Resolution Scaler

Five modes, one source resolution

Everything here shares the same source W×H at the top — set it once and the modes stay in sync as you switch tabs. Scale to box answers “what size does this become in that container” three honest ways: fit (contain, bars allowed), fill (cover, crop allowed) and stretch (exact dims, distortion included). Percentage does the simple ×N resize — and reports the pixel count too, because 50%-per-side is a quarter of the data, a fact resize dialogs never volunteer. Res ladder lays out the standard rungs for your aspect — 4K, 1440p, 1080p, 720p and below — with each row tagged downscale or upscale so encoding-ladder decisions are visible at a glance. Social presets carries the current platform specs (YouTube thumbnails to Pinterest pins, verified October 2026) and shows what your source looks like fitted and filled into each. Will it fit is the sanity check: does this file go into that slot without inventing pixels?

The math, exactly

No approximations hiding anywhere — every number reduces to arithmetic you can check:

• Fit (contain): s = min(boxW ÷ srcW, boxH ÷ srcH), then W' = srcW × s, H' = srcH × s. The smaller factor wins, so one axis always touches the box.
• Fill (cover): s = max(boxW ÷ srcW, boxH ÷ srcH) — same formula, bigger factor, so the box is fully covered and the overflow gets cropped.
• Stretch: W' = boxW, H' = boxH independently — shown with both axis factors because when they differ, shapes distort by max(sx÷sy, sy÷sx).
• Letterbox/pillarbox bars: (box − scaled) ÷ 2 on the axis that didn’t touch — the letterbox math guide derives where every black bar comes from.
• Ladder rungs: landscape sources scale by rung height (that’s what “1080p” means), portrait sources by rung width — and the computed side rounds to even pixels because H.264/H.265/AV1 refuse odd dimensions.
• Aspect ratios everywhere reduce by gcd(w, h) — 1920×1080 → 16:9, and 1366×768 → 683:384, which is why it almost fits 16:9 boxes but not quite.

Honest defaults

ResCal calls an upscale an upscale: any mode that enlarges the source flags it, because stretching pixels means inventing data — the upscale vs downscale guide explains why down is nearly free and up never is. And the social spec table is generated from the same data the calculator uses, so the table can’t drift from the math.

Deeper reading: fit vs fill vs stretch · social-media image sizes · upscaling vs downscaling · letterbox & pillarbox math. If what you actually need is pixel density — PPI, DPI, print dimensions — that’s the sister tool PixelCalc’s job; ResCal stays purely in pixel geometry.

Everything runs in your browser — nothing you measure is transmitted, logged or stored.

Frequently asked questions

What's the difference between fit, fill and stretch?

Fit (a.k.a. contain) scales uniformly until the image touches the box on one axis — nothing is cropped, but empty bars can appear. Fill (a.k.a. cover) scales until the box is fully covered — no bars, but the overflow gets cropped. Stretch ignores the aspect ratio and forces the exact box size, distorting shapes. The full comparison walks the formulas.

How do I scale 1920×1080 down to 720p?

Both are 16:9, so it's a clean uniform scale: 1280×720 is exactly ⅔ of 1920×1080 — every 3×3 block of pixels becomes a 2×2 block. Type the source above, load the 1280×720 box preset and the scaler shows ×0.6667. Same-aspect downscales like this are the friendliest kind of resize.

Does the percentage apply to width and height or to total pixels?

Each side — so 50% scale on a 1920×1080 gives 960×540, which is 25% of the pixels, not 50%. The percentage panel shows both numbers so there's no ambiguity.

Why does the resolution ladder round to even numbers?

Because video codecs demand it: H.264, H.265 and AV1 all encode on even-aligned blocks and reject odd dimensions outright. A 1280×719 export simply fails, so the ladder's computed side rounds to the nearest even pixel — which also matches how real encoding ladders (YouTube's, for instance) are specified.

Can this tool resize the actual image file for me?

No — ResCal does the geometry, not the resampling. It tells you exactly which dimensions to export; the actual resize happens in your editor, FFmpeg (scale=1280:720), or ImageMagick (-resize 1280x720). That split keeps the page instant and means nothing you measure ever leaves the tab.

Are the social-media sizes current?

The preset table was verified against platform documentation and cheat sheets in October 2026 and the page is date-stamped accordingly — platforms do revise their specs, so check the social sizes guide for the notes behind each number before a big export run.

Do the numbers I type get sent anywhere?

No. All five modes are plain JavaScript running in this tab — there's no server doing the math, nothing is logged, and the page keeps working offline after it loads. The privacy policy has the whole picture.