Aspect Ratio Calculator
What This Calculator Does
This aspect ratio calculator computes the simplified ratio between width and height dimensions. It also provides a resize tool that lets you enter a new width or height and automatically calculates the other dimension to maintain the same proportions. A visual preview shows the shape of the ratio.
Inputs Explained
- Width (px): The horizontal dimension of your image, video, or screen in pixels.
- Height (px): The vertical dimension in pixels.
- Common Presets: Quick buttons for standard ratios like 16:9, 4:3, 1:1, 9:16, and 21:9.
- New Width / New Height (Resize Tool): Enter one value and the other is calculated to preserve the aspect ratio.
How It Works
The calculator finds the Greatest Common Divisor (GCD) of the width and height using the Euclidean algorithm, then divides both dimensions by the GCD to produce the simplified ratio. For the resize tool, it uses the proportion formula: new height = new width × (original height ÷ original width), and vice versa.
Formulas Used
GCD = Euclidean algorithm applied to Width and Height
Simplified Ratio = (Width ÷ GCD) : (Height ÷ GCD)
Decimal Ratio = Width ÷ Height
Resize: New Height = New Width × (Original Height ÷ Original Width)
Resize: New Width = New Height × (Original Width ÷ Original Height)
Simplified Ratio = (Width ÷ GCD) : (Height ÷ GCD)
Decimal Ratio = Width ÷ Height
Resize: New Height = New Width × (Original Height ÷ Original Width)
Resize: New Width = New Height × (Original Width ÷ Original Height)
Calculate dimensions and simplify ratios.
Ratio
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Decimal
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Total Pixels
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Step-by-Step Example
Width: 1920 px
Height: 1080 px
Step 1: Find GCD of 1920 and 1080. GCD(1920, 1080) = 120.
Step 2: Divide both by GCD. 1920 ÷ 120 = 16, 1080 ÷ 120 = 9.
Simplified Ratio: 16:9
Decimal: 1920 ÷ 1080 = 1.778
Total Pixels: 1920 × 1080 = 2,073,600 (approximately 2 megapixels).
Resize Example: To resize to width 1280, new height = 1280 × (1080 ÷ 1920) = 720 px.
Use Cases
- Video production: Ensure footage matches target formats like 16:9 (YouTube, TV), 9:16 (TikTok, Instagram Reels), or 21:9 (cinema).
- Web design: Maintain image proportions when creating responsive layouts or hero banners.
- Photography: Crop images to standard print sizes (4:3 for 8×6, 3:2 for 6×4) without distortion.
- Social media: Resize images to platform-specific dimensions while preserving the aspect ratio.
- Display shopping: Compare monitor and TV aspect ratios (16:9 vs. 21:9 ultrawide) before purchasing.
Assumptions and Limitations
- The calculator works with integer pixel dimensions. Sub-pixel values are rounded to the nearest whole number.
- The GCD simplification may produce large numbers for non-standard dimensions (e.g., 1366:768 simplifies to 683:384, not a clean ratio).
- The resize tool maintains exact mathematical proportions. Actual rendered sizes may differ due to display scaling or CSS constraints.
- Only width and height are considered. The calculator does not account for pixel aspect ratio (PAR) used in some video standards.
Frequently Asked Questions
Aspect ratio describes the relationship between an image's or screen's width and height, written as two numbers separated by a colon. So 16:9 means for every 16 units of width, there are 9 units of height. It doesn't tell you the actual size — a 16:9 image could be 1280×720, 1920×1080 or 3840×2160 — only the shape. Common ratios you'll come across: 16:9 for TVs and YouTube, 4:3 for older monitors and presentations, 1:1 for Instagram squares, and 9:16 for vertical phone videos and Reels.
1920×1080 is 16:9 — the standard widescreen ratio used for Full HD video, YouTube, most monitors and televisions. To work it out yourself, find the greatest common divisor of 1920 and 1080, which is 120. Divide both numbers by 120 and you get 16:9. The same ratio applies to other Full HD-derived sizes like 1280×720 (HD), 2560×1440 (QHD) and 3840×2160 (4K UHD) — they're all 16:9, just at different pixel densities. If you're choosing dimensions for a video project, sticking to 16:9 keeps things compatible with most displays.
