Estimate how big your MP4 will be from its bitrate and length — or reverse it to find the bitrate that fits a limit. Checks TikTok, Instagram, Discord, email, X, and YouTube. Free, no sign-up.
ClipSpeedAI exports small, platform-ready clips — captioned, correctly sized, no re-encoding headaches. Paste any video and get 9:16 shorts that drop straight into TikTok, Reels, Shorts, and X without a size-limit error.
Turn Videos Into Clips FreeVideo file size comes down to one simple idea: a video is a stream of data, and bitrate is how much data that stream carries every second. Multiply the bitrate by how many seconds the video runs and you have the total amount of data — the file size. This calculator adds your video bitrate and audio bitrate together, multiplies by the clip length, and converts the result from bits into megabytes and gigabytes. The exact formula is: file size in bytes = (video kbps + audio kbps) × 1000 ÷ 8 × total seconds. The ÷ 8 converts bits to bytes, and dividing by 1,048,576 turns bytes into megabytes.
Flip the tool into reverse mode and it solves the same equation backwards. Give it a target size and a duration, and it returns the total bitrate that will land you exactly at that size. This is the fastest way to answer questions like "what bitrate keeps my clip under 25 MB for Discord?" — you set the limit and the length, and the number to type into your export settings falls right out.
A common mistake is assuming 4K files are always huge and 720p files are always small. Resolution influences the bitrate you should use, but bitrate is what actually determines size. A 10-second 4K clip exported at 45,000 kbps is about 56 MB. A 10-minute 1080p clip at 8,000 kbps is about 600 MB — more than ten times larger, at a quarter of the resolution. Duration and bitrate are the two levers that matter. Resolution, frame rate, and codec only matter because they change what bitrate you need to look good.
When a file won't fit a platform's cap, work through these levers in order of impact. First, lower the bitrate — it is the most direct control and dropping it in half roughly halves the file. Second, trim the clip: a shorter video is a smaller video, and most short-form platforms reward tighter edits anyway. Third, drop the resolution or frame rate — 1080p30 instead of 4K60 dramatically cuts the bitrate you need. Fourth, switch to a more efficient codec: H.265/HEVC and AV1 achieve the same visual quality as H.264 at a noticeably lower bitrate. Use this calculator's reverse mode to find the precise bitrate ceiling for your target, then apply it in your editor's export dialog.
Every platform enforces both a maximum file size and a maximum length, and either one can block your upload. TikTok accepts roughly 500 MB in the mobile app and up to about 4 GB through the web uploader, with clips up to 60 minutes. Instagram Reels top out around 1 GB and 3 minutes. Discord caps free uploads at 25 MB and Nitro uploads at 500 MB. Gmail attaches files up to 25 MB inline before pushing them to Google Drive. X accepts up to 512 MB and 2 minutes 20 seconds for standard accounts. YouTube is effectively unlimited for creators — 256 GB or 12 hours, whichever comes first. The tool checks your estimated size against all of these at once and flags whether the blocker is size or length.
Video file size is bitrate multiplied by duration. Add your video bitrate and audio bitrate (both in kilobits per second), multiply by the number of seconds, then divide by 8 to convert bits to bytes: file size in bytes = (video kbps + audio kbps) × 1000 ÷ 8 × seconds. Divide by 1,048,576 for megabytes. For example, an 8,000 kbps video with 128 kbps audio running 60 seconds is about 58 MB. The container and codec add a small overhead of a few percent, so treat the result as a close estimate.
File size is driven almost entirely by bitrate and length, not by resolution alone. A high bitrate stores more data per second, so a short 4K clip at 45,000 kbps can be far bigger than a long 1080p clip at 8,000 kbps. Long duration, 60fps instead of 30fps, high-motion footage, an old or inefficient codec, and an unnecessarily high export bitrate all inflate the file. If a file feels too big for its length, its bitrate is almost always the reason.
Lower the bitrate first — it has the biggest impact. Dropping from 12,000 to 6,000 kbps roughly halves the file. Beyond that: trim the clip shorter, drop the resolution (1080p instead of 4K), export at 30fps instead of 60, and use an efficient codec like H.264 or H.265/HEVC. Use this calculator in reverse mode: enter your target size and duration to get the exact bitrate that will fit, then set that bitrate in your export settings.
Discord's free upload limit is 25 MB per file. Nitro subscribers can upload up to 500 MB per file (Nitro Basic is 50 MB). To fit a video under the 25 MB free cap, use this calculator's reverse mode: enter 25 MB and your clip length to get the maximum bitrate. For a 60-second clip that works out to roughly 3,500 kbps total; a 30-second clip can go up to about 7,000 kbps.
It depends on length, because 25 MB is a fixed budget of data spread across the clip. The formula is total kbps = 25 × 1024 × 1024 × 8 ÷ seconds ÷ 1000. That gives about 7,000 kbps for a 30-second clip, 3,500 kbps for 60 seconds, 1,750 kbps for 2 minutes, and roughly 1,150 kbps for 3 minutes. Subtract about 128 kbps for audio to get your video bitrate. The 25 MB target matters because it is both the Gmail attachment limit and the free Discord upload limit.
| Platform | Max file size | Max length | Notes |
|---|---|---|---|
| TikTok (in-app) | ~500 MB | 60 min | Mobile upload, 9:16 .mp4/.mov |
| TikTok (web upload) | ~4 GB | 60 min | Desktop uploader accepts larger files |
| Instagram Reels | ~1 GB | 3 min | 90s standard, up to 3 min |
| Discord (free) | 25 MB | — | Per-file limit on the free tier |
| Discord (Nitro) | 500 MB | — | Nitro Basic is 50 MB |
| Gmail / email | 25 MB | — | Larger files auto-upload to Google Drive |
| X (Twitter) | 512 MB | 2:20 | Standard accounts, .mp4/.mov |
| YouTube | 256 GB | 12 hr | Whichever limit is reached first |
Instead of exporting, checking the size, and re-encoding when it's too big, let ClipSpeedAI do it once. It cuts your long video into scored, captioned 9:16 shorts sized correctly for TikTok, Reels, Shorts, and X — and can schedule them for you. No size-limit errors, no manual re-exports.
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