What it solves
Given an aspect ratio and one dimension (width or height), compute the other. Used constantly in video, image, and UI work.
The 7 presets cover the common cases:
- 16:9: Full HD, 4K, most modern displays and YouTube
- 4:3: older TVs, presentations, most printers
- 1:1: square Instagram posts, profile pictures
- 9:16: vertical/portrait, TikTok, Instagram Reels, Stories
- 21:9: ultra-wide cinematic, gaming monitors
- 3:2: DSLR camera default, photo prints
- 5:4: older computer monitors, some prints
For custom ratios, type any two integers in the ratio fields. The simplified ratio (using GCD) shows in the corner.
Why aspect ratio matters
Three real cases:
- Video editing: setting a YouTube thumbnail to exactly 1280×720 (16:9) requires this math.
- Responsive images: when scaling an image to fit a container while preserving aspect ratio.
- Print layout: knowing what 4×6 inches becomes at 300 DPI.
Common pixel sizes derived from aspect ratios:
- 16:9: 1920×1080 (HD), 3840×2160 (4K), 1280×720 (HD ready)
- 9:16: 1080×1920 (vertical 1080p)
- 1:1: 1080×1080 (Instagram)
- 4:5: 1080×1350 (Instagram portrait)
- 21:9: 3440×1440 (ultrawide monitor)
Cropping vs scaling
This calculator gives you the math for proportional resize. If you have a 1920×1200 image (16:10) and want it at 16:9, you can either:
- Crop: cut off 60 pixels from the height to get 1920×1080. Loses content.
- Scale and letterbox: resize to fit (1920×1080) and add black bars to fill the original frame. Preserves all content.
- Scale and pillarbox: resize the height to match (1080) and let width come out wherever (1728). Different ratio.
The right choice depends on context. For video upload, most platforms expect specific ratios, match them or accept letterboxing.
Frequently asked questions
Why are some aspect ratios written as decimals? 21:9 is technically 7:3 (some sources say 64:27). Cinematic ratios often use approximate forms because the underlying spec is irrational (CinemaScope is 2.39:1, sometimes called 2.40:1). For pixel work, integer approximations are close enough.
What’s the “Academy ratio”? 1.375:1, the standard for film between 1932 and the 1950s. Now mostly obsolete.
What aspect ratio is best for portfolios? Square (1:1) is safest for grids, looks consistent on every device. 16:9 if you want video-style hero images. Mix them deliberately.
How do I figure out my screen’s ratio? Divide width by height. 1920/1080 = 1.777 ≈ 16:9. 2560/1080 = 2.37 ≈ 21:9. Most consumer monitors are 16:9; gaming/productivity monitors often go ultrawide.