Video Size Estimator
Estimate the output file size of an H.264 encode from resolution, frame rate, duration, audio bitrate, and CRF quality. Uses empirical bits-per-pixel modeling for libx264. Pure math — runs instantly in your browser.
Lower = larger and higher quality. Balanced is CRF 23.
Estimated output size
- File size
- 373.88 MBestimate ±20%
- Video bitrate
- 4,977 kbps
- Audio bitrate
- 128 kbps
- Total bitrate
- 5,105 kbps
- Bits per pixel
- 0.08
- Frame area
- 2,073,600 px²
About Video Size Estimator
Video Size Estimator is built for video editing and conversion jobs that fit cleanly into a browser tab. Estimate the output file size of an H.264 encode from resolution, frame rate, duration, audio bitrate, and CRF quality. Uses empirical bits-per-pixel modeling for libx264. Pure math — runs instantly in your browser. The processing runs in the page itself, which is why the controls update instantly when you change settings and why a freshly loaded page is ready to do real work the moment it becomes interactive.
The engine behind the page is FFmpeg compiled to WebAssembly. It reads your file in-memory and writes the result back into the browser. Supported inputs include MP4, WebM, MOV, MKV, AVI, FLV, and OGV. For 500 MB and below the work usually completes in seconds; larger files mostly depend on how much spare RAM your device has.
Most people land on Video Size Estimator via a search at the moment they actually need the tool. That shapes the design: the page is a single screen with the input on one side, the controls in the middle, and the result on the other, so a first-time visitor can complete the job without reading documentation.
Because everything runs in the page, the tool scales the same way for one user or a million — there is no per-user backend cost. The page is static, the engine is the same JavaScript bundle for every visitor, and the work happens on the visitor's own device. That keeps the tool free and keeps it fast on the first interaction.
Constraints worth knowing about: inputs are capped at 500 MB to keep memory usage in a sensible range, one input is processed per run, and the tool must be loaded over HTTPS for the in-browser engine to work. These are properties of the architecture.
If your task needs more than one step, chain Video Size Estimator with Video Compressor, Video Bitrate Calculator, and Video File Size Calculator. Each tool produces output that is a clean input to the next, so multi-step workflows are just a matter of opening the next tool in a new tab and continuing.
Video Size Estimator is shaped around the recurring needs of two audiences: educators editing lecture clips, who use it as a quick utility between bigger tools, and teams compressing demo recordings, who use it as their primary way of getting the job done. Both groups get the same defaults and the same speed.
Once the engine finishes, `{name}-edited.{ext}` is offered as an immediate download. There is no preview gate, no email-wall, and no "register to download" intermediary — the file is yours the moment it is ready.
Video Size Estimator is honest about scope: it handles a single, well-defined video editing and conversion step. Specialist edge-case work — uncommon formats, very large inputs, or pipelines that need scripting — is what dedicated desktop apps are for. This page handles the common case quickly.
Video Size Estimator is structured around the idea that a useful tool should be its own page. Open the page, do the work, close the tab — the page is the entire product. There is no onboarding flow because there is nothing to onboard into.
Pro tip: Video Size Estimator works just as well in a private/incognito window as in a normal one, which is occasionally useful when you want zero browser-history footprint of the job. Another tip: if the tool ever feels slow, it is almost always because the browser tab is competing for CPU with another tab — pausing or closing the heavy ones gives the engine room to work.
Video Size Estimator fits the gap where opening a desktop app feels heavy and writing a script feels overkill. The page handles the common video editing and conversion task with sensible defaults so a single visit usually completes the job; for highly specialised work, a dedicated desktop application can offer more knobs to turn.
For most failure modes, refreshing the page and re-running the job is enough — the engine has no persistent state to corrupt. If the same input fails twice in a row, the input itself is most likely the problem (a truncated file, an unexpected variant of the format, or a stream the engine does not recognise).
If Video Size Estimator solved your problem, sharing the page link with someone who has the same problem is the most useful thing you can do. The catalog grows mostly through word of mouth; visitors arriving through a recommendation tend to be the ones the tool serves best.
How it works
- 1Land on the Video Size Estimator page. The tool is ready to use the moment the page renders.
- 2Drop a MP4, WebM, MOV, MKV, AVI, FLV, and OGV file onto the upload area, or click to pick one from your device.
- 3Tweak the controls if the defaults are not quite right for your input. The options are kept short and labelled in plain language.
- 4Click to start the job. The engine (FFmpeg compiled to WebAssembly) processes the input in the page; you can watch the progress indicator until it completes.
- 5Download the result as `{name}-edited.{ext}`. The file is generated in your browser and saved through your normal download flow.
- 6Re-run with different settings as often as you want. Each run produces a fresh output and the original file on disk is never modified.
Common use cases
- Trim the silent intro from a screen recording before sharing it using Video Size Estimator.
- Cut a long meeting recording down to the relevant five minutes.
