Video File Size / Bitrate

Estimating File Size From Bitrate and Duration

A video’s file size is its bitrate multiplied by its duration, converted from bits to bytes. Enter a bitrate and duration to estimate file size, or a target size and duration to find the required bitrate.

The Formula

$$\text{File Size (bytes)} = \frac{\text{Bitrate (Mbps)} \times 1,000,000 \times \text{Duration (s)}}{8}$$

Worked Example

1. $\frac{10 \times 1,000,000 \times 300}{8} = 375,000,000 \text{ bytes} \approx 358 \text{ MB}$

Key Factors to Consider

• Video codec (compression format) has a major effect on file size for the same visual quality. A more modern, efficient codec (like H.265/HEVC) can deliver comparable visual quality to an older codec (like H.264) at a meaningfully lower bitrate — the bitrate figure entered here should reflect the specific codec actually being used, since “good quality” bitrate varies significantly by codec.
• Resolution and frame rate both directly influence how much bitrate is needed for a given visual quality. Higher resolution (like 4K vs 1080p) and higher frame rate (like 60fps vs 30fps) both generally require more bitrate to maintain the same perceived sharpness and smoothness — this is exactly why streaming platforms recommend different bitrates for different resolution/frame-rate combinations.
• Variable bitrate (VBR) encoding, common in modern video, means the actual bitrate fluctuates throughout a video rather than staying perfectly constant. This calculator computes based on an AVERAGE bitrate over the whole duration — a scene with lots of motion or detail typically uses more instantaneous bitrate than a static, simple scene, even within the same overall file.
• Audio takes up file size too, separate from the video bitrate itself. A complete video file’s total size includes both the video stream’s bitrate and a separate audio stream’s own bitrate — for a precise total file size estimate, both should be accounted for, though video typically dominates the total for most consumer video content.

Common Mistakes

• Mixing up megabits and megabytes. Bitrate is conventionally given in megabits per second (Mbps), while file size is conventionally shown in megabytes (MB) — since a byte is 8 bits, skipping that conversion overstates the estimated file size by a factor of 8.
• Forgetting to include audio bitrate in a total file size estimate. Entering only the video stream’s bitrate slightly understates the real exported file size, since the audio track adds its own bitrate on top.
• Treating a variable-bitrate (VBR) export’s actual result as if it should match a constant-bitrate estimate exactly. This calculator’s formula assumes a flat, constant bitrate — a real VBR export’s file size is an approximation around that number, not an exact match, since the encoder allocates more bits to complex scenes and fewer to simple ones.

Useful to Know

• Planning to upload or stream this file rather than just export it? Bandwidth (Download Time) Calculator estimates how long a file that size takes to transfer over a given connection speed.
• Curious how a recurring stream (not just one export) adds up in data usage over time? Streaming Data Usage Calculator covers that separately.