How to Calculate MP3 File Size From Bitrate and Duration

Will a week of podcast episodes fit on an old phone? How long will a download take on hotel Wi-Fi? Questions like these have a precise answer, and you do not need a calculator app. A single multiplication does the job, and once you know it you can estimate any audio file in your head.

The formula

File size in bytes equals bitrate multiplied by duration, divided by eight.

• Bitrate is measured in bits per second.
• Duration is measured in seconds.
• Dividing by eight turns bits into bytes.

Written out: size (MB) = kbps x seconds / 8 / 1000.

Walk through one example

Take a 15-minute recording at 128 kbps.

1. 15 minutes is 900 seconds.
2. 128 x 900 = 115,200 kilobits.
3. Divide by 8: 14,400 kilobytes.
4. That is about 14.4 MB.

The same recording at 192 kbps gives 192 x 900 / 8 = 21,600 KB, or 21.6 MB. At 320 kbps it is 320 x 900 / 8 = 36,000 KB, or 36 MB.

A handy table for 15 minutes

Bitrate	Approximate size
128 kbps	14.4 MB
192 kbps	21.6 MB
320 kbps	36 MB

Fifteen minutes is the longest video the converter accepts, which makes these three numbers a useful ceiling for any single file from this site.

Shortcut for mental math

Memorize cost per minute:

• 128 kbps is about 1 MB per minute (precisely 0.96).
• 192 kbps is about 1.4 MB per minute.
• 320 kbps is about 2.4 MB per minute.

A 40-minute lecture at 128 kbps is therefore roughly 38 MB. Multiply minutes by the figure and you are done.

Megabytes versus mebibytes

Storage devices and operating systems sometimes disagree on what a megabyte is. Manufacturers count 1 MB as 1,000,000 bytes, while some operating systems use 1,048,576. The difference is about five percent, which does not matter for planning but explains why a 14.4 MB file may show as 13.7 in a different window.

Planning storage

Once you can estimate one file, you can scale up:

1. Count total listening time in minutes.
2. Multiply by the per-minute figure for your bitrate.
3. Add ten percent for cover images and tags.

Example: 50 hours of audiobook-style speech at 128 kbps is 3,000 minutes, so roughly 2.9 GB. On a phone with 2 GB spare, you would pick a lower number of hours or accept a smaller bitrate for narration.

Estimating download time

Divide the file size in megabits by your connection speed. A 21.6 MB file is 172.8 megabits. On a 10 Mbps connection, that takes about 17 seconds in ideal conditions, and perhaps double in practice. If you convert on the converter, the file is deleted after one hour, so make sure your connection is stable enough to finish the download in that time. It always is, for files this small.

Why numbers can differ slightly

Real files add a little overhead for headers, tags and cover art. A tag with a large image can add several hundred kilobytes. Variable-bitrate files will not match the simple formula exactly. For planning, treat results as accurate to within a few percent.

Using the knowledge

When you choose a quality on the converter page, you now know what you are trading. If the goal is a speech archive, 128 kbps saves about 40 percent compared with 192 and almost 60 percent compared with 320. For the full discussion of what each bitrate sounds like, see the bitrate explainer in the blog.

Rights note

Only convert content whose rights you hold, or that is Creative Commons licensed or royalty-free and used according to its terms. The service accepts videos up to 15 minutes and 10 conversions per IP per hour. Read the terms of use for the details.