the live tool with our own 3 second test clips and probed every download. The table below is the result: every mismatch is handled, and one catch is left: the frame rate follows the first clip (a variable-frame-rate average within 2% of a standard rate snaps to it, for example 59.87 becomes 59.94 fps). The silent-clip row was re-measured on October 25, 2026 after the tool was fixed.

How the table was made: one pair of our 3 second test clips per row, run on the updated tool as a local build on October 24, 2026, each download checked with ffprobe; the same pairs before the update are in the test log. The silent-clip row was re-measured on the fixed tool on October 25, 2026 (details in the last section). Reading it: the output always has one frame size, one frame rate (the first clip's) and one audio format (AAC, 48 kHz, stereo). The tool re-encodes everything, so there is always a small quality cost; it uses H.264 at the encoder default quality (read from the tool page).

A 720p clip in a 1080p frame is the same 16:9 shape, so it is scaled up 1.5 times and has no bars. Bars only appear when the shapes differ, and then they are black. If the black bars are not what you want, crop the odd clip to the frame shape first with the crop tool linked below.

If one clip has no sound, the tool now adds generated silence of that clip's length, so the other clips keep their sound. We measured it on October 25, 2026: with a silent clip first, last or in the middle, the sound clips measured -22 to -34 dB in the download and the silent clip's span -91 dB (silence). Only if every clip is silent is the result silent (no audio stream). If the tool cannot read a silent clip's length it still joins the pictures only. To add music over silent parts, use the add audio guide linked below.

What frame rate do I get when my clips have different frame rates?

The frame rate of the first clip. After the tool update of October 24, 2026, we merged 24 then 30, 30 then 24, 60 then 30, 30 then 60 and 25 then 30 fps pairs, plus a three-clip run (30, 24 and 60 fps). Every run finished in 11 to 23 seconds and every file had one frame rate: 24, 30, 60, 30, 25 and 30 fps in that order, each equal to the first clip's. The picture length stayed 6.000 s (two clips) or 9.000 s (three clips).

So choose the order on purpose (our suggestion): put the clip whose frame rate you want to keep first. A 60 fps clip first gives a 60 fps file, and the 30 fps clip after it plays at its normal speed. We checked the frame counts and rates in the files (for example 60 then 30 fps gave 360 frames in 6.000 s), not how motion looks after the conversion.

This is new: before the update, the same pairs stayed at "Merging 0%" with no file and no error, for 75 seconds to 7 minutes of waiting. The test log keeps those old runs, marked as before the update.

The update also adds a safety stop: if merging makes no progress for 90 seconds, the tool cancels with a message ("Merging stopped making progress and was cancelled"). We did not trigger that stop in our tests, so we cannot say how it behaves on a real stuck file. If you see it, convert the clips to one frame rate with the optional command below and merge again.

Phone clips can have a variable frame rate; we did not test real phone recordings. On October 25, 2026 we tested VFR clips derived from animation (see the last section): an average of 59.87 fps came out as 59.94 fps, while 40.6 fps, not near a standard rate, was kept as 40.62 fps. Run the ffprobe line above: if r_frame_rate differs between clips, expect the first clip's value, and convert first if you need a fixed rate.

ffmpeg -i clip24.mp4 -vf fps=30 -c:v libx264 -pix_fmt yuv420p -preset veryfast -c:a copy clip24_30fps.mp4

ffprobe -v error -show_entries stream=codec_name,width,height,r_frame_rate,sample_rate,channels -of compact clip.mp4

Is there one command that merges clips of any size, rate and audio format?

Yes, if you can use ffmpeg and want to control the frame rate yourself. This command scales each clip into a 1920x1080 frame, pads it, forces 30 fps and one audio format, then joins them. We ran it on 30 plus 24 fps clips and the result was 1920x1080, 30 fps, 180 frames (6.000 s) with 48 kHz stereo AAC. The same recipe, generated for other inputs, also joined 60 plus 30 fps, 25 plus 30 fps, an MPEG-4 AVI plus H.264, a mono 44.1 kHz plus stereo 48 kHz pair and three clips. Every clip needs an audio track in this command (we did not test silent clips with it; the browser tool adds the silence for you).

This is close to what the browser tool does since the October 24, 2026 update, except that the tool uses the first clip's frame rate and generates silence for a silent clip, while this command fixes the rate at 30. FFmpeg's documentation says the concat filter needs the same number of streams of each type in every segment, which is why the audio step normalizes every clip to one format and why the browser tool generates silence for a clip that has no track.

ffmpeg -i clip1.mp4 -i clip2.mp4 -filter_complex "[0:v]scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,setsar=1,fps=30[v0];[0:a]aresample=48000,aformat=channel_layouts=stereo[a0];[1:v]scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,setsar=1,fps=30[v1];[1:a]aresample=48000,aformat=channel_layouts=stereo[a1];[v0][a0][v1][a1]concat=n=2:v=1:a=1[v][a]" -map "[v]" -map "[a]" -c:v libx264 -pix_fmt yuv420p -preset veryfast -c:a aac -b:a 160k merged.mp4

Can I merge clips without losing quality?

