Diagram tool free options developers actually keep

A diagram tool free enough to keep, for a developer who stores pictures in git, is one you can open when you need a figure and close without a leftover seat. That includes a whiteboard you already have, the drawing tools in an office suite, and a text file that renders. I am not going to rank them. "Free" is not "best," and a tool that costs nothing and produces a file nobody will update is an expensive hobby. MermaidViewer is one option in the text category. It is free to start. It is not a verdict on the others.

Free is a category

I sort tools by what the file becomes, not by a badge on a homepage. A free whiteboard becomes a board that lives on someone else's site, or a local file, depending on the product. A free office drawing becomes a slide or a document. A free text diagram becomes source you can diff. Those are different outcomes. Picking from a list titled "top free diagram tools" usually means picking the one the author had installed. I would rather you pick from the outcome.

Price is part of the outcome and I will not invent it. Some whiteboards are free until the export, or until a second editor, or until the board is older than a trial. Some office suites are free because your employer already pays, which is not the same as free to you next year if you change jobs. Some text tools are free because they are open source and you run them yourself. Check the current terms the day you choose. A blog that writes "$0" next to a product name it doesn't control will be wrong, and it will stay wrong until someone notices.

What I will say about this site is narrow. The editor is free and it does not ask you to sign up. PNG export is on every plan, with the free plan at 1× and a small watermark. Starter is $6.99 a month for watermark-free PNG up to 4×. JPG, SVG, and PDF are Pro-only at $11.99 a month. Five complimentary AI uses in total, then hand editing, which stays free. The pricing page is the list to trust if you are comparing numbers. I don't claim those numbers make the tool the right one. They make the limit visible.

The editor comparison is about Mermaid editors specifically, which is a smaller question than "diagram tool." Use it when you have already chosen text and you are choosing where the preview runs. Don't use it as a ranking of whiteboards. They are not in that race.

Whiteboards

A whiteboard is the right free tool when the diagram doesn't exist yet and more than one person is pointing. You need a box to appear while someone is talking. You need to delete it without a pull request. The picture is allowed to be wrong for an hour. That hour is the point.

I stop using the whiteboard when the hour is over. The board is a bad system of record. It has no obligation to match the code, and its history, when it has history, is not the history of the repository. I copy the surviving boxes into text the same day, while I still remember which arrow was a joke. If I wait a week, the joke becomes the architecture, because it was still on the board and the board looked official.

Whiteboard exports vary. Some give you a PNG. Some give you a file their app can reopen. Few give you a Mermaid fence, and I don't depend on one. If an export to text exists in the version you have, try it on a five-box board and read every label. Importers drop words. A missing "not" on a decision is worse than redrawing five boxes by hand. Redraw. It is short when the board was honest.

Don't put credentials, customer data, or an unreleased incident timeline on a whiteboard whose sharing settings you haven't read. A free tool can still be a public link. I assume a link is broader than I intended until I check. That is not a feature list. It is a habit.

Office suites

Slides and documents are where diagrams go to be seen by people who will not open a repo. The drawing tools are already there. Insert a shape, connect it, put the file in the deck you were going to send anyway. For a one-off explanation in a meeting that will not be repeated, this is the correct amount of tool. I am not going to give you a menu path. Desktop apps and browser apps move the insert command. Check the version, look for shapes, and stop if the hunt takes longer than writing the list.

The failure is the deck that becomes the spec. Someone asks for the picture, you paste it into a slide, the slide is emailed, and six months later the code has changed and the slide is still in the kickoff folder. Nobody diffs a rectangle in a presentation. I treat office drawings as exports. The source, if the picture describes the system, lives in the repo. The slide is generated or rebuilt when the meeting is real. If you will not keep a source, don't call the slide the design. Call it a meeting artifact and date it in the corner.

Office tools are also where org charts and slide timelines pretend to be flowcharts. A box with a person's name is not a step. If the deck needs a process, use a decision shape and label both outcomes, or put a real flowchart image on the slide. The image can be a PNG from a text tool. Free 1× with a watermark is often good enough for an internal deck. If the watermark is embarrassing in front of a customer, that is what the paid PNG is for, or use the office shapes and accept that you now maintain them by hand.

Text in the repo

Text is the category I bias toward once the diagram has a second edit. Mermaid covers the pictures I need in docs: flowcharts, sequences, class diagrams, ER diagrams, state machines, and a few layouts like block diagrams when the row matters. The diagram index is the list of those types. I don't need a separate app for each. I need a fence and a preview.

The template library is how I start when the picture is a known shape, a login or a set of layers, and I would rather rename than invent. Templates are not a different tool. They are files. Copy one, delete half the nodes, commit the rest. A template you don't delete down to your system is how generic boxes ship in a README and confuse the next hire. "API" from a template is not your API until you change the label.

