Bitrate File Size Calculator: Formula & How to Use

This calculator works out the file size of an audio or video file from its bitrate and duration. You enter the bitrate (in kilobits per second, kbps) and the length of the media (in minutes or hours), and it returns the approximate file size in megabytes and gigabytes. It is a straightforward, useful tool for planning recordings, estimating storage needs, or checking whether a file will fit on a disc or memory card.

Formula

The core formula is:

File size (in bytes) = Bitrate (bits per second) × Duration (seconds)

Then convert bytes to MB or GB. In this calculator, the bitrate is entered in kbps (kilobits per second) and duration in minutes. So the full expression is:

File size (MB) = (Bitrate (kbps) × Duration (min) × 60) / 8 / 1024

Where:

• Bitrate – the amount of data used per second of media, in kilobits per second (kbps). 1 kbps = 1,000 bits per second.
• Duration – the length of the media in minutes (converted to seconds by multiplying by 60).
• 8 – converts bits to bytes (8 bits = 1 byte).
• 1024 – converts bytes to kilobytes (KB) and then to megabytes (MB) if you divide again by 1024. This is the binary definition used by most operating systems.

Worked Example

Using the default values: bitrate = 128 kbps, duration = 60 minutes.

1. Convert duration to seconds: 60 min × 60 = 3,600 s
2. Multiply by bitrate: 128 kbps × 3,600 s = 460,800 kb
3. Convert kilobits to bits: 460,800 kb × 1,000 = 460,800,000 bits (but we can keep it in kb and divide by 8 to get kilobytes)
4. Convert to kilobytes: 460,800 kb / 8 = 57,600 KB
5. Convert to megabytes: 57,600 KB / 1024 = 56.25 MB

So a 128 kbps MP3 of 60 minutes is about 56.25 MB. The calculator shows 56.25 MB and 0.05 GB, which matches.

Why Bitrate Matters

Bitrate is the single biggest factor in file size for a given duration. Higher bitrate means better quality (more data per second) but a larger file. For example:

• 128 kbps MP3 – common for spoken word or low-quality music.
• 256 kbps MP3 – decent music quality, about double the size of 128 kbps.
• 320 kbps MP3 – highest MP3 quality, about 2.5× the size of 128 kbps.
• Video: 1,000–8,000 kbps (1–8 Mbps) for standard HD, much higher for 4K.

This calculator works for any digital media where bitrate is constant (CBR). Many codecs use variable bitrate (VBR), where the bitrate changes over time – the result is then an estimate based on the average bitrate.

How to Use the Calculator

Enter the bitrate and duration you expect. If you are encoding a file, the bitrate is often set in the encoder (e.g., LAME for MP3, x264 for video). If you have an existing file, you can find its average bitrate in a media player or file properties. Then the calculator gives you the approximate size. If you have a target file size and want the bitrate, you can rearrange the formula: Bitrate (kbps) = (File size in MB × 8 × 1024) / (Duration in min × 60).

Common Mistakes and Misconceptions

• Mixing up bits and bytes: Bitrate is in bits, file size in bytes. Always divide by 8.
• Decimal vs. binary units: Hard drive manufacturers use decimal (1 MB = 1,000,000 bytes), but operating systems often show binary (1 MB = 1,048,576 bytes). The difference is about 5%, so a file might show as slightly smaller in Explorer than the calculator says.
• VBR vs. CBR: This calculator assumes constant bitrate. For variable bitrate, the actual size can differ by 10–20% depending on the content.
• Container overhead: The formula gives the raw media size. Real files (MP4, MKV, etc.) have container headers, metadata, and possibly multiple audio tracks, adding a few KB to a few MB – negligible for large files.

When to Use This Calculator

• Recording audio: If you record a podcast at 128 kbps for 45 minutes, you can estimate the file size before uploading.
• Video editing: Knowing the output size helps plan storage for long recordings.
• Streaming or uploading: Estimate upload time or data usage.
• Fitting media on a disc: A 700 MB CD can hold about 12 hours of 128 kbps audio, or about 74 minutes of 1,500 kbps video.

Limitations of the Formula

The formula is exact for constant bitrate streams with no container overhead. In practice, video codecs like H.264 or H.265 often use variable bitrate, and the actual size can vary. Also, audio codecs like AAC or Opus may have a lower bitrate for the same perceived quality compared to MP3. The calculator gives a solid estimate, but always check the final file size after encoding if precision is critical.

Frequently Asked Questions

How do I find the bitrate of an existing file?

Open the file in a media player (like VLC) and look at the codec information, or use a tool like MediaInfo. It will show the average bitrate in kbps for audio and video streams. You can then plug that into the calculator with the duration.

What is a good bitrate for audio?

For spoken word, 64–128 kbps is usually enough. For music, 192–320 kbps is common for MP3. Lossless formats like FLAC have bitrates around 800–1,200 kbps, but they are not covered by this simple formula because they are not constant bitrate.

Does this work for video?

Yes, if you know the total bitrate (video + audio). For example, a 1080p video at 8,000 kbps (8 Mbps) for 2 hours (120 min) would be about 8,000 × 120 × 60 / 8 / 1024 / 1024 = 6.87 GB. The calculator gives the same result.

Why is the file size from the calculator different from what my computer shows?

Two reasons: your computer may use binary MB (1,048,576 bytes) while the calculator uses the same, but some programs use decimal MB (1,000,000 bytes). Also, the file you have may be VBR, so the actual average bitrate is different from what you entered.

Can I use this to calculate how long a file can be on a device with limited storage?

Yes. Divide the free space (in MB) by the bitrate (in kbps) and then convert: Duration (min) = (Free space in MB × 8 × 1024) / (Bitrate in kbps × 60). For example, 1 GB (1024 MB) at 128 kbps gives about 1024 × 8 × 1024 / (128 × 60) = 1092 minutes (18.2 hours).

Is higher bitrate always better?

Not necessarily. Beyond a certain point, the human ear or eye cannot perceive the extra detail. For example, 320 kbps MP3 is indistinguishable from 128 kbps for most people on typical speakers. Higher bitrate mainly wastes storage and bandwidth.

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in br | Bitrate | kbps | 128 | 0
in t  | Duration | min | 60 | 0
out fs_mb | File size (MB) | MB | (br * t * 60) / 8 / 1024 | 2
out fs_gb | File size (GB) | GB | (br * t * 60) / 8 / 1024 / 1024 | 2