How this instrument works
Video bitrate describes how many megabits of data are encoded for every second of footage, and file size is simply that rate multiplied by how long the video runs, converted from megabits to megabytes by dividing by eight. A higher bitrate packs more data, and usually more visible detail, into every second, at the direct cost of a larger file for the same running time.
Bitrate is the dial video encoders actually turn to trade quality against file size — resolution and frame rate matter too, but for a fixed resolution and frame rate, bitrate is the single number that determines both detail and size most directly. Streaming platforms typically publish recommended bitrate ranges for each resolution, since going far below that range produces visible compression artifacts, while going well above it makes the file larger with only marginal quality gains.
This formula estimates a constant-bitrate encode, where the same bitrate is targeted evenly across the whole video. Variable-bitrate encoding, common in modern streaming and delivery formats, spends more bits on complex, fast-moving scenes and fewer on static ones, so a variable-bitrate file's actual size will typically land close to, but not exactly at, what this formula predicts from its average bitrate.
- Enter Video bitrate (Mbps) — check your encoding settings, or use a platform's recommended bitrate for your target resolution.
- Enter Duration (seconds) — convert minutes or hours to seconds first, since an hour-long video is 3,600 seconds.
- Read File size (MB) — the estimated file size at that bitrate and duration.
- For a variable-bitrate encode, treat this as an estimate based on the average bitrate rather than an exact figure.
Worked example — a 1-hour video at 8 Mbps
Enter 8 for Video bitrate and 3600 for Duration — a one-hour video encoded at 8 Mbps, a common bitrate for 1080p streaming content. Multiplying gives 8 x 3600 = 28,800 megabits of total encoded data.
Dividing by 8 to convert to megabytes, File size (MB) reads 3600.0 MB, or roughly 3.5 GB, for the full hour. Doubling the bitrate to 16 Mbps for higher quality would double the file size to 7,200 MB for the exact same one-hour running time.
Questions
How big is a 1-hour video at 8 Mbps?
About 3,600 MB, or roughly 3.5 GB. Multiplying the 8 Mbps bitrate by 3,600 seconds gives 28,800 megabits, and dividing by 8 to convert to megabytes gives (8 x 3600) / 8 = 3,600 MB for the full hour.
What bitrate should I use for 1080p video?
Streaming platforms commonly recommend somewhere around 6-12 Mbps for standard-frame-rate 1080p video, with higher motion content or high-frame-rate footage often needing more to avoid visible compression artifacts. 8 Mbps sits comfortably in that typical range for general-purpose 1080p delivery.
Does this work for variable-bitrate encodes?
Approximately. This formula assumes a constant bitrate held evenly across the whole video, while variable-bitrate encoding shifts more data to complex, fast-moving scenes and less to static ones. Entering a variable-bitrate encode's average bitrate here gives a close estimate of the final file size, though the exact figure can differ by a modest margin.
How much does doubling the bitrate increase file size?
It exactly doubles it, for the same duration, since bitrate is a direct multiplier in the formula. An hour-long video grows from 3,600 MB at 8 Mbps to 7,200 MB at 16 Mbps — twice the data rate for the same running time always means twice the file.
How do I convert my video's duration to seconds?
Multiply minutes by 60, or hours by 3,600. A 90-minute movie is 90 x 60 = 5,400 seconds, and a 2-hour video is 2 x 3,600 = 7,200 seconds — enter that total seconds figure into Duration.