Resizing an image without warping it is straightforward arithmetic — keep the new-width-to-new-height ratio equal to the old-width-to-old-height ratio — but doing it by hand is the kind of small mistake that leaks into thumbnails, social-media previews, and printed marketing material. This calculator does the arithmetic, names the aspect ratio when it matches a common standard, and reports the GCD-simplified pixel ratio for cases that do not.

How it works — the formula

aspect = w/h (invariant under proportional scaling)
ratio = w/gcd(w,h) : h/gcd(w,h)
megapixels = w·h / 1e6

Aspect ratio is invariant under proportional scaling. The GCD-reduced fraction gives the canonical name (1920:1080 → 16:9).

Sources: SMPTE ST 274-2 — HD aspect ratio standard · ITU-R BT.709 — HDTV signal parameters · Instagram for Business — image sizes guide

Worked examples

Example 1

1920×1080 → 1280-wide

Inputs:: w=1920, h=1080, mode=width, newW=1280
Output:: 1280×720 (16:9, 0.92 MP)

Example 2

4032×3024 → 50%

Inputs:: w=4032, h=3024, mode=percent, pct=50
Output:: 2016×1512 (4:3, 3.05 MP)

Example 3

3000×3000 → 1080-high

Inputs:: w=3000, h=3000, mode=height, newH=1080
Output:: 1080×1080 (1:1, 1.17 MP)

Limitations

Integer rounding may cause sub-pixel drift on extreme aspect-ratio-mismatched targets.
Aspect-ratio name detection uses ±0.01 tolerance; unusual ratios are reported by their GCD-reduced fraction.
No file-size estimation — that depends on encoder, codec, and content complexity.

Pure arithmetic — works for any input range up to 100,000 px per axis.

Frequently asked

What aspect ratio is HD video?+

16:9 — the SMPTE ST 274 standard for HDTV and the default for nearly every modern display, smartphone-shot landscape video, and YouTube upload.

What is the difference between resolution and aspect ratio?+

Resolution is the absolute pixel count (e.g. 1920×1080); aspect ratio is the proportion (16:9). 1920×1080 and 1280×720 are both 16:9 but very different resolutions.

How do I scale to an Instagram post?+

Instagram square posts are 1:1 (1080×1080). Stories/Reels are 9:16 (1080×1920). Use the calculator with target 1080×1080 or 1080×1920 to see if your source needs cropping.

Why does my video look stretched after resizing?+

Almost certainly because the player resized one axis without matching the other — e.g. forcing a 16:9 source into a 4:3 frame. This calculator keeps the ratio constant.

Does this account for DPI / PPI?+

No — DPI is a print attribute. Pixel dimensions are correct for any display. Divide pixel dim by target DPI for printed size in inches.