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.

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.

Image Dimensions + Aspect Ratio Calculator

Resize an image to a target dimension while preserving aspect ratio. Reports the GCD-simplified ratio (16:9, 4:3 …) and total megapixels.

Result

Resized dimensions

800 × 450 px

Aspect ratio 16:9 (HDTV widescreen) preserved.

Original1920 × 1080 px (2.07 MP)
Aspect ratio16:9 (HDTV widescreen)
Pixel ratio16:9
Resized800 × 450 px (0.36 MP)
Scale factor41.67% width / 41.67% height
Diagonal918 px

Source: SMPTE ST 274-2 — 16:9 HDTV aspect ratio; ITU-R BT.709.

Note — Integer rounding applied on the scaled axis; for sub-pixel precision use a graphics tool that supports fractional dimensions.

Step-by-step

GCD(1920, 1080) = 120 → simplified ratio 16:9.
Lock width to 800; height = width · (originalH/originalW) = 450.

How to use this calculator

Type the original pixel dimensions.
Choose what to lock — width, height, or a percent scale.
Read the resized dimensions and the recognised aspect-ratio name.

About this calculator

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.