TL;DR: In our view, Intervals.icu is one of the best free tools in endurance sport. David Tinker, a developer and cyclist in Cape Town, started it in 2018 as an analysis extension for Strava (about Intervals.icu), and it now reports 160,000+ active athletes and 193 million+ analyzed activities (Intervals.icu). The fitness chart, power curve, automatic interval detection and custom charts are free, with no trial or time limit, and the optional Supporter tier costs 4 US dollars per month (pricing). Intervals.icu is an analysis platform first and a coaching platform second. Coaches can follow and coach athletes (athletes), chat with them (chat) and list themselves in a coach directory (coaches). In its documentation we did not find tools for billing your coaching clients, a lactate step test protocol, or courses for coaches (features, pricing). For self-coached, data-minded athletes it is close to perfect. For coaches running a professional practice, it covers the analysis and planning and leaves the business side to other tools.
What Is Intervals.icu?
Intervals.icu is a web-based training analysis and planning platform for cyclists, runners, triathletes, swimmers and other endurance athletes. It started in mid-2018 as a side project by David Tinker, a software developer and cyclist based in Cape Town, who wanted better analysis of his Strava rides. By 2024 it had grown to over 100,000 athletes and 111 million analyzed activities, and David went full-time on it in September 2024 (about Intervals.icu). Supporter subscriptions fund development, which the site says keeps the platform independent (pricing).
That origin explains most of what the platform is: built by an athlete who loves data, for athletes who love data. It now counts more than 160,000 active athletes and more than 193 million analyzed activities (Intervals.icu). In our view the community around it is warm, and one testimonial on its homepage sums it up: “The only constant across all the different cycling forums is the love for Intervals.icu.”
The platform covers activity analysis, fitness and fatigue tracking, a training calendar with a structured workout builder, wellness data, and coaching features that let a coach follow and manage athletes. It connects to Garmin, Polar, Suunto, COROS, Wahoo, Strava, Zwift, MyWhoosh, Rouvy, Hammerhead, Oura, WHOOP, Concept2, Amazfit, Huawei and more, and its homepage counts 250+ third-party apps built on its open API (Intervals.icu).
This review looks at what Intervals.icu does well, where its limits are, and who should build their training or coaching around it.
Key Features
Fitness, Fatigue and Form Tracking
The fitness chart is the heart of Intervals.icu. It uses the classic Performance Management Chart model: fitness (CTL), fatigue (ATL) and form (TSB) over time, with training load from power, heart rate, pace or RPE and configurable time constants (fitness chart). On TrainingPeaks an athlete needs Premium to see this chart on their own dashboard (TrainingPeaks help); for a Basic athlete, only the coach sees it (TrainingPeaks coach pricing). On Intervals.icu it is free.
The platform also estimates FTP from single maximal efforts in your data (eFTP), so zones stay current without a formal test. You can download historical data from Strava, Garmin, TrainingPeaks and more (fitness chart); importing your full Strava history is part of the Supporter tier (pricing).
Power and Pace Analysis
For cyclists, the analysis goes deep:
- Power curve: Compare seasons, view watts per kilogram, estimate MAP, and model your profile with eFTP, Morton’s 3-parameter model and Monod and Scherrer (power curve)
- Automatic interval detection: The platform finds your efforts in each activity, with spike detection and correction for power and heart rate data (activity timeline)
- Decoupling analysis: Cardiac drift and the Power/HR Z2 metric track aerobic fitness over time (decoupling)
- Zone time distribution: Time in zone and power histograms per activity (power charts), time in zone for custom zones (custom zones), plus a polarization index (forum announcement)
Runners, swimmers and rowers get separate zones and settings per sport, and runners get gradient-adjusted pace with elevation correction (multisport).
Wellness
Intervals.icu tracks weight, sleep, HRV, readiness, menstrual cycle, stress, mood, SpO2 and more, synced from Garmin, Polar, Suunto, COROS, Huawei, Amazfit, Oura and WHOOP, and you can add your own wellness fields (wellness).
Custom Everything
This is where Intervals.icu stands out in our view. You can create zones for any activity stream, anchored to FTP, LTHR or your own custom fields (custom zones), build charts from 70+ metrics (custom charts), and write JavaScript that runs on Intervals.icu’s servers to compute activity fields, interval fields, activity streams from FIT file data and Plotly charts, then share them with the community (custom JavaScript). Completely custom zones are part of the Supporter tier (pricing).
If you can define a metric, you can probably track it.
Training Calendar and Workout Builder
The calendar supports drag-and-drop planning, a workout library, weekly totals, third-party calendar imports and sync to devices and platforms (training calendar). The workout builder is a quick, text-based editor and supports steps targeting power, heart rate, pace or cadence. Workouts import and export in Zwift (ZWO), FIT, MRC and ERG formats (workout builder).
