Aspect Ratio Calculator

Scale width and height while keeping the same aspect ratio.

This tool runs entirely in your browser. Your data is never uploaded, never stored, and never leaves your device.

A resolution calculator that holds a set of proportions while you change the size: give it the width and height you are starting from, then whichever half of the new pair you know, and the missing one is filled in. Underneath, the original pair is reduced by its greatest common divisor to show the ratio in lowest terms.

How to use it

1Enter Original Width and Original Height, or press 16:9, 4:3, 1:1, 21:9 or 3:2 — a preset writes its own two numbers into all four boxes, so type your real dimensions afterwards.
2Type into New Width or New Height; the other one updates as you type, always from the original pair.
3Read the ratio line beneath the boxes.

Example

Input: Original 1366 × 768, new width 1000
Output: New height 562 — ratio 683:384

That ratio line surprises people and it is correct: 1366 × 768, the old laptop panel size, is not quite 16:9, so it reduces to 683:384. Ultrawides behave the same way — 3440 × 1440 comes out as 43:18 and 2560 × 1080 as 64:27, though both are sold as 21:9, and pressing the 21:9 preset itself displays 7:3, the same fraction in lowest terms. The calculated dimension is rounded to a whole pixel, so it is a very close match rather than an exact one, and it can land on an odd number: 1920 × 1080 with a new height of 400 returns a width of 711, which most video encoders reject until you round it to 712. Type whole pixel counts — a decimal such as 2.35 is rounded before the ratio is reduced, and reports itself as 2:1.

What happens to your data

Four numbers in four boxes, multiplied and divided in the page. No image is opened and no file is read, so this works from a spec sheet or a line in a client's email as well as from something you have on disk, and there is nothing to upload. Nothing is stored either: reload and the boxes return to 1920 × 1080.

Last updated August 2026

The brief gives you one number. The banner has to be 1200 pixels wide, or the video has to come out 1080 tall, and what you are starting from is some other size entirely. Get the second number wrong and nothing breaks loudly — the picture looks a little off, faces slightly narrow or slightly round, in a way people notice without being able to name.

Work out first which of two jobs you are doing, because only one is arithmetic. Scaling keeps the proportions and changes the size: both dimensions move by the same factor, and the missing number can be worked out exactly. Changing the ratio is a different operation — 4:3 artwork does not become 16:9 by multiplication, and the choice is between cropping, padding with bars, or stretching. This is built for the first job, so if your target ratio and your source ratio disagree, the number it gives you answers a question you were not asking.

The other thing to settle is that a ratio is a proportion, not a size. 16:9 describes the shape of the frame and nothing about how big it is; 1280×720, 1920×1080 and 3840×2160 are all 16:9, and a platform asking for one of them will not take another. The common mistake is trusting the name on the box rather than reducing the pixel counts you actually have.

How it works

Toolvore does the whole job with one multiplication and one division. Type a new width and the new height becomes that width times the original height, divided by the original width, rounded to a whole pixel; type a new height and the same sum runs the other way. Both new figures derive from the original pair rather than from each other, so the original stays the reference. The ratio line is Euclid's algorithm on the two originals, dividing each by their greatest common divisor. Rounding to whole pixels makes the answer a close match rather than an exact one. The originals are rounded before the divisor is found, so a decimal is quietly turned into an integer. Editing an original after the new boxes are filled pushes no correction through: the ratio line moves, the new pair does not, until you retype one of them.

Common use cases

Getting an export height when a client specifies only a width
Scaling a hero image down for mobile without squashing it
Checking whether a screen resolution is genuinely 16:9
Sizing a video render for a platform that quotes one dimension
Working out thumbnail dimensions that match a full-size image
Confirming two assets share the same proportions

Frequently asked questions

How do I resize an image without stretching it?＋

Change one dimension and let the other follow by the same factor. Halve the width and the height must halve too; multiply the width by 0.7 and the height goes the same way. That is the sum being done for you here — enter the size you have under Original, then the one figure you know under New, and its partner appears. The distortion people complain about comes from typing both new numbers by hand, or from dragging a corner handle without holding whatever key your editor uses to constrain proportions. Faces and circles betray it first; straight edges hide it well enough that a stretched image can ship.

