Three numbers decide almost everything about how a video looks and how large it is: its resolution, its aspect ratio and its bitrate. They are easy to confuse and surprisingly easy to get wrong. People upload 4K footage that a platform immediately shrinks, squeeze a widescreen clip into a vertical frame and end up with black bars, or export a ten-minute video at a bitrate that produces a file too large to send.
This guide explains each number in plain terms, shows the arithmetic behind common problems, and points to two calculators that do the sums for you in the browser: the Video Resolution Calculator and the Aspect Ratio Calculator (in its video mode). Both run locally, so nothing you enter or load is uploaded.
Resolution: counting pixels
Resolution is the frame's width and height in pixels. Names such as 1080p refer to the height, while 4K refers loosely to a width near 4,000. What matters practically is the total number of pixels, because every extra pixel is data an encoder must describe.
| Name | Size | Pixels | Versus 1080p |
|---|---|---|---|
| 720p HD | 1280×720 | 921,600 | 0.44× |
| 1080p Full HD | 1920×1080 | 2,073,600 | 1× |
| 1440p QHD | 2560×1440 | 3,686,400 | 1.8× |
| 2160p 4K UHD | 3840×2160 | 8,294,400 | 4× |
| 4320p 8K UHD | 7680×4320 | 33,177,600 | 16× |
A 4K frame contains exactly four 1080p frames' worth of pixels, and 8K contains sixteen. That is why 4K files are so much heavier and why an ordinary laptop struggles to edit them. The resolution calculator shows this comparison for any size you enter, including the megapixel count.
One more detail: the label 4K covers two sizes. Consumer televisions and streaming use 3840×2160. Cinema projection uses 4096×2160. They share a name but not a shape.
Aspect ratio: the shape of the frame
Aspect ratio is width divided by height, written as two small numbers. You get it by dividing both sides by their greatest common divisor: 1920 and 1080 share a divisor of 120, giving 16:9. Not every size divides so neatly. A 1366×768 laptop screen works out to 1.7786, a hair from 16:9's 1.7778, and is treated as 16:9 for practical purposes.
| Ratio | Common size | Typical use |
|---|---|---|
| 16:9 | 1920×1080 | YouTube, TV, laptops |
| 9:16 | 1080×1920 | Reels, Shorts, TikTok |
| 1:1 | 1080×1080 | Square feed posts |
| 4:5 | 1080×1350 | Portrait feed posts |
| 4:3 | 1440×1080 | Classic TV, tablets |
| 2.39:1 | 2048×858 and similar | Widescreen cinema |
Platforms revise their recommendations regularly, so treat this list as a starting point and check current guidance before publishing. The direction of the trend is clear, though: vertical and near-square shapes take up more of a phone screen, and most viewing happens on phones.
What it costs to change shape
Every change of shape forces a trade-off. Take a 1920×1080 clip and a 1080×1920 vertical canvas:
- Fit. Scale the video until its width matches the canvas. It becomes 1080×608, leaving bars of about 656 pixels above and below. Nothing is lost, but only a third of the canvas shows picture.
- Fill. Scale until the height matches and crop the sides. You keep a 608×1080 slice, about 32% of the original, and you enlarge it by roughly 78%, which softens it.
- Stretch. Force both dimensions. Everything fits, and everything is distorted: people look impossibly thin.
Fit and fill are almost always better than stretching. The aspect ratio calculator draws the source and the result side by side, with a circle that stays round in fit and fill and becomes an oval when stretched, and reports the bar thickness, the crop rectangle and the share of the picture you keep. To perform the crop, use the Video Cropper, or read the trimming and cropping guide for planning tips.
Why sizes should be even numbers
H.264 and similar codecs store colour at half resolution in each direction and divide frames into blocks. Odd widths or heights break that scheme, and many encoders and editors reject them. The height 1080 is not even a multiple of 16, so encoders quietly code 1088 rows and mark the extra eight as cropped. Both calculators round outputs to even numbers for this reason, and report how far rounding moves the shape, usually a fraction of a percent.
