How we picked these tools
We judged each converter on where it processes your files, how private it is, how many formats it handles, its free limits, and how quickly you get a result. Some links to commercial services are affiliate links, so we may earn a commission if you sign up — at no extra cost to you. Read our affiliate disclosure if you want the full picture. Commissions never change our rankings or the order tools appear in.
Client-side vs server-side: the difference that matters
This is the single most important thing to understand before you pick a converter. A client-side tool runs the conversion in your own browser using your device's power, which means your file is never uploaded, there is no queue to wait in, and there is nothing sitting on someone else's server afterward. A server-side service does the opposite: it sends your image to a remote machine, processes it there, and returns the result. Server tools can offer enormous format ranges and APIs, but you are trusting a third party with your file. For anything private — IDs, contracts, screenshots, client work — that distinction is the whole game.
1. PNGifier — best private, client-side converter
PNGifier is our own converter, and it is fully client-side: every conversion happens inside your browser, so your image never travels across the internet. There is no upload, no account, no watermark, and nothing to delete from a server later. It is free and instant for the jobs people actually do — turning files into PNG or running them through the image converter in seconds.
Being honest about the trade-off: because it works locally, PNGifier targets the common image formats most people need rather than the long tail of niche or non-image types, and it is not meant to power server-side automation. For private, everyday image conversion in the browser, it is the most discreet option here. Just open a tool and drop a file in.
2. CloudConvert — best server-side range and API
CloudConvert is the most versatile server-based pick. It converts between more than two hundred formats spanning images, documents, audio, and video, and its well-documented API makes it a favourite for developers wiring conversion into their own apps. The output quality and granular options are excellent.
Because it is server-side, your files are uploaded and processed remotely, and the free tier is metered by conversion minutes per day, so steady use needs a paid plan. CloudConvert deletes files after processing, which helps, but if privacy is your priority a client-side tool still keeps your image off the wire entirely.
3. Convertio — simplest server-side converter
Convertio keeps server-side conversion refreshingly simple: choose a file, pick a target, download. It covers a broad spread of formats and even pulls files in from Dropbox or Google Drive, and the free tier needs no account for an occasional job.
The free plan caps file size at 100 MB and limits how many conversions you can run before it nudges you toward a paid tier, and every file is uploaded to its servers for processing. For a clean, no-fuss one-off conversion it is great; for private or high-volume work it is not the tool to lean on.
4. Zamzar — longest-running format workhorse
Zamzar has been converting files online since the mid-2000s, and that longevity shows in the breadth of formats it supports across images, documents, and media. It is dependable, familiar, and offers an API for teams that need conversion as a service.
It is firmly server-side, so files are uploaded, and the free tier is limited on file size and daily conversions before a subscription is required. As a reliable catch-all for an unusual format you cannot open anywhere else, it earns its place — just not for sensitive images.
5. Squoosh — best client-side compression tool
Squoosh is a free, open-source converter from the Google Chrome Labs team that, like PNGifier, runs entirely in your browser — no uploads. Its standout feature is a live side-by-side preview with a slider, so you can dial in quality and watch the file size change in real time while comparing formats like WebP, AVIF, and JPEG.
It is built around compression and modern web formats for a single image at a time rather than bulk batches or exotic file types, and it has no account or API. For privately squeezing an image down to the smallest sensible size with full control, it is a brilliant client-side companion.
How to choose an online converter
Lead with privacy: if the image is sensitive, choose a client-side tool like PNGifier or Squoosh so it never leaves your device. If you need a format no browser tool supports — or you are converting documents, audio, or video — a server-side service such as CloudConvert, Convertio, or Zamzar is the right reach, accepting that the file gets uploaded. Then check the free limits against how often you convert, and whether you need an API for automation. For the great majority of everyday image jobs, the most private free tool is also the fastest.
Why client-side wins for most jobs
Today's browsers are powerful enough to convert images without any server in the loop. That means no upload time, no processing queue, no account, and no copy of your file living on someone else's infrastructure. It is faster for routine conversions and far safer for anything you would not want to hand to a stranger — which is precisely why we built PNGifier to run client-side from the start.
Frequently asked questions
- What is the difference between a client-side and a server-side image converter?
- A client-side converter does all the work inside your browser, so your image never leaves your device — there is nothing to upload and nothing stored on a server. A server-side converter uploads your file to a remote machine, converts it there, and sends the result back. Client-side is more private and instant; server-side can support more exotic formats and very large jobs.
- Are online image converters private and safe?
- Client-side tools like PNGifier and Squoosh are private by design because your file is processed locally and never transmitted. With server-upload services, safety depends on the provider's policies — check how long they keep files and when they delete them, and avoid uploading anything sensitive.
- Do I have to create an account to use an online converter?
- Not for the everyday tools. PNGifier and Squoosh need no account at all. Server-based services like CloudConvert, Convertio, and Zamzar let you do occasional conversions without signing up, but they ask you to register once you hit their free daily limits or want larger files and an API.
- Which online converter handles the most formats?
- Server-based services win on sheer breadth — CloudConvert and Zamzar convert hundreds of formats including documents, audio, and video, not just images. Client-side tools focus on common image formats, which covers nearly every everyday need while keeping your files on your own device.