You can build training plans with drag-and-drop, link them to several athletes and apply them to calendars in bulk. An annual plan generator works back from your target race to Base, Build and Peak phases (plan your training), and the annual training plan builder is part of the Supporter tier (pricing).
Coaching and Communication
Intervals.icu has coaching features, and they matter for this review. Anyone on the platform can follow (read only) or coach (write access) another athlete once that athlete accepts. Coaches get tags, a status overview of all athletes, an activity feed, a compliance overview and a plan summary (athletes). Chat shares workouts, activities and screenshots in group messages (chat) and private chats (news, September 2026), and every activity has its own chat (activity timeline). Ask A Coach connects athletes with professional coaches inside the platform (Ask A Coach), and a public coach directory lists coaches by country and language (coaches).
Teams and coaching organisations, along with bulk athlete configuration, are part of the Supporter tier (pricing).
Open API and Integrations
The REST API supports personal API keys and OAuth 2.0, uploads and downloads of activities, wellness data, planned workouts and webhooks (open API). Intervals.icu counts 250+ third-party apps integrated through it, including workout generators, AI coaches and coaching tools (app integrations). For technically minded coaches and developers, it is in our view one of the most open platforms in endurance sport.
Community and Languages
Translations, custom fields, charts and many feature ideas come from the community on the Intervals.icu forum, and Supporter feature requests move up the to-do list (pricing). The platform is available in 21 languages, including English, Dutch, French, German, Spanish, Italian and Portuguese (Intervals.icu).
Strengths
The Core Is Free
The free tier is not a trial or a teaser. Fitness and fatigue tracking, activity analysis and interval detection, the calendar, the workout builder, device integrations and custom charts cost nothing, with no credit card (pricing). The Supporter tier at 4 USD per month adds extras such as advanced weather analysis, the annual plan builder, full Strava history import, completely custom zones and teams, and funds development. The pricing page does not say whether VAT is included. One testimonial on the homepage reads: “I canceled my TP Premium and support them now” (Intervals.icu).
Analysis Depth
In our view, Intervals.icu competes on raw analysis with paid platforms such as TrainingPeaks Premium, which lists 19.95 USD per month billed monthly (TrainingPeaks pricing). Standard metrics (Coggan training load, power curve models) combined with deep customization mean serious athletes rarely hit a ceiling.
Open by Design
The API, the export formats, the custom JavaScript and the third-party integrations make Intervals.icu a platform in the true sense (open API). You can get your data out, and the ecosystem around it keeps growing.
A Developer Who Listens
Feature ideas come from the forum, the developer answers there (example), and new features ship in regular news posts (news, September 2026). The platform grew from 111 million analyzed activities in 2024 (about Intervals.icu) to 193 million+ today (Intervals.icu). In our view, few products at any price have this feedback loop.
Community-Powered Multilingual Support
Twenty-one interface languages (Intervals.icu), translated by volunteers (about Intervals.icu), is in our view more than most competitors offer. Because the translations are volunteer work, depth may vary by language, but the breadth is impressive.
Weaknesses
It Is Not a Coaching Business Platform
This is the central limitation for professional coaches. Intervals.icu lets you coach: follow athletes, plan workouts, chat, and organize teams on the Supporter tier. In its feature pages we did not find tools to run the business side of coaching: athlete billing or subscriptions, invoicing, a client onboarding flow, or branded reports for paying clients (communicate, pricing). Coaches handle those pieces elsewhere.
Lactate Testing Lives Elsewhere
Intervals.icu estimates thresholds from field data (eFTP is useful), and since February 2026 it imports continuous lactate data from Q-LAC sensors into the activity timeline (forum announcement). In its documentation we did not find a module for blood lactate step tests with threshold detection and zones set from the result (features). You can log spot lactate values in a custom interval field, as the developer explains on the forum (forum). Coaches who base zones on a full step test run the threshold analysis in separate software and enter the results by hand.
The Learning Curve Is Real
In our view, Intervals.icu is dense. The interface puts information before polish and the settings run deep, which is exactly what data-minded athletes love. Athletes who want a simple, guided experience may find it a lot at first, and coaches should factor that in for every athlete they invite.
No Official Native App
Intervals.icu is web-first. On its forum, a moderator states in March 2026 that there is no native Intervals.icu app yet and points to third-party apps built on the API (forum). The web app runs in a mobile browser, and the September 2026 update reworks it for phones (news, September 2026). In the same thread the developer writes that a native iOS and Android app is in progress, and that you can already install the web app to get push notifications (forum). For athletes who live on their phones, that is worth checking before you commit.
A Small Team and Forum-Based Support
Intervals.icu is run by Intervals.icu Ltd, based in London (privacy policy), with a team of seven people, David Tinker included (about Intervals.icu). That size brings speed and coherence. Questions go mainly to the community forum, where volunteer moderators help answer them (about Intervals.icu). We did not find a service level agreement in their published pages. For individual athletes this rarely matters. For a coach placing a professional workflow on the platform, it is worth weighing.
