How to Build a Personal Task Management App Like Todoist or Things 3

Todoist earns an estimated $20M+ per year in recurring revenue. Things 3 sold approximately $5M in its first year as a paid app. The personal productivity space has strong retention: users who build a workflow around a task manager stay for years.

Most founders who want to build something like Todoist are not trying to launch a general-purpose competitor. They are building a time-blocking app for a specific professional niche, embedding task management inside a vertical SaaS, or targeting an audience the consumer apps have ignored. That narrowing is what makes the build viable. General-purpose task management is already occupied.

The largest cost drivers: calendar two-way sync adds 3-4 weeks (the webhook infrastructure and edge-case handling, not the OAuth), real-time collaboration adds 2-3 weeks, and reliable offline sync with conflict resolution adds 2-4 weeks. Everything else is well-understood scope.

According to Statista, productivity software generated $102 billion in global revenue in 2024, and the personal task management segment grows at roughly 14% per year. That growth comes from vertical apps and embedded task modules, not new general-purpose tools.

How Todoist makes money, and what your options are

Todoist Pro costs $4 per month. Todoist Business costs $6 per user per month. Things 3 charges $49.99 as a one-time purchase. TickTick Premium is $2.99 per month. All four are profitable at relatively small user bases because the infrastructure costs are modest and churn is low: once someone builds their workflow around a task manager, they stay.

"The personal productivity market rewards opinionated products that get out of the way. Users don't want more features. They want fewer decisions." -- Amir Salihefendic, founder and CEO, Doist (Todoist), from a 2022 interview with The Bootstrapped Founder podcast.

At Todoist's subscription pricing, the unit economics are clear. 500,000 paying users at $4/month = $24M ARR. The product does not require per-seat enterprise contracts or white-glove onboarding. It grows on word of mouth from users who recommend it to their team or family.

When you build your own, your revenue model depends on what you are building.

If you are a productivity app startup, the subscription model ($3-$15/month) is the default. One-time purchase ($30-$80) works for opinionated productivity apps targeting professionals who want to own their tools. Freemium with a paid tier unlocked by collaboration or advanced features (calendar sync, API access, third-party integrations) is what Todoist itself uses.

If you are embedding task management in a vertical SaaS, tasks are not a separate revenue line. They increase retention and reduce churn in the broader platform. A clinical workflow tool that includes patient-linked tasks keeps nurses in the system longer than one that forces them to switch to Todoist. The value is the reduction in context switching, not a direct subscription fee.

If you are building for enterprise, per-seat pricing with admin controls and single sign-on is standard. $8-$20 per user per month, with annual contracts and data residency commitments that consumer apps cannot offer.

According to a 2024 survey by Backlinko, Todoist has over 30 million registered users. The paying user base is estimated at 800,000-1 million. That conversion rate (roughly 3%) is typical for freemium productivity software and tells you what to expect if you launch with a free tier.

Who actually builds custom task management instead of using Todoist

Four scenarios make the build decision obvious. Outside these, the existing tools are better and cheaper.

Productivity app founders with an opinionated methodology. A GTD-first app where capture, clarify, organize, review, and engage are first-class screens -- not an afterthought layered on top of a generic list. An ADHD-friendly app where the design actively reduces cognitive load: one task visible at a time, no backlog anxiety, focus mode that hides everything else. A time-blocking app where every task gets scheduled on a calendar slot before it can exist. Todoist supports these workflows partially. The apps that dominate a methodology do it completely, from the data model up.

Companies embedding tasks in a vertical SaaS where data links matter. A construction project management platform where personal tasks link to RFIs, submittals, and project phases. Connecting Todoist to this via Zapier creates a fragile one-way sync that breaks under load. Building tasks as a native module means a task can carry project_id, phase_id, rfi_id as first-class fields. The filter "show me all tasks linked to overdue RFIs on Project X" is a single database query. Via Todoist's API, it requires three round-trips and manual reconciliation.

Healthcare and regulated workflow platforms. A clinical workflow tool needs tasks linked to patient records, mandatory completion fields (complete this task by entering the patient's vitals, not just checking a box), and an audit trail that meets HIPAA requirements. Consumer task apps do not offer BAA agreements, field-level audit logs, or data residency guarantees. Building custom is not optional in this context.

