Guides · updated October 22, 2026

Target size MP3

The bitrate math behind a target file size, worked examples for podcasts and music, and the cases where the target is impossible.

Tested: 22 October 2026 · 10, 30 and 60-minute fixtures · measured output vs estimate

Short answer

Target size is bitrate math: target bytes × 8 ÷ duration. For a 60-minute file and a 50 MB target, that is about 111 kbps, so the tool uses 112 kbps CBR. If the target is too small for the duration, the lowest MP3 rate still produces a larger file; trim the audio or accept the larger size.

The formula

A CBR MP3 is a stream of bits at a fixed rate. The size is:

bytes = bitrate (bits/s) × duration (s) ÷ 8

To go the other way:

bitrate (bits/s) = target bytes × 8 ÷ duration (s)

The converter treats 1 MB as 1,024 × 1,024 bytes and snaps the result down to a standard MP3 rate. A decimal-MB target will land a little under the number you typed; the difference is about 5% at 50 MB.

Worked examples

Duration	Target	Computed bitrate	Rate used	Estimated output
10 min	10 MB	140 kbps	128 kbps	~9.2 MB
30 min	25 MB	117 kbps	112 kbps	~24.2 MB
60 min	50 MB	116 kbps	112 kbps	~48.4 MB
60 min	100 MB	233 kbps	224 kbps	~96.8 MB
120 min	50 MB	58 kbps	56 kbps	~48.4 MB
2 min	50 MB	3,495 kbps	320 kbps	~4.8 MB
120 min	10 MB	12 kbps	32 kbps	~28.8 MB

The last two rows are the edge cases: one target is above the MP3 ceiling, the other is below the floor.

When the target is impossible

MP3’s useful floor in this tool is 32 kbps. A two-hour file at 32 kbps is about:

32,000 × 7,200 ÷ 8 = 28,800,000 bytes ≈ 28.8 MB

If the target is 10 MB, no MP3 setting can reach it without cutting the audio. The page says the file will be larger than requested. The honest options are:

Trim the recording.
Accept the larger file.
Use a more efficient codec (not MP3) if the destination supports it.

When the target is above the ceiling

320 kbps is the maximum. A short file with a generous target will be smaller than the target. That is usually fine: the target was a maximum, not a quota. The tool does not pad the file to hit the number.

CBR, not VBR

The target mode uses constant bitrate. CBR makes the size predictable; VBR would spend more bits on complex passages and less on quiet ones, so the final size would depend on the content. If you need VBR, use ffmpeg or a DAW.

A practical workflow

Check the duration. The panel shows it before conversion.
Enter the target in MB.
Read the estimate. It tells you the bitrate and the approximate output.
If the bitrate is below 64 kbps, consider whether speech or music; 32–56 kbps is voice territory.
Convert and check the actual size. If it is over the hard limit, lower the target or trim.

The honest limit

Target size is an estimate, not a contract. Container overhead, frame padding and the Xing header move the final byte count by a small amount. For a hard upload limit, leave a margin: target 48 MB for a 50 MB limit.

FAQ

How does the target size calculation work?

It converts the target size to bits, divides by the duration in seconds, and divides by 1,000 to get kbps. The result is snapped down to a supported MP3 bitrate. A 60-minute file with a 50 MB target: 50 × 1,024 × 1,024 × 8 ÷ 3,600 ÷ 1,000 ≈ 116 kbps, snapped to 112 kbps.

Why is the output slightly different from the target?

MP3 frames have headers and padding, and the file includes a Xing header. The encoder also works in fixed frame sizes. The result is usually within a few percent of the estimate, not exact to the byte.

What if the target is too small?

At 32 kbps, a 2-hour file is still about 28.8 MB. If the target is 10 MB, the tool cannot reach it without trimming. The page says so instead of silently producing a file over the limit.

What if the target is larger than the maximum bitrate allows?

320 kbps is the MP3 ceiling. A 2-minute file with a 50 MB target would need about 3.3 Mbps, so the tool caps at 320 kbps and the file is much smaller than the target. That is fine for a maximum, not for a minimum.

Can I use target size for a batch?

The target-size control is single-file because it depends on the duration. The batch queue uses one fixed bitrate for every file. For per-file targets, convert the files individually.