What is the difference between resizing an image and changing its aspect ratio?＋

Resizing changes how many pixels there are. Changing the ratio changes the shape of the frame, and no multiplication takes you from one shape to another. To put 4:3 material into a 16:9 slot you have three options and each costs something. Crop, and the top and bottom of the picture are gone. Pad, and you get bars down the sides — pillarboxing, the vertical twin of letterboxing. Stretch, and everything grows fatter. Nothing here crops or pads; the arithmetic assumes the shape is staying and only the size is moving.

Why does my new height come out as an odd number, and does it matter?＋

Rounding to whole pixels is unavoidable, since there is no such thing as two-thirds of a pixel, and the result sometimes lands odd. For a still image that is harmless. For video it often is not: most delivery codecs store colour at half resolution horizontally and vertically, so they want both dimensions divisible by two, and an encoder tends to refuse an odd one rather than fix it. Some workflows prefer multiples of eight or sixteen, to suit the blocks compression works in. Adjust by a pixel yourself before exporting — one pixel of shape error is invisible.

What aspect ratio should I use for video and social posts?＋

Landscape video is overwhelmingly 16:9, and the vertical formats built for phones use its inverse, 9:16. Square, 1:1, still appears in feeds, and 4:5 portrait suits a scrolling column. Photography keeps its own conventions — 3:2 from most cameras, 4:3 from phone sensors — and cinema goes wider again. Five of those have buttons here: 16:9, 4:3, 1:1, 21:9 and 3:2. For anything else, including every vertical format, type the two numbers into the original boxes yourself. A preset is a shortcut for common cases, not a list of what is allowed.

How do I use a ratio written as a decimal, like 2.39:1?＋

Cinema ratios are usually written as one decimal against one, and 1.85:1 and 2.39:1 are the two common projection formats. A decimal typed into the original boxes will not survive, because the pair is rounded to whole numbers before the ratio is reduced. Scale both sides up instead: for 2.39:1 enter 239 as the original width and 100 as the original height, and for 1.85:1 use 185 and 100. The proportion is identical and the arithmetic behaves. Put your intended width under New and the matching height appears.

Why does my video file say 1920×1080 but play wider than that?＋

Because the pixels in some formats are not square. A frame stored as 1920×1080 can be flagged to display at 2.39:1, and a player that honours the flag stretches it on the way to the screen; DVD-era formats did the same at much lower resolutions. The stored numbers describe the file, the display ratio describes what you are meant to see, and the two differ by whatever the pixel aspect ratio says. Any calculation from stored dimensions assumes square pixels, so if your source is anamorphic, work from the display dimensions your editor reports.

Why does a 6×4 print cut the top off my photo?＋

Because 6×4 inches is 3:2 and most phones shoot 4:3, so something has to give — and a print lab crops rather than leave white edges, usually taking a slice off the long sides. The same collision runs through the standard sizes: 5×7 is 7:5, 8×10 is 4:5, and A-series paper is a proportion no camera shoots. Check what will be lost before ordering, because a head near the top edge is the usual casualty. Enter your image's pixel dimensions as the original pair and read the ratio line to see which print size it sits closest to.

Do I have to upload the image to work out its new size?＋

No. There is nothing to upload. No image is opened, there is no file picker and no drop area — the only inputs are four numbers you type. The sums run in your browser as you type, so there is no request to send and no server to receive one. Nothing is remembered either: the values live in the page's own state, so a reload returns the boxes to their defaults and keeps no record of what you calculated. There is no copy button beside the result either, so the figures have to be read off and retyped.