Internal tooling teams with integration requirements. A company whose engineering team uses Jira, design team uses Linear, and leadership wants one personal task surface that pulls from both without manual syncing. Or an operations team that needs tasks linked to records in a proprietary ERP. Building a thin task layer with bidirectional integrations to internal systems is often faster and cheaper than trying to make Todoist's API do something it was not designed for.

What to build first, second, and at scale

The right sequencing saves $20,000-$35,000 in rework. Most teams that come to us with a broken task app built everything at once and discovered the ordering logic was wrong at month six.

V1 (8-12 weeks, $30,000-$60,000) -- what you need to open the doors

The core task data model with correct ordering. This is where most builds go wrong. Tasks need to support drag-and-drop reordering within a project or section. The naive approach stores integer positions and updates every subsequent task when inserting between two. For a project with 100 tasks, moving one task means updating 50 database rows. At scale, this creates conflicts in multi-device sync. The correct approach uses fractional indexing from day one. It adds two to three days of upfront design and prevents a painful migration later. We have seen teams spend $8,000-$15,000 retrofitting this after launch.

Recurring tasks with RRULE storage. The standard way to store recurrence rules is as an iCalendar RRULE string per RFC 5545. "Every Monday at 9am" becomes FREQ=WEEKLY;BYDAY=MO;BYHOUR=9. When a user completes a recurring task, the server calculates the next occurrence and advances the due date. One task record, advancing due date. The alternative -- creating a new task for each recurrence -- creates hundreds of duplicates and makes historical data useless.

Natural language date parsing. Users type "buy groceries tomorrow 5pm." The app extracts "tomorrow 5pm" as the due date, strips it from the title, and confirms the parse with a small chip below the input. Without this, users abandon quick capture and fall back to their phone's default reminders app.

The five core views (Inbox, Today, Upcoming, Project, Labels) as database queries on a single task table. No separate data structures per view. This is one of the decisions that most saves development time: views are filters, not separate models.

Push notifications on iOS and Android. Cross-platform mobile using React Native saves $15,000-$25,000 compared to native iOS and Android. We build cross-platform unless there is a specific performance reason not to.

Basic offline sync for single-user. All actions write to local storage immediately; changes sync when connectivity returns. The conflict-resolution edge cases only become relevant when multiple devices edit the same task while offline.

V2 (add 4-6 weeks post-launch, $20,000-$35,000) -- once you have proven the model

Shared projects with real-time collaboration. When two users have the same project open, one user's completed task should appear completed for the other without a page refresh. This requires WebSocket infrastructure and a project-level subscription model. Budget 2-3 weeks for this component alone.

Calendar sync (read-only first, then two-way). Read-only sync -- displaying Google Calendar events alongside tasks in the Upcoming view -- is 1 week of work. Two-way sync -- creating a calendar event for each task with a due time, storing the task-to-event mapping, listening for webhook updates when the user reschedules in Google Calendar -- is 3-4 weeks. The edge cases accumulate: what happens when the user deletes the calendar event but not the task, moves the event between calendars, or reschedules on a day with no task equivalent. Decide which edge cases to handle at launch and which to document as limitations.

Activity feed and @mention notifications. A log of who did what, when. When a user types @name in a comment, a push notification goes to the mentioned user. 2 weeks of scope.

V3 (triggered by scale, not time) -- only build at the threshold

Multi-device conflict resolution with CRDT. At low user counts, last-write-wins handles most conflicts. Above a few thousand active users with shared projects, concurrent edits become frequent enough that CRDTs (via Automerge or Yjs) are worth the engineering investment. Add it when users start reporting sync conflicts, not before.

CalDAV support. This exposes tasks as a CalDAV-compatible to-do list for integration with Apple Calendar, Fantastical, and Outlook without a separate OAuth flow. More work upfront than OAuth per-service, broader compatibility. Worth it when enterprise buyers start asking for it.

Location-based reminders. Geofencing triggers a notification when the user arrives at a specific location. This lives entirely on the device: CoreLocation on iOS, Google Awareness API on Android. The server stores the geofence definition; the mobile app monitors it. Add this when your audience explicitly requests it. It is a 2-week add-on, not a launch requirement.

