OneTaskBy Tim King · Updated
OBS Scene Size Calculator
Work Out the Fit, Fill and Downscale Size for an OBS Canvas
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How the OBS Scene Size Calculator works
This OBS scene size calculator tells you how big to make a source on your canvas and where to put it so it sits dead centre. Enter the canvas and source sizes and choose fit, fill or stretch. It also shows the scaled output resolution for the downscale factor you stream or record at.
OBS works with two sizes. The base canvas is the scene you arrange sources on. The output resolution is what is actually encoded, and OBS scales the canvas down to it. Sources such as a 4:3 webcam, a vertical phone feed or a 1440p game rarely match the canvas, so each has to be scaled, positioned and sometimes cropped. Dragging handles by eye leaves a pixel or two of gap; typed values do not.
The OBS scene size calculator formula
The formula is: scale = canvas width ÷ source width or canvas height ÷ source height, taking the smaller of the two to fit inside and the larger to fill. The size on canvas is the source size × scale, and the position is (canvas − size) ÷ 2 on each axis. Output size is the canvas ÷ the downscale factor, rounded down to even numbers, which video encoders need.
Worked example, a 4:3 webcam fitted to 1080p: 640 × 480 on 1920 × 1080 gives scales of 3 and 2.25. Fit uses 2.25, so the webcam becomes 1440 × 1080 at position 240, 0, with 480 pixels of bars.
Worked example, the same webcam filling the canvas: fill uses 3, so it becomes 1920 × 1440 at position 0, −180, and 360 pixels of height are cropped off.
How many pixels does downscaling throw away? Divide 1 by the factor squared: 1.5× keeps 44% of the pixels, 2× keeps 25%. The table lists common canvases and the output each factor gives.
Output resolution for common canvases and downscale factors
| Base canvas | Downscale | Output | Pixels kept |
|---|---|---|---|
| 1920 × 1080 | 1.00× | 1920 × 1080 | 100.0% |
| 1920 × 1080 | 1.20× | 1600 × 900 | 69.4% |
| 1920 × 1080 | 1.50× | 1280 × 720 | 44.4% |
| 1920 × 1080 | 2.00× | 960 × 540 | 25.0% |
| 1920 × 1080 | 2.50× | 768 × 432 | 16.0% |
| 1920 × 1080 | 3.00× | 640 × 360 | 11.1% |
| 2560 × 1440 | 1.00× | 2560 × 1440 | 100.0% |
| 2560 × 1440 | 1.20× | 2132 × 1200 | 69.4% |
| 2560 × 1440 | 1.50× | 1706 × 960 | 44.4% |
| 2560 × 1440 | 2.00× | 1280 × 720 | 25.0% |
| 2560 × 1440 | 2.50× | 1024 × 576 | 16.0% |
| 2560 × 1440 | 3.00× | 852 × 480 | 11.1% |
| 3840 × 2160 | 1.00× | 3840 × 2160 | 100.0% |
| 3840 × 2160 | 1.20× | 3200 × 1800 | 69.4% |
| 3840 × 2160 | 1.50× | 2560 × 1440 | 44.4% |
| 3840 × 2160 | 2.00× | 1920 × 1080 | 25.0% |
| 3840 × 2160 | 2.50× | 1536 × 864 | 16.0% |
| 3840 × 2160 | 3.00× | 1280 × 720 | 11.1% |
| 1080 × 1920 | 1.00× | 1080 × 1920 | 100.0% |
| 1080 × 1920 | 1.20× | 900 × 1600 | 69.4% |
| 1080 × 1920 | 1.50× | 720 × 1280 | 44.4% |
| 1080 × 1920 | 2.00× | 540 × 960 | 25.0% |
| 1080 × 1920 | 2.50× | 432 × 768 | 16.0% |
| 1080 × 1920 | 3.00× | 360 × 640 | 11.1% |
Entering the numbers in OBS
Right-click the source, choose Transform, then Edit Transform. Set Position to the x and y shown and Size to the width and height. Leave Positioning Alignment at top left so the position refers to the top-left corner. For Fill and crop, also set Bounding Box Type to Scale to outer bounds with the canvas size, or crop the overhang in the Crop fields.
Tips and pitfalls
Canvas aspect should match the output. If the base canvas is 16:9, pick an output that is 16:9 too, or OBS will stretch the whole scene.
Downscale with Lanczos. In Video settings, the Lanczos filter keeps text sharper than bilinear when the output is smaller than the canvas.
Upscaling a small source looks soft. A 640 × 480 webcam at 225% will never be sharp. Use the camera's native high resolution in its properties first.
How to read the result
Size and position are in canvas pixels. Empty bars or cropped pixels show what fit or fill costs, and the output line is the resolution to set in Settings, Video, Output (Scaled) Resolution.
FAQ
What is the difference between base and output resolution in OBS?
The base (canvas) resolution is the scene you compose. The output (scaled) resolution is what OBS encodes for the stream or recording. When the output is smaller, OBS downscales every frame, which saves bitrate and encoder load at the cost of detail.
How do I fit a 4:3 webcam into a 16:9 scene?
Scale it by the canvas height divided by the webcam height, so it fills the height, then centre it. On a 1920 × 1080 canvas a 640 × 480 webcam becomes 1440 × 1080 at x = 240, with bars at the sides.
Should I stream at 1080p or downscale to 720p?
If your bitrate is below about 6,000 kbit/s for fast games, 720p usually looks cleaner, because each pixel gets more data. The stream bitrate recommender shows the bitrate each resolution needs.
Why must the output resolution be an even number?
Most video encoders store colour at half resolution in both directions, known as 4:2:0 chroma subsampling. An odd width or height cannot be halved cleanly, so encoders round or refuse it.
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