A single photo from a modern iPhone or camera can be 3–10 MB. That's too big for most upload forms, slow to email, and heavy on a website.
You don't need an online compressor to fix it. This guide covers the settings that actually matter and three ways to compress images on your Mac.
What makes an image file big?
Three things decide an image's file size:
- Pixel dimensions. Halving the width and height leaves a quarter of the pixels. This is the single biggest saving.
- Format. JPG and WebP throw away detail your eye won't miss. PNG keeps every pixel exactly, which is great for screenshots and terrible for photos.
- Quality setting. For JPG and WebP, lowering the quality from 100% to around 80% often cuts the size in half with no visible change.
Photos from an iPhone may also be HEIC, which is already efficient but not always accepted. If that's your problem, see how to convert HEIC to JPG on Mac.
Method 1: Compress with Preview
Preview shows you the file size before you save, so you can find the right balance.
- Open the image in Preview.
- To reduce the dimensions, choose Tools → Adjust Size, enter a smaller width (keep Scale proportionally on), and click OK.
- Choose File → Export, set Format to JPEG, and drag the Quality slider. The estimated file size updates as you go.
- Click Save.
Tip: Preview edits the open file directly. Choose File → Duplicate first if you want to keep the original.
Method 2: Resize many images from Finder
On macOS Monterey and later, Finder's Convert Image Quick Action can shrink a batch of photos in one go.
- Select the images in Finder.
- Right-click and choose Quick Actions → Convert Image.
- Choose JPEG and an Image Size of Small, Medium, or Large.
- Click Convert. Smaller copies appear next to the originals.
It's quick, but you only get three preset sizes and no quality control, so results can be larger or smaller than you need.
Method 3: Compress from the right-click menu with FinderBuddy
FinderBuddy adds Compress Image and Resize Image to Finder's right-click menu. Select one image or a hundred, right-click, and choose the tool. It compresses JPG and PNG files without visible loss, resizes to exact dimensions or percentages, and can convert to WebP when you need the smallest file for a website.
Everything runs locally, so private photos and work files never leave your Mac. See pricing or read the setup guide.
Best settings for common jobs
- Email or messaging: about 1600px on the long side, JPG at 75–80%. Usually 200–500 KB.
- Website images: the largest size the page displays (often 1200–2000px), WebP or JPG at 70–80%.
- Forms with a size limit: start at 1200px and JPG 80%, then lower the quality until it fits.
- Screenshots: keep PNG if it has text and flat colors; switch to JPG if it's mostly a photo.
Compressing a PDF full of scanned pages works on the same principles. See how to reduce PDF file size on Mac.
Frequently asked questions
- Does compressing an image reduce its quality?
- Lossy formats like JPG and WebP discard some detail, but at 70–85% quality most people can't see a difference. Resizing to a smaller width also removes detail you often don't need.
- What is the best free image compressor for Mac?
- Preview is built in and lets you control JPG quality while watching the estimated file size. For batches, Finder's Convert Image Quick Action is free too.
- Should I use PNG or JPG?
- Use JPG (or WebP) for photos and PNG for screenshots, logos, and images with sharp text or transparency.
- Can I compress images without uploading them?
- Yes. Preview, Finder Quick Actions, and FinderBuddy all process images on your Mac, so nothing is sent to a server.