Only if every clip has exactly the same codec, size, frame rate and audio settings, for example clips cut from one recording. Then ffmpeg can join them without re-encoding, which keeps the picture identical and takes a fraction of a second. FFmpeg's documentation says the files must have the same streams (same codecs, same time base, and so on).

We measured both sides. Two identical 720p30 clips joined in 0.08 s into a valid 6.000 s file with 180 frames at 1280x720. A 1080p clip plus a 720p clip joined the same way produced a file whose header says 1920x1080 while the second half decodes at 1280x720 (90 frames at each size): players may show it wrongly. A 30 fps plus 24 fps pair gave a 4.8 s picture track with 6.02 s of audio and decode warnings about non-monotonic timestamps, so the picture and sound no longer line up. We did not test how players handle these files.

Rule of thumb from these tests (our suggestion): same settings, copy; anything else, re-encode, either in the browser tool or with the command above.

ffmpeg -f concat -safe 0 -i list.txt -c copy merged.mp4

How do I combine videos on an iPhone?

Not tested on a real iPhone: we had no iPhone, so we did not run the browser tool in Safari or an iPhone recording through it, and we did not run any iPhone app. The steps below are Apple's own for the free iMovie app (official, read October 25, 2026, not run by us).

1. Open or create a movie project in the iMovie app and scroll the timeline so the white playhead sits where the next clip should go. The clip is added before or after the existing clip nearest the playhead.
2. Tap "Add Media", then "Video", then a category, and tap the clip you want. If a Download button appears (iCloud Photos), tap it first.
3. To use only part of the clip, drag the yellow trim handles, then tap "Add to Project". Repeat for each clip.
4. Save the finished movie as a video file with Apple's "Share your movie as video file" steps (we did not read that page in full).

Plan: we will test the browser tool with a real iPhone recording (variable frame rate, HDR) when we have one; until then iPhone clips in the tool are an open risk. Our trimming guide for iPhone and the apps comparison are linked on this page.

How do other online video joiners compare?

We read two popular joiners on October 24, 2026 and did not run them. Clideo's page lists crop options, several resolutions and formats, and its FAQ asks whether videos with different aspect ratios can be combined, whether you can add transitions and background music, and whether clips can be rearranged. The Online Video Cutter merger says it can combine files with different input formats, aspect ratios and frame sizes, and offers a Transform step to remove black bars. Neither page's visible text said anything about frame rate or showed measured results for mixed clips (we read the FAQ questions and answers in the page text).

Where they are ahead of us: Clideo's FAQ mentions transitions, background music and rearranging clips, and the Online Video Cutter has a Transform step, none of which the Koldflux tool has. The Koldflux tool has no reorder control (clips go in the order of the list), no transitions, no per-clip trim and no audio mixing, but it needs no upload and no account. If you need transitions or a timeline, use an editor from the apps page below.

Limits and how we checked

• Tested by us on October 24, 2026: the Koldflux merge tool (https://koldflux.com/tools/video/merge) in headless Chromium, 22 runs on the tool as it was live before its update, then 20 runs on the updated code served as a local build (the same code that ships), each download probed with ffprobe 6.1.1. The commands and every output are in the test log linked in the sources.
• Our first test clips are synthetic: 3 second ffmpeg-generated clips (solid color, label, frame counter, sine tone).
• Footage test, October 25, 2026: 15 runs of the fixed tool (local dev build, headless Chromium, ffprobe and ffmpeg volumedetect on each download) with clips cut from "Big Buck Bunny" (Blender Foundation, CC BY 3.0, source: Wikimedia Commons, File:Big_Buck_Bunny_4K.webm, first 80 MB of the 480p VP9 file). It is animation, not phone recordings. We derived a silent clip next to sound clips (first, last, middle, all silent), a variable-frame-rate clip (40.6 fps and 59.87 fps averages made by dropping frames), a 90 degree rotation flag (result portrait 1080x1920), an HEVC 10-bit clip with HDR tags but SDR pixels (result 8-bit H.264; the tool does not tone-map), a 5.1 audio clip (result stereo) and a 99.1 MiB file (30 s 1080p, joined with a silent clip in 148 s on the dev build). All 15 finished with no errors; at 390 px width the page did not overflow. Audio and picture lengths differ by up to 0.08 s. We did not listen to the files.
• Still not tested: real phone variable frame rate and HDR (HLG, Dolby Vision) recordings, Safari, Firefox, phones, files above 100 MB, more than 3 clips, and lip-sync of long clips.
• Frame rates: the old hang (5 of 5 pairs) was in the tool before its update and we never found the cause; on the updated code all 6 mixed-rate runs finished (3 more runs in the earlier fix check also finished). The new 90 second no-progress stop was not exercised, so it is untested.
• Not covered: transitions, trimming, reordering, merging audio files, adding music, and subtitles (the tool has none of these). Other joiners (Clideo, Online Video Cutter) were read, not run, and the page text may have changed.
• Calculations are ours: the 31.6% picture share is 1920x1080 scaled to fit inside 1080x1920 (607.5 by 1080 pixels).
• Last checked October 25, 2026.