Other text languages exist. Graphviz DOT is the one I still use for a large generated graph. This post isn't the comparison. A free online diagram tool writeup and a create a diagram online writeup cover the browser path in more detail. The flowchart online tool post is the flowchart-specific version. A visual editor is the middle ground: you see the picture while the source stays text. That middle ground is the one I want for day-to-day edits. A pure whiteboard has no source. A pure file with no preview makes me mis-count arrows.

AI generation sits in this category too. Describe the boxes, get a fence, edit the fence. Five uses total on the free allowance. I spend them when I don't know the shape yet. I don't spend them to add a node. The preview updates when I type. Typing is the free part that doesn't run out.

A flowchart you can paste

This is the decision I use when someone asks which free tool to open. It is not a ranking. It is a fork. If the picture will change when the code changes, I want text in git. If it will not, a canvas is fine, and free is enough, and I should not build a pipeline around it.

flowchart TD
    need[Need a picture] --> change{Will it change with the code?}
    change -->|Yes| text[Keep text in git]
    change -->|No| canvas[Use a canvas]
    text --> preview[Preview before you commit]
    canvas --> dateBox[Date the export]

The diamond is the whole policy. "Will it change with the code?" is a question about the next month, not about your identity as a person who likes text. A conference poster will not change when you rename a function. A request path will. I have used a canvas for the poster and Mermaid for the path in the same week without feeling inconsistent. The fork is per picture.

dateBox is the id because the label is about dating the file, and I won't use end as an id. Mermaid uses end to close subgraphs. A future edit that wraps this fork in a subgraph should not break because I named a node badly today. The label can say "Date the export" so the canvas path has a concrete action. An undated PNG in a wiki is how a stale architecture survives a reorg.

"Preview before you commit" is the text path's action. A fence that doesn't render is a broken doc, the same way a broken test is a broken branch. I look at the preview. I don't assume the last edit was harmless because it was "only a label." Labels are the content.

A map of the choice

A mindmap is the wrong picture for that fork, and it is the right picture for the topics on the table. Nothing here is an order. Whiteboard, office, and text are siblings. Under text I keep Mermaid and DOT because those are the two languages I actually open. Under office I keep slides and docs. Under whiteboard I keep a sketch and a workshop, which are uses, not products.

mindmap
  root((Diagram tools))
    Whiteboard
      Sketch
      Workshop
    Office
      Slides
      Docs
    Text
      Mermaid
      DOT

The indent is the relationship. Sketch sits under Whiteboard because a sketch is a way of using a board, not a third category. If I indent it wrong, Mermaid will promote it to a sibling and the map will say a sketch is a peer of the whole whiteboard category. That is the only syntax that matters here. There are no arrows to debug. Count two spaces.

I don't put prices on the map. A mindmap with "free" and "paid" as children starts to look like a fact and becomes a stale one. The fork flowchart already made the maintainability choice. The map is the index. If a fourth category shows up, a CAD tool for a physical board, it gets a branch the day we have a diagram that needs it. It doesn't get a branch so the map looks balanced.

Read the map as topics, not steps. People scan a mindmap left to right and invent a sequence. There isn't one. You can start in a workshop, move to text, and export a slide. You can also start in text and never touch the others. Both are allowed. The flowchart is the sequence. Don't ask the mindmap to be one.

What I keep on the free plan

I keep the editor, the repo, and PNG at 1× when a ticket needs an image. The watermark is small. If a reader can't see past it, the diagram was too dense anyway. I don't pay to feel like the diagram is official. I pay, or I use a local renderer, when the file format is the requirement: an SVG in a docs site, a PDF in a packet, a JPG where a tool rejected PNG. Those downloads are Pro here. Starter is the middle step when the only complaint is the watermark or the resolution.

I don't keep a second free tool "just in case" if I won't remember its file format. A graveyard of accounts is not a strategy. One whiteboard for discovery, the office suite I already have for decks, and Mermaid for anything that describes the system. DOT stays available for generated graphs and doesn't need to be a daily app. The DOT language versus Mermaid is that choice when the graph is generated rather than written. That is four things, and three of them might already be installed. Adding a fifth because a roundup listed it is how the week disappears.

Share links are optional. They help a review that happens outside the pull request. They are not the archive. The archive is the fence in the commit. If the link and the commit disagree, the commit wins. I have said that in other posts because I keep relearning it.

If you are choosing today, run the flowchart on the actual picture you need this afternoon, not on a hypothetical platform. One picture. One branch. Then open the tool that branch names. A roundup you finish by subscribing to everything has failed, even if every subscription was free.

The editor is the text side of that branch when Mermaid is the language. Paste the fork if you want to see it render, then replace the labels with your own picture's names. If the diamond's question doesn't fit your picture, you don't have a tool problem yet. You have a "why are we drawing this" problem, and no free signup will answer it.