How to Hit an Exact Video File Size (Target-Size Compression Guide)

Q: "How do I compress a video to an exact file size?"

"Use a target-size compressor: enter your desired size, and the tool calculates the video bitrate from your duration, then re-encodes to land within a few percent of the target. Our free tool does this in your browser."

Q: "What is the formula for video file size?"

"File size ≈ (video bitrate + audio bitrate) × duration ÷ 8. Rearranged: bitrate (kbps) = target size (MB) × 8192 ÷ duration (seconds). That is the math every target-size tool uses."

Q: "Does hitting a target size reduce quality?"

"It depends on the target. Shrinking a 500MB video to 480MB is invisible; squeezing it to 25MB will visibly soften detail. The tighter the target relative to the original, the more quality you trade."

Q: "Why is my compressed video slightly over the target size?"

"Container overhead, variable-bitrate encoding, and audio tracks add a small margin. Good tools undershoot slightly or run a second pass; expect results within 2–5% of the target."

October 14, 2026 · 5 min read · By the DollarWise team

How to Hit an Exact Video File Size (Target-Size Compression Guide)

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Email attachments cap at 25MB. A upload form rejects anything over 100MB. Your messaging app downscales anything chunky. And your video sits at 237MB, mocking you. Generic “compress my video” tools pick a quality level and hope — but when you need to land under a specific number, you need target-size compression: you name the size, the tool does the math, and the output lands on target. Here is how it works and how to do it free.

The bitrate math behind every target size

Video file size is not mysterious. It is almost entirely determined by two numbers: the bitrate (how much data per second) and the duration. The formula:

File size (bits) = (video bitrate + audio bitrate) × duration in seconds

Flip it around to find the bitrate for any target size:

Video bitrate (kbps) = (target MB × 8,192 ÷ duration seconds) − audio bitrate

A worked example: you have a 5-minute (300-second) video and need it under 100MB, with audio at 128kbps.

Total budget: 100 × 8,192 ÷ 300 = 2,730 kbps
Subtract audio: 2,730 − 128 = 2,602 kbps for video
Encode at ~2,600 kbps video + 128 kbps audio → file lands just under 100MB

That is the entire trick. Every target-size compressor is just doing this arithmetic and then running an encoder at the computed bitrate. Understanding it means you can sanity-check any tool’s output — and do it manually with ffmpeg if you ever need to.

Step by step: compress to your target size

Step 1: Know your exact limit. “Under 25MB” and “under 25.0MB” are the same; “about 50MB” is not a target. Check the actual cap of the email provider, form, or platform — common ones are 25MB (email attachments), 100MB, 256MB, and 2GB.

Step 2: Check your video’s duration. Right-click the file → Properties (Windows) or Get Info (Mac), or open it in any player. You need duration in seconds for the math.

Step 3: Use a target-size compressor. Our free video target-size compressor takes your file and your desired size, computes the bitrate automatically, and re-encodes right in your browser — nothing uploads anywhere, so private footage stays private.

Step 4: Pick sensible audio settings. For speech-heavy video, 96–128kbps AAC is plenty; for music, 128–192kbps. Audio is a small slice of the budget, but trimming it from 320kbps to 128kbps frees meaningful room for the video stream.

Step 5: Verify the result. Check the output file size before sending. Good tools land within 2–5% of the target. If you are cutting it close to a hard limit, aim 5% under to leave margin.

The quality trade-off (and how to minimize it)

Target-size compression always trades quality for size when the target is much smaller than the original. The relationship is not linear:

Shrinking by 10–30%: virtually invisible. Do this without worry.
Shrinking by 50%: slight softening in fast motion and fine detail; acceptable for casual sharing.
Shrinking by 70–90%: visibly blocky, especially in dark scenes and gradients. Watchable on a phone, rough on a big screen.

Ways to get more quality per megabyte:

Use H.265 (HEVC) instead of H.264 when the recipient’s device supports it — roughly 30–50% smaller at the same quality. For maximum compatibility, stick with H.264.
Lower the resolution before crushing the bitrate. A 720p video at a healthy bitrate looks better than a starved 4K encode at the same file size.
Use two-pass encoding if the tool offers it: the first pass analyzes the video, the second spends bits where they matter most.
Trim dead air first. Cutting 30 seconds off a video shrinks every target-size calculation proportionally — free quality.

Common target sizes and when you need them

Under 25MB — email attachments (Gmail, Outlook) and many contact forms. The most common target by far.
Under 16–100MB — messaging apps and social DMs, which vary wildly; 25MB is a safe default.
Under 100–256MB — portfolio submissions, job applications, and upload forms.
Under 2GB — platform upload caps (YouTube and similar); rarely needs aggressive compression.
Exact sizes — some archival or disc-authoring workflows want a file to fill a size rather than just fit under it; set the target a hair below the medium’s capacity.

When target-size compression is the wrong tool

Sometimes the smarter move is not compressing at all. If the recipient can accept a link, uploading the original to cloud storage and sharing a link preserves full quality with zero effort. If only a short clip matters, trimming to the essential 30 seconds beats compressing 10 minutes. And if the video is destined for a platform that re-encodes everything anyway (YouTube, social feeds), upload the highest quality you reasonably can and let their encoder do its job — pre-compressing just adds a generation of quality loss.

Frequently asked questions

How do I compress a video to an exact file size?

Use a target-size compressor: enter your desired size, and the tool derives the video bitrate from your video’s duration, then re-encodes to land within a few percent. Our free tool performs the whole process in your browser with no upload.

What is the formula for video file size?

File size ≈ (video bitrate + audio bitrate) × duration ÷ 8. Rearranged for targeting: bitrate (kbps) = target size (MB) × 8192 ÷ duration (seconds), minus your audio bitrate. That is the math every target-size tool applies.

Does hitting a target size reduce quality?

Only relative to how aggressive the target is. Cutting 20% off is invisible; cutting 90% will visibly soften the video. Use H.265 when compatible, consider dropping resolution before bitrate, and trim dead footage first to protect quality.

Why is my compressed video slightly over the target size?

Container overhead, variable-bitrate encoding decisions, and the audio track add a small margin. Quality tools compensate by undershooting slightly or running a second pass — expect results within about 2–5% of the target, and aim 5% under hard limits.

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Written by the DollarWise team

We turn confusing money topics into plain-English guides for beginners. Every article is written to be actionable on day one — no jargon, no hype, no sales pitch.