Bitrate: the data budget
Bitrate is the amount of data used for each second of video, in kilobits or megabits per second. File size follows directly:
Size in bytes = (bitrate in bits per second × seconds) ÷ 8
A one-hour video at 8 Mbps is about 3.6 GB, plus around 60 MB for 128 kbps audio. Working backwards is just as useful. To fit a ten-minute clip into 25 MB, take 25 × 8,192 = 204,800 kilobits, divide by 600 seconds to get about 341 kbps in total, and subtract your audio bitrate. That leaves very little for the picture, and the calculator flags it when the result falls far below normal guidance for the resolution.
Why downscaling can look better than compressing
A useful way to judge a bitrate is bits per pixel per frame: the bitrate divided by pixels times frame rate. For H.264, many encoders treat around 0.1 as a comfortable baseline. Consider a fixed 2 Mbps budget at 30 fps:
| Resolution | Bits per pixel per frame | Result |
|---|---|---|
| 4K | 0.008 | Blocky, smeared |
| 1080p | 0.032 | Soft, visible artefacts |
| 720p | 0.072 | Clean and acceptable |
The same 2 Mbps looks terrible at 4K and fine at 720p, because the encoder has nine times as many bits to spend on each pixel. When a file must be small, lowering resolution before lowering quality is nearly always the right move. The video compression guide shows how to apply this to size limits on Discord, WhatsApp and email.
Suggested bitrates
YouTube publishes recommended upload bitrates for standard dynamic range video. They are a useful reference even for other uses.
| Resolution | 24-30 fps | 48-60 fps |
|---|---|---|
| 360p | 1 Mbps | 1.5 Mbps |
| 480p | 2.5 Mbps | 4 Mbps |
| 720p | 5 Mbps | 7.5 Mbps |
| 1080p | 8 Mbps | 12 Mbps |
| 1440p | 16 Mbps | 24 Mbps |
| 4K | 35-45 Mbps | 53-68 Mbps |
These figures suit uploads that will be re-encoded by the platform. For sizes not on the list, the resolution calculator scales from 1080p by pixel count and labels the answer as an estimate. HDR video and high-motion content usually need more.
Reading the numbers from your own file
You can learn a file's average bitrate from two facts you already have, its size and its length. A 250 MB clip that runs five minutes holds 250 × 8,192 = 2,048,000 kilobits over 300 seconds, an average of about 6,800 kbps including audio. Load a video into the resolution calculator and it fills these values for you.
Phones and cameras often record at bitrates well above upload guidance, because they favour speed and safety over size. That is why re-encoding a phone clip for sharing can cut its size dramatically with little visible harm, provided the new bitrate suits the resolution.
How much resolution do viewers need?
Most video is watched on phones and laptops, where 1080p already exceeds what the eye can resolve at normal viewing distance. Higher resolution earns its cost in three cases: large televisions, footage you intend to crop or reframe later, and archival masters. For a talking-head clip that will be watched on a phone, 1080p at a sensible bitrate is nearly always the best balance, and 720p is often good enough.
Mistakes that catch people out
- Swapped width and height. A vertical video is 1080 wide and 1920 tall. Entering it the other way round reverses the ratio, which is why both calculators include a swap button.
- Rotation flags. Some phone clips are stored as landscape with a note to rotate them. Different tools report the stored or the displayed size, so confirm which one you are looking at.
- Confusing p and K. 1080p describes height, while 4K describes width. They are not measured the same way.
- Trusting file size alone. Two files of equal size can differ enormously in quality if one has a short duration and one a long one. Compare bitrates instead.
The upscaling trap
Scaling a 720p video to 1080p or 4K makes the numbers bigger, not the picture. Enlarging adds pixels by interpolation, so edges look softer and file size grows. If the source is 720p, deliver 720p. The scale tab of the calculator warns whenever the target is larger than the source, and shows the percentage change in pixel count so you can see what you are paying for.
A practical workflow
- Load your file in the resolution calculator to read its size, length and average bitrate.
- Decide the destination shape and check the crop or bars in the aspect ratio calculator.
- Choose a resolution your bitrate can support, scaling down if the file must be small.
- Confirm the frame rate matches your plan, using the frame rate guide.
- Resize or crop with the Video Resizer or Video Cropper, then compress if needed.
Understanding the three numbers takes the guesswork out of exports. Once you can estimate pixels, shape and data before you start, uploads stop being surprises.
How we checked this: the YouTube bitrates above were confirmed from search results quoting Google's support page on 21 October 2026, not from the live page. Please check YouTube Help before relying on them.
