Calculate Video File Size: Formula and Examples

Calculate an estimated video file size by multiplying its total bitrate by its duration, then dividing by 8 to convert bits to bytes. Enter the video duration and average bitrate to estimate the file size in megabytes or gigabytes; the actual exported file can differ slightly because of audio, metadata, and encoding settings.

Formula

The basic formula is:

File size in bytes = bitrate in bits per second × duration in seconds ÷ 8

For the calculator, bitrate is entered in megabits per second (Mbps), so the unit conversion is built into the expressions:

• d = video duration in seconds.
• b = average total bitrate in megabits per second (Mbps).
• Estimated size in MB = d × b ÷ 8. This uses decimal megabytes: 1 MB = 1,000,000 bytes.
• Estimated size in GB = d × b ÷ 8,000. This uses decimal gigabytes: 1 GB = 1,000 MB.

The factor of 8 converts bits to bytes, and the megabit-to-megabyte conversion contributes a factor of 1,000,000. For instance, 1 Mbps sustained for one second is 1,000,000 bits, or 125,000 bytes, equal to 0.125 decimal MB.

Worked Example

With the calculator’s default values, the duration is 60 seconds and the average bitrate is 8 Mbps.

1. Multiply bitrate by duration: 8 Mbps × 60 seconds = 480 megabits.
2. Convert megabits to megabytes: 480 ÷ 8 = 60 MB.
3. Convert decimal megabytes to decimal gigabytes: 60 ÷ 1,000 = 0.060 GB.

The estimated file size is 60.00 MB, or 0.060 GB in decimal units. These are two representations of the same estimated size, not separate files.

What Bitrate and Duration Mean

Duration is the playback length, expressed in seconds. Convert minutes to seconds before applying the formula: a 12-minute video lasts 720 seconds. For a duration containing hours, minutes, and seconds, calculate total seconds as (hours × 3,600) + (minutes × 60) + seconds.

Bitrate is the amount of data encoded per second. A higher bitrate generally means more data and a larger file for the same duration, but does not guarantee a visibly better result. Resolution, frame rate, motion, encoder, codec, and quality settings all affect the bitrate needed to represent a particular video.

Use the average total bitrate when possible. A video file can contain separate video and audio streams, and its total bitrate is approximately the video bitrate plus the audio bitrate and any other included streams. If you have only the video bitrate, the estimate will omit audio and may come out low.

How to Use the Estimate

This calculation is useful for estimating storage or transfer requirements before encoding, and for comparing possible durations or bitrates. Enter duration in seconds and bitrate in Mbps; do not enter a resolution, frame rate, or file size in place of bitrate. If your encoding settings state a bitrate in kilobits per second, divide that number by 1,000 to convert it to Mbps. For example, 6,000 kbps is 6 Mbps.

To estimate a target bitrate from a desired file size, rearrange the same formula: bitrate in Mbps = file size in MB × 8 ÷ duration in seconds. This gives a rough total bitrate budget. If sound is included, reserve part of that budget for audio rather than assigning all of it to video.

For a fixed bitrate, doubling a video’s duration doubles its estimated size. Likewise, doubling the average bitrate doubles the estimate at the same duration. This makes the formula useful for quick planning, but it does not determine which bitrate will produce acceptable quality.

Why Actual File Sizes Differ

The formula assumes a steady average bitrate. Constant-bitrate encoding aims to keep the data rate relatively consistent, while variable-bitrate encoding allocates more data to complex scenes and less to simple ones. Variable-rate files can still have an average bitrate, but a nominal or maximum bitrate is not necessarily that average.

Actual size may also include audio, subtitles, captions, container indexes, and metadata. Some of these additions are small compared with a long, high-bitrate video, but their impact is more noticeable in short clips or files with multiple audio tracks. An encoder may also produce an average rate that differs from its target.

File-size units cause another common mismatch. This calculator uses decimal units: 1 MB is 1,000,000 bytes and 1 GB is 1,000,000,000 bytes. Some operating systems report sizes using binary units, where 1 MiB is 1,048,576 bytes and 1 GiB is 1,073,741,824 bytes, even when the display labels them MB or GB. That can make the displayed number look different without changing the underlying file.

Common Estimation Mistakes and Limits

• Mixing units: Do not multiply seconds by a bitrate in kbps as though it were Mbps. Convert first, or use a formula with matching units.
• Forgetting audio: Add audio and other stream bitrates when estimating the complete file, not just the video track.
• Using a peak rate: A maximum bitrate can overstate a variable-bitrate file’s size; use average bitrate for a closer estimate.
• Rounding too early: Keep the full values during the calculation and round only the final displayed result.

The formula estimates encoded media data from bitrate and duration; it cannot predict quality, codec efficiency, exact container overhead, or the encoder’s final output. For a precise size, encode the video or inspect a completed file. If the resulting file size is known, its average total bitrate can be estimated by reversing the formula, provided duration and byte units are consistent.

Frequently Asked Questions

How do I calculate video file size from bitrate?
Multiply average bitrate in bits per second by duration in seconds, then divide by 8. To get decimal megabytes directly from Mbps, use: Mbps × seconds ÷ 8.

How many MB is a one-minute video at 10 Mbps?
At a steady average bitrate of 10 Mbps for 60 seconds, the estimate is 10 × 60 ÷ 8 = 75 decimal MB, before any additional file overhead.

Does resolution determine video file size?
No. Resolution affects how much data may be needed for a given visual result, but file size is determined by bitrate and duration. Two videos at the same resolution can have different sizes.

Should I include audio bitrate?
Yes, if you want to estimate the complete file and your stated bitrate covers video only. Add the audio bitrate to the video bitrate, using the same units, before calculating.

Why is my file smaller or larger than the estimate?
The actual average bitrate may differ from the entered value, especially with variable-bitrate encoding. Audio, subtitles, metadata, container overhead, and decimal-versus-binary unit displays can also explain the difference.

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in d | Duration | seconds | 60 | 0
in b | Average bitrate | Mbps | 8 | 0
out size | Estimated file size | MB | d * b / 8 | 2
out gb | Estimated file size | GB | d * b / 8000 | 3