Find the greatest common divisor (GCD) of the width and height, then divide both numbers by it. The simplified pair is your aspect ratio. For 1600×1200, the GCD is 400 — divide both and you get 4:3. For 1920×1080, the GCD is 120, giving 16:9. If the result still looks ugly (like 683:384), the original dimensions probably aren't a clean ratio, and you might want to round to the nearest standard one. The calculator does all this in one click, so you don't have to factor numbers by hand.
The trick is to keep the same aspect ratio when changing the size. Decide either the new width or the new height, then calculate the other one using the original ratio: new height = new width × original height ÷ original width. So if a 1920×1080 image needs to be 800 pixels wide, the new height should be 800 × 1080 ÷ 1920 = 450 pixels. Most image editors and resize tools have a "lock aspect ratio" or chain icon that does this automatically — keep it on unless you specifically want to stretch the image.
16:9 isn't a fixed pixel size — it's a shape. Many different pixel dimensions share the 16:9 ratio. The most common ones: 1280×720 (HD), 1366×768 (standard laptop screens, slightly off-spec), 1600×900, 1920×1080 (Full HD), 2560×1440 (QHD), 3840×2160 (4K UHD) and 7680×4320 (8K). They all show the same shape on screen, just with different sharpness depending on the display. When you're picking dimensions for a video, presentation or banner, choose based on where it'll be displayed — 1080p is still the safest default for most use cases.
Instagram supports several ratios depending on the post type. For square feed posts use 1:1 (1080×1080). For portrait feed posts use 4:5 (1080×1350) — these take up more vertical space and tend to get more engagement. For Stories and Reels use 9:16 (1080×1920), the full vertical phone screen. For landscape feed posts use 1.91:1 (1080×566), which matches the Facebook link preview ratio. Profile photos display as circles but should be uploaded square. Always design with safe zones in mind — Stories, especially, can have UI elements covering the top and bottom 250 pixels or so.
4:3 is the older, squarer ratio used by traditional TVs, early computer monitors and many PowerPoint templates. 16:9 is wider — the standard for HD video, modern displays and YouTube. If you put a 4:3 image on a 16:9 screen, you get black bars on the sides; the reverse gives you bars on top and bottom (letterboxing). 4:3 is closer to a 1.33 width-to-height ratio while 16:9 is about 1.78. For new content today, 16:9 is the safe choice unless you're specifically designing for older systems or square formats.
Use the formula: new height = new width × original height ÷ original width. So if your image is 1200×800 (a 3:2 ratio) and you want it to be 600 wide, the new height becomes 600 × 800 ÷ 1200 = 400 pixels. The image stays proportional — no stretching or squashing. Most tools have an aspect-lock toggle that does this automatically when you change either dimension. If you're doing it by hand for a CSS layout or batch resize script, the formula is quick to apply and works for any pair of dimensions.
Pixel aspect ratio (PAR) describes the shape of a single pixel — its width compared to its height. Most modern displays use square pixels (1:1 PAR), so this rarely matters. But older video formats like NTSC DV used non-square pixels (around 0.91:1 or 1.21:1) to fit more visual data into limited storage. If you ignore PAR when editing such footage, the picture stretches or squashes. PAR is different from image aspect ratio, which is the shape of the whole frame. For modern web, mobile and digital video, PAR is almost always 1:1.
Multiply width by height — that's it. So a 1920×1080 image contains 1920 × 1080 = 2,073,600 pixels, which rounds to about 2.07 megapixels. A 4K UHD image (3840×2160) is 8,294,400 pixels, or roughly 8.3 megapixels. Camera megapixel counts work the same way — a 12 MP phone sensor captures around 12 million pixels per shot, usually at a 4:3 ratio like 4032×3024. Knowing the total pixel count helps when comparing image quality, calculating file sizes or working out storage needs for batches of photos or video frames.
Sources and References
- ITU-R BT.709 – HDTV Standard — International standard defining the 16:9 aspect ratio for high-definition television.
- SMPTE – Society of Motion Picture and Television Engineers — Professional body establishing standards for digital cinema aspect ratios and video formats.
- W3C CSS Images Module Level 3 – Image sizing — Web standard covering how images are sized and displayed in browsers.
- Euclidean Algorithm – Khan Academy — Educational reference for the GCD algorithm used to simplify ratios.
- Schema.org WebPage Specification — Structured data standard used for the page metadata.