Reduce MP4 file size
There are four ways to make an MP4 smaller, and they save very different amounts. Trimming saves in proportion to what you cut. Lowering the resolution cuts the video roughly in half per step. Aiming at a target size lets Wave choose the bitrate for you. Dropping the audio saves the least, usually a few percent. All four are in Wave's Options panel once you drop a file, they run in your browser, and nothing gets uploaded.
What actually changes
What a file size buys the picture
- Video
- Audio 96 kbps
- 5% kept in reserve
Pick a size and a length to see the resolution Wave would choose.
Size is bitrate multiplied by duration, so every method works on one of those two numbers. Cutting a 3-minute clip down to the 45 seconds you need removes three quarters of the file without touching the picture, and Wave's fast cut copies the kept part without re-encoding, so it takes seconds. A fast cut starts on the nearest keyframe (a complete picture the video stores every few seconds) at or before your start time, so it can begin slightly early; tick Exact cut for a frame-accurate start, which re-encodes the clip.
Lowering the resolution cuts the pixel count: 1080p to 720p keeps 44% of the pixels, and 720p to 480p keeps 44% of those again. Wave re-encodes at a bitrate matched to the new size - for 30 fps video its standard H.264 rates are about 5 Mbps at 1080p, 1.9 Mbps at 720p and 1 Mbps at 480p - so each step down shrinks the video to roughly 40-50% of the size before.
A target size is the most direct method when the destination has a hard limit. Pick 16 MB for WhatsApp, 18 MB for email or 20 MB for Discord, or type any number. Wave divides the size by the length to get a bitrate, keeps 5% back as a safety margin, gives the audio 64-128 kbps and the rest to the video, and steps the resolution down when the bitrate is too thin for the original size. The formula, if you want to check it: target megabytes x 8,000 / seconds = total kbps; subtract the audio rate to get the video rate. A 3-minute clip aimed at 20 MB gets about 840 kbps in all, enough for clean 480p, which is the size Wave would pick.
Dropping the audio saves the least. A typical AAC track runs at 128-192 kbps, so in a phone video at 10-20 Mbps it's 1-2% of the file. It matters more on small, already-compressed files: in a screen recording at 500 kbps, a 128 kbps audio track is a fifth of the size. Tick Remove the audio track under Audio; on its own it's an instant copy with no re-encode.
Three steps.
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Step 1
Drop the MP4 onto Wave above and open Options.
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Step 2
Set any combination: a Trim range, a lower Resolution, a File size target, or Remove the audio track. The plan and size estimate update as you change them.
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Step 3
Select Convert to MP4. Trims and audio removal on their own are instant copies; a size target or a resolution change re-encodes the video.
Answers, specifically.
Which method loses the least quality?
Trimming and removing the audio lose none: the video you keep is copied byte for byte. Lowering the resolution and aiming at a size both re-encode, which always costs some detail; how much depends on how far you push it.
Is it better to lower the resolution or the bitrate?
At small sizes, the resolution. 720p at 1.5 Mbps looks better than 1080p at 1.5 Mbps, because the encoder spreads the same bits over fewer pixels. Wave applies this rule itself when you set a size, and the plan tells you which resolution it chose.
Why is my converted file bigger than the original?
When Wave re-encodes a heavily compressed source - an HEVC phone clip or a low-bitrate download - without a size target, it uses its standard H.264 bitrate for that resolution, which can be higher than the source's. Set a File size target or a lower resolution and the output will be smaller.
Does re-encoding lose quality even at high bitrates?
A small amount, technically. Every H.264 encode is lossy, so encoding a file that was already compressed adds a little more loss. At bitrates close to the source's, it stays well below what you can see; it only becomes visible after several rounds of re-encoding, as in some editing workflows.
The file is 4 GB and needs to be 50 MB. Realistic?
Only if the source is a high-bitrate camera recording (a drone or a camera at 100 Mbps) or a raw format. A 5-minute clip at 100 Mbps is about 4 GB, and at 50 MB it gets about 1.3 Mbps - fine at 720p. A 4 GB file that was already efficiently compressed, like a 2-hour film at 4.5 Mbps, can't shrink 80 times without falling apart; at that size, cut it into the part you need.
Can Wave make it smaller without re-encoding?
Only by trimming or dropping the audio. A plain conversion to MP4 rewraps the existing video, so the size stays almost the same; to actually shrink the picture data, it has to be re-encoded at a lower bitrate.