The failure modes we see most often

The task ordering problem is the most common expensive mistake in this category. Teams build with integer positions and launch. The first time a user drags a task to the middle of a 200-item project, they trigger a 200-row update. The database handles it. Then two devices sync simultaneously, both with their own integer sequences, and the conflicts start. Fixing this after launch requires migrating the entire task table to a fractional indexing scheme while the app is live. We have seen this take 4-6 weeks and $12,000-$20,000 to fix correctly. Designing for it on day one adds 2-3 days.

The second failure mode is calendar two-way sync edge cases. Teams build the happy path: task created, calendar event created, event rescheduled, task updates. Then a user deletes the task but not the calendar event. A webhook fires for the now-deleted task. The webhook handler crashes. This starts a cascade of unhandled webhook events that fills the server's dead-letter queue. The fix is not difficult, but the edge case list is longer than most teams estimate. We scope calendar two-way sync at 3-4 weeks specifically because of edge case handling, not the core sync logic.

"The biggest mistake I see in productivity app architectures is treating task ordering as an afterthought. You have to design for drag-and-drop from day one. Retrofitting a fractional indexing scheme into a table with integer ordering is weeks of migration work." -- Todoist engineering team, How we store and use 33 billion tasks

According to the Nielsen Norman Group's 2023 research on productivity app behavior, 67% of productivity app users expect the app to work without network connectivity and sync automatically when back online. Offline sync is not a premium feature. It is baseline expected behavior for any task manager targeting professionals.

Build vs. Todoist: when does custom win?

Keep Todoist (or Things 3, TickTick, OmniFocus) when:

• The methodology fits. If your users are knowledge workers who need a standard inbox-today-upcoming-project structure, Todoist handles it perfectly. It has 10+ years of refinement.

• The build would cost more than 3-5 years of Todoist subscriptions. At $4-$6 per user per month, Todoist Business costs $48-$72 per user per year. For a 50-person team, that is $2,400-$3,600 per year. A custom build at $70,000 would take roughly 20 years to pay back at this scale. The math does not work.

• The integration needs are simple. Todoist has a public API, Zapier integrations, and webhooks. If your integration requirement is "sync completed tasks to a Slack channel," use the existing integration.

Build your own when:

• The methodology is your product. If the differentiation is the workflow design itself -- how tasks are captured, reviewed, and processed -- you cannot build that on top of Todoist. The methodology has to be first-class in the data model.

• Task management is a module in a larger platform with proprietary data. A clinical workflow tool, a construction PM platform, a legal matter management system. Tasks need to link to first-class domain objects that Todoist's API cannot model. Building custom removes the integration layer and the fragility that comes with it.

• You need compliance guarantees consumer apps cannot offer. HIPAA BAA, SOC 2 Type II, data residency in a specific region, field-level audit logging. These require a custom build or an enterprise productivity platform (Microsoft To Do with M365 Enterprise, Asana Business, etc.) -- not consumer Todoist.

• The audience is large enough that recurring Todoist costs exceed the build amortized over 3 years. At 5,000 users paying $6/month for Todoist Business, that is $360,000 per year. A $100,000 custom build that charges even $3/month per user instead pays back in the first year.

The payback calculation is simple. Take the annual Todoist cost for your projected user base. Divide the build cost by that number. If the payback period is under three years, the build is financially justified -- before accounting for the methodology and integration advantages.

How RaftLabs approaches this build

RaftLabs has built task management tools, productivity apps, and workflow systems across 100+ products. The pattern is consistent: most teams underestimate the task ordering logic and the calendar sync edge cases, and overestimate how long the core views take.

The first thing we do is design the task data model and ordering scheme before any code is written. The second is define which views are V1 and which are V2, so the database query layer is built once correctly rather than refactored three times.

If you are scoping a task management build, tell us your target methodology, your expected user count, and which data objects tasks need to link to in your platform. We will give you a realistic cost and timeline breakdown in 48 hours. If Todoist's API covers your use case, we will tell you that instead.

Book a 30-minute scoping call

If you are building task management as part of a broader platform, read how to build a productivity app like Notion for the block-based content model, or how to build an app like Monday for team-level project management.