- Add a quick caption overlay before posting to social.
- Slow down a section of footage to highlight a detail.
- Rotate a portrait phone clip into landscape for embedding on a site.
- Compress a demo video small enough to attach to an email.
- Strip the audio track from a screencast for a silent loop.
- Stitch several short clips into a single uploadable video.
Frequently asked questions
How does the estimator work?
Pure JavaScript math: file size = (width × height × fps × bits-per-pixel × duration + audio bits) / 8. The bits-per-pixel value is an empirical constant for each H.264 CRF setting derived from real-world encoding tests.
Why use this tool?
Predict the size of a planned encode before actually running it. Useful when choosing between resolution and quality trade-offs, planning storage, or budgeting upload time.
Why is this not slower in browser?
Unlike video transcoding, this is pure arithmetic — there is no FFmpeg WebAssembly involved. Results are instant and identical to any server-side tool.
How accurate is the estimate?
Within ±20% for most content. Action footage with lots of motion encodes larger than the estimate; talking-head footage encodes smaller. The estimate assumes "average" content complexity.
Why is this better than the Video File Size Calculator?
The File Size Calculator needs you to know the bitrate. This Estimator works directly from human-friendly inputs (resolution, fps, quality preset) so you can reason about the encode without knowing the exact bitrate.
What is "bits per pixel" (BPP)?
BPP measures how many encoded bits each pixel takes on average. Lower CRF (higher quality) = higher BPP = bigger files. Real BPP varies with scene complexity, but for libx264 the values we use are reliable averages.
Does this account for audio?
Yes — pick your audio bitrate and it is added to the total. Audio is usually a small fraction of the total file size.
Is anything logged?
No. The estimator runs entirely in your browser tab. Nothing is transmitted, stored, or logged.
Can I use Video Size Estimator on iOS or Android?
Video Size Estimator runs in any modern mobile browser — Safari, Chrome, Firefox and the in-app browsers in most messaging apps all support the underlying APIs. Performance depends on the device: a recent phone handles typical inputs nearly as fast as a laptop, while older devices may take a few seconds longer near the 500 MB ceiling. The interface lays out cleanly on small screens, so you do not need to pinch-zoom to see the controls.
Does Video Size Estimator work in Safari, Firefox, Chrome and Edge?
Video Size Estimator works in any modern browser released in the last few years — Chrome, Edge, Firefox, Safari, Brave, Arc and the major Chromium derivatives are all supported. The underlying engine relies on widely-supported web APIs, so there is nothing exotic to install. If you are on a very old browser version and the tool fails to load, updating to the latest release of your preferred browser is the only fix needed.
Is Video Size Estimator really free?
Video Size Estimator is free to use. The processing runs in your browser, which keeps the per-user cost low enough that the tool can be offered openly. The download is the same file the engine produced — you can use it for as many runs as you need.
Does Video Size Estimator ask for any browser permissions?
Video Size Estimator only needs the standard web platform — file picker access for the inputs you choose to load, and optionally clipboard access if you copy the result rather than downloading it. There is no microphone, camera, geolocation or background-permission request, because none of those are needed for the work the tool does. Some browsers prompt the first time WebAssembly is compiled; that is a normal one-time event, not specific to Favtoo.
Can Video Size Estimator run inside a corporate firewall?
Video Size Estimator is a static page running an open-source engine in your browser, so a typical corporate firewall does not get in the way as long as it allows JavaScript and WebAssembly to load from Favtoo. For teams that need to host it themselves on an internal network, the underlying engine (FFmpeg compiled to WebAssembly) is open-source and can be packaged into a private build with the same behaviour. Reach out via the Contact page if that is something you are exploring.
Can I use Video Size Estimator offline?
Once the page is loaded, Video Size Estimator can complete jobs without an active internet connection — the engine is bundled with the page, so there is no per-job network call. The initial page load does require a connection (to fetch the static assets), but after that you can disconnect entirely and the tool will still work. This is a side-effect of the local-first architecture, not a deliberate "offline mode" feature.
What should I do if Video Size Estimator fails on my file?
Failures usually fall into one of three buckets: the input is in an unsupported format, the input is over the size cap, or the input is structurally malformed (a truncated download, a partial export, or a stream the engine does not recognise). The first two are easy to confirm — check that your file is one of MP4, WebM, MOV, MKV, AVI, FLV, and OGV and that it is below 500 MB. For the third, opening the file in its native viewer first is the fastest way to confirm the source is intact.
How accurate is Video Size Estimator?
Video Size Estimator is built on FFmpeg compiled to WebAssembly, which is the same class of engine used by professional video editing and conversion pipelines. For deterministic operations, the output is byte-identical to what an equivalent CLI run would produce; for operations involving a codec or a model, the result is well within the range of what comparable tools generate. If you have a specific reference output you need to match, run a small test job first to confirm the configuration produces what you expect.