Aspect Ratio Calculator
Calculate width or height for a target aspect ratio (16:9, 4:3, 1:1, 21:9). Useful for video, design, and screen sizing.
What this calculates
Aspect ratio is the proportional relationship between width and height. 16:9 is HD video; 4:3 is older TV and tablets; 1:1 is Instagram squares; 9:16 is vertical/mobile video. This calculator finds the missing dimension when you know one, or simplifies a given pixel size to its closest standard ratio.
Formula & how it works
Width = height × (ratio_w / ratio_h). Height = width × (ratio_h / ratio_w). To find ratio from pixels: simplify width:height by GCD. 1920:1080 → divide both by 120 → 16:9. Decimal form: 16/9 = 1.778, 4/3 = 1.333, 21/9 = 2.333.
Worked example
Want a 16:9 image 1920 wide. Height = 1920 × 9/16 = 1080. Common HD. Reverse: have 1280 × 720. GCD(1280, 720) = 80. Divided: 16:9. Or 2560 × 1080: GCD 40 → 64:27. Not standard — sits between 21:9 and 16:9.
Frequently asked questions
Which ratio for video?
16:9 for landscape (YouTube, TV, monitors). 9:16 vertical (TikTok, Reels, Stories). 1:1 square (older Instagram feed). 4:5 portrait (modern Instagram feed). 21:9 cinematic ultrawide.
Do I need to crop?
Often yes when switching ratios. Going from 4:3 to 16:9 means either cropping top/bottom or letterboxing left/right. Both lose information; only re-shoot avoids it.
Pixel aspect vs display aspect?
Modern digital video uses square pixels (1:1 pixel aspect). Older formats (NTSC, PAL DV) had non-square pixels — same pixel count rendered at different display ratio. Ignore unless dealing with broadcast video.
Common photo aspects?
DSLR 3:2 (matches 35 mm film). Phone 4:3 or 16:9. Square 1:1 (Hasselblad). Medium-format 6:7 or 4:5. Each crops differently for print sizes.