File Size Estimator

Estimate output file size from CRF, resolution, codec and duration before you encode.

Resolution

Codec

Duration (seconds)

CRF Value (23) — lower = higher quality & larger file

18 (high quality)23 (default)36 (low quality)

—

⚠ Calibrated against real encodes at CRF 18–36 (FFmpeg 8.1, i9-14900KF) — the commonly quoted 2^((23−CRF)/6) rule is off by 18–33% at the ends of that range. CRF is still content-adaptive, so your file size depends heavily on motion, noise and gradients; see how much the source material changes the result.

Popular uses

Estimate file size before postingCompare CRF and bitrate optionsPlan storage needs

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What this tool does

H.264 / H.265 coefficients
Try several CRF / resolution combos
No actual encode required

How to use

1

Enter duration

Seconds or HH:MM:SS.
2

Pick codec / resolution / CRF

Use a preset or custom values.
3

Read the estimate

Approximate MB output.

What each setting means

Resolution

Vertical pixels.

CRF

Quality target — lower means bigger files.

Codec

H.265 ≈ 1.5× more efficient than H.264.

Recommended settings

YouTube CRF 23 1080p

~10min ≈ 100–500MB

Varies with source complexity.

Size-limited sharing

Target-MB mode with the actual cap

A CRF value alone cannot fix the final file size.

Common pitfalls

Symptom: Estimate doesn't match reality

Cause: CRF varies with scene complexity.

Fix: Treat ±30% as expected error.

Equivalent FFmpeg commands

Reference commands you can run on the desktop FFmpeg CLI.

Historical log: 2026-09-11, 1 successful example executions and 0 excluded (desktop FFmpeg 8.1, fixtures). This log does not verify later edits, browser controls or visual quality.

Encode and check

ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset fast output.mp4

Then ls -lh output.mp4

Browser support & limits

Estimates only (±30%)
AV1 / VP9 less accurate

Privacy

This tool only does math in your browser. Inputs stay on your device — nothing is uploaded. Read the privacy policy →

Frequently asked questions

Why not exact?

CRF is a quality target, not a bitrate. Real bitrate depends on the content.

H.264 or H.265?

Same quality, H.265 is ~30–50% smaller. Use H.264 for max compatibility.