No Coach Education
We did not find courses or other coach education on the Intervals.icu site. For growing a practice, the platform offers the coach directory and Ask A Coach (coaches, Ask A Coach).
Who Is Intervals.icu Best For?
Intervals.icu is the right choice if you:
- Coach yourself and love data: The platform was built for exactly you, and the core is free
- Want Performance Management Chart analytics without a subscription: CTL/ATL/TSB, power curves and interval detection at no cost
- Customize everything: Custom fields, charts and JavaScript reward tinkering
- Are a coach with data-minded athletes: The follow and coach workflow works well when athletes already use the platform
- Build tools: The open API and the app ecosystem around it give you room to work
Ideal Intervals.icu user: A self-coached cyclist, runner or triathlete who reads their own power curve, enjoys configuring charts, and wants deep analysis without paying for it. Or a coach whose practice is analysis guidance for athletes like that, and who handles billing and testing with other tools.
Who Should Look Elsewhere?
Intervals.icu may not be the right fit if you:
- Base training zones on lactate testing and want threshold detection and zone updates built into your platform
- Coach athletes who need a simple, guided app rather than a data workbench
- Want PDF reports to hand to paying athletes (we did not find them in Intervals.icu’s documentation)
- Want a support team you contact directly for your professional workflow
- Want coach education alongside your software
Coachbox as an Alternative
Coachbox and Intervals.icu share the fundamentals: structured workouts, device sync, training load, wellness data such as HRV and sleep, and coach-athlete chat. Athletes use both for free. The difference is the job each platform was built for. In our view, Intervals.icu is built for the athlete analyzing their own training, and Coachbox is built for the coach running a professional practice.
Where Coachbox goes further for coaches:
- Lactate testing: Enter your step test values or upload a photo of a paper protocol, and Coachbox detects LT1 and LT2 automatically in under a minute, then generates the athlete’s training zones. Every coaching plan includes 5 to 20 free tests at the start of the license, then 6 to 3 EUR per test excl. VAT depending on the plan
- A coaching workflow: Messaging, workout feedback and check-ins, reactions to individual workouts, injuries logged as milestones, and a test history to compare lactate tests over time
- Client output: One-click PDF reports of lactate test results, with custom branding, that you email to the athlete
- Coachbox AI: An assistant that answers questions about your athletes and writes changes straight into their training plan, plus an MCP connection to ChatGPT, Claude and other assistants. Intervals.icu connects to third-party AI coaches through its API (app integrations)
- Coachbox Academy: Coach education on training methodology and lactate testing
What Coachbox doesn’t match Intervals.icu on:
- Price: Nothing beats free. Coachbox coaching plans start at 19 EUR/month billed yearly (25 EUR billed monthly), excl. VAT, with 5 athletes included
- Raw analytical customization: We have no equivalent of custom JavaScript, custom streams and Plotly charts
- API openness: Intervals.icu gives every user a personal API key (open API) and counts 250+ integrated apps (app integrations)
- Athlete billing: Coachbox does not bill your clients for you yet; that is coming soon. We did not find client billing in Intervals.icu’s documentation either (pricing)
The honest split: if you are a self-coached athlete, Intervals.icu is probably all you need, and we say that as a competitor. If you are a coach whose practice includes lactate testing, client reports and a growing roster, Coachbox brings those into one platform. For the full feature-by-feature breakdown, see Coachbox vs Intervals.icu.
Coachbox has 25,000+ users who have logged 12 million+ workouts in 35+ countries. It syncs with Garmin, Polar, COROS, Suunto, Wahoo, Apple Watch, Fitbit and TomTom, and lists its prices in EUR (Coachbox pricing). Athletes coached on Coachbox use it for free, and self-coached athletes pay 4 EUR/month billed yearly for the One plan, excl. VAT (4.76 EUR incl. 19% German VAT). Intervals.icu lists its Supporter tier at 4 USD per month and does not say whether that includes VAT (pricing).
The Bottom Line
In our view, Intervals.icu is a rare thing: an excellent product with a free core, improved constantly, and loved by its community for good reasons. For self-coached endurance athletes who want deep analysis, it is the best value in the sport.
Its limits are limits of scope rather than quality. It is an analysis workbench with coaching features, not a coaching business platform. We did not find lactate step testing, client billing, client-ready reporting or coach education in its documentation, support runs mainly through the forum, and the learning curve can put off less technical athletes.
If you coach yourself, start using it today. If you coach others professionally, use it for what it does well, and look at what a dedicated coaching platform adds on top. We’ve compared the main options in Intervals.icu Alternatives, examined the money question in Intervals.icu Pricing, and if you’re weighing it against what we see as the market leader, read Intervals.icu vs TrainingPeaks.
