On This Page
- Shape, not size
- Three modes
- Ratio cheat sheet
- Finding the ratio by hand
- Resizing without stretching
- Pixels, sensors and CSS
- How the ratio is verified
- FAQ
Shape, Not Size
Aspect ratio is width compared with height, written W:H. A 16:9 screen is 16 units wide for every 9 tall, whether it is a 5-inch phone or an 85-inch TV. 1920×1080 and 1280×720 are the same shape at different resolutions, which is why you can scale one to 800 wide and know the height must be 450. It says nothing about sharpness or pixel count.
Three Modes
- Find aspect ratio: enter width and height in the same unit. You get the simplified ratio, the decimal, and the closest standard ratio if you are near one.
- Find new height: enter the original size and a new width, for resizing images or thumbnails without distortion.
- Find new width: the same in reverse, for a fixed-height slot such as a banner or sidebar.
Ratio Cheat Sheet
| Ratio | Decimal | Where you see it | Typical size |
|---|---|---|---|
| 16:9 | 1.778 | HD and 4K TVs, YouTube, Zoom | 1920×1080, 3840×2160 |
| 9:16 | 0.5625 | TikTok, Reels, Shorts | 1080×1920 |
| 4:3 | 1.333 | Older TVs, iPads, Micro Four Thirds cameras | 1024×768 |
| 3:2 | 1.5 | DSLR and mirrorless sensors, 6×4 prints | 6000×4000 |
| 16:10 | 1.6 | MacBooks and business monitors | 2560×1600 |
| 1:1 | 1.0 | Profile photos, app icons, album art | 1080×1080 |
| 4:5 | 0.8 | Instagram portrait posts | 1080×1350 |
| 21:9 | 2.333 | Ultrawide monitors | 3440×1440 |
Cinema adds a few more: 2.39:1 for scope films, 1.85:1 for flat, and about 1.43:1 for native IMAX. For exact 2026 pixel sizes, safe zones and cropping one shoot for every platform, see what aspect ratio to use for YouTube, Instagram, TikTok & Shorts.
Finding the Ratio by Hand
The GCD is the largest number that divides both. Euclid's method keeps dividing and taking the remainder. For 1920×1080: 1920 ÷ 1080 leaves 840, 1080 ÷ 840 leaves 240, 840 ÷ 240 leaves 120, and 240 ÷ 120 leaves 0, so the GCD is 120 and the ratio is 16:9. For 6000×4000 the GCD is 2000, giving 3:2.
The faster check is width ÷ height, then match the decimal: 1.333 is 4:3, 1.5 is 3:2, 1.6 is 16:10, 1.778 is 16:9 and 2.333 is 21:9. Rise over run on the Slope Calculator simplifies fractions the same way.
Resizing Without Stretching
- A 1280×720 thumbnail at 1920 wide: 1920 × 0.5625 = 1080 high, a 1.5× scale.
- A 3:2 photo printed 12 inches wide: 12 × 2 ÷ 3 = 8 inches high, a standard 12×8 print.
- A portrait phone video at 1080×1920: GCD 120 gives 9:16.
To express how much bigger a new canvas is as a percentage, use the Percentage Calculator. The screen diagonal is a right-triangle problem for the Triangle Calculator.
Pixels, Sensors and CSS
- Resolution is not aspect ratio. 4K and 1080p are both 16:9 but have very different pixel counts.
- Non-square pixels. Old DV video at 720×480 looks 3:2 by pixel count but displays at 4:3, because each pixel is 0.9 as wide as it is tall. That is the usual cause of footage looking squashed in an editor.
- Sensors. Full-frame cameras shoot 3:2 because 35mm film frames were 36 × 24 mm, and 36 ÷ 24 = 1.5. Micro Four Thirds cameras shoot 4:3.
- CSS.
aspect-ratio: 16 / 9;keeps a box in proportion. The old padding hack usedpadding-top: 56.25%, which is 9 ÷ 16.
How the Ratio Is Verified
The ratio shown as the main result comes from a Euclidean GCD loop: it repeatedly replaces the larger number with the remainder of dividing by the smaller until nothing is left over, then divides both width and height by that final value. A resolution like 1000 x 625 reduces through GCD(1000, 625) = 125 to 8:5, and the main result shows exactly that, even though the twelve named presets the calculator keeps for reference would describe the same shape as 16:10. That preset list only feeds an optional "closest standard ratio" note when the decimal is within 0.015 of a known value; it never substitutes for the computed fraction itself.
Frequently Asked Questions
What aspect ratio is 4K?
Standard consumer 4K UHD (3840×2160) is 16:9. Cinema 4K (4096×2160, DCI 4K) has a ratio of 256:135, often described as approximately 17:9. Most 4K TVs and monitors use 16:9. If someone says "4K" without qualification, they almost always mean 3840×2160 at 16:9.
How do I calculate tire aspect ratio?
Tire aspect ratio is the sidewall height as a percentage of the tread width. On a tire labeled 225/55R17, the 55 is the aspect ratio: the sidewall height is 55% of the 225mm tread width. Sidewall height = 225 × 0.55 = 123.75mm. This is a different calculation from image or screen aspect ratios, closer to a standard percentage problem than a ratio simplification.
What is the aspect ratio of an A4 sheet of paper?
A4 paper is 210mm × 297mm. GCD(210, 297) = 3, giving 70:99. The decimal ratio is 1:1.414, which is the square root of 2. The entire ISO 216 paper series (A0, A1, A2, A3, A4...) is designed so that folding any sheet in half gives the next smaller size with the same aspect ratio. That is a clever consequence of the square-root-of-2 ratio.
References
- Adobe Photoshop: Image Size and Resolution: Official Adobe documentation covering pixels, aspect ratio, resolution, and scaling in practical image editing.
- MDN Web Docs: CSS aspect-ratio property: Technical reference for the CSS aspect-ratio property and how browsers handle proportional scaling of elements.
- B&H Photo: Understanding Aspect Ratios in Photography: Photography-focused overview of sensor formats, common ratios, and their effect on print and digital output.