TL;DR: These are two different kinds of product. Intervals.icu calls itself a “free online training platform” to track fitness, analyze activities, plan workouts and coach other athletes (Intervals.icu). David Tinker started it in Cape Town in 2018 (About), it reports 160,000+ active athletes (Intervals.icu), and in our view it is above all an analysis platform, with the deepest customizable analytics in endurance sport. Coachbox is a coaching platform built in Belgium: lactate testing with automatic LT1/LT2 detection, Coachbox AI on your athletes’ data, coach-athlete communication, branded test reports for your clients, coach education in the Coachbox Academy and an athlete app in seven languages, from EUR 19 per month billed annually, excl. VAT. Self-coached data lovers should use Intervals.icu, and we mean that. Coaches who run a professional practice built on testing get lactate analysis, reports and education in the same platform as their planning.
An Analysis Platform and a Coaching Platform
Coachbox and Intervals.icu both run in an endurance athlete’s browser, both sync with the big device brands, and both hold a season of structured training (Intervals.icu). From there, they diverge, because each one grew around a different question.
Intervals.icu grew around the question: what is my data telling me? It started in mid-2018 as a side project and analysis extension for Strava; David went full-time on it in September 2024, and a small team now builds it (About). The result is a data workbench: a fitness, fatigue and form chart (Fitness Chart), power curves with several modelling algorithms, automatic interval detection, custom zones, and user-written JavaScript for custom fields, streams and charts (Intervals.icu). The core platform is free, and a Supporter tier at $4 per month adds features (Pricing). It reports 193M+ activities analyzed (Intervals.icu). Coaching features exist too: coaches follow and coach athletes and chat with them (Communicate), and build training plans for several athletes (Plan).
Coachbox grew around the question: what does a professional coach need to run their practice? It is built for coaches who manage cyclists, runners and triathletes as a business, and who base training on physiological data. Lactate testing with automatic LT1/LT2 detection, dated zones, athlete communication, branded PDF test reports, the Coachbox Academy for coach education, and athlete matchmaking are the core.
The useful way to compare them is to keep asking: whose job is this tool for? The athlete analyzing their own training, or the coach responsible for someone else’s?
At a Glance
| Coachbox | Intervals.icu | |
|---|---|---|
| Built in | Belgium | Developer based in Cape Town (About); company Intervals.icu Ltd, London (Intervals.icu) |
| Product type | Coaching platform | Free training platform (Intervals.icu) |
| Lactate testing | LT1/LT2 detected automatically in under a minute | Lactate data from Q-LAC sensors in FIT files (forum); we did not find step-test analysis in their documentation (Features) |
| AI assistant | Coachbox AI, plus an MCP connection for ChatGPT, Claude and other assistants | We did not find a built-in assistant; third-party AI coaches connect via the API (App Integrations), and an official MCP server is “at the spec stage” (forum) |
| Threshold estimation | From lactate test data | Automatic eFTP from single maximal efforts (Fitness Chart) |
| Device sync | Garmin, Polar, Wahoo, Suunto, COROS, Apple Watch, Fitbit, TomTom | Garmin, Polar, Suunto, Coros, Wahoo, Strava, Zwift, MyWhoosh, Rouvy and more (Intervals.icu) |
| Languages | EN, NL, FR, DE, IT, ES, PT | 21, translated by the community (Intervals.icu) |
| Free athlete account | Yes, free forever | Core platform free (Pricing) |
| Coach price | From EUR 19/month billed annually, excl. VAT | Free; Supporter $4/month, VAT basis not stated (Pricing) |
| Support | Support team by email | Community forum with Questions & Help and Bug Reports categories (Forum), and support@intervals.icu as the contact address in its privacy policy (Privacy Policy); we did not find a helpdesk or published response times on their site (About) |
Feature Comparison
Analytics and Data
| Feature | Coachbox | Intervals.icu |
|---|---|---|
| Load and fitness tracking | Training load monitoring | CTL/ATL/TSB chart with several load models and configurable time constants (Fitness Chart) |
| Power analysis | Power data arrives with each workout through the device sync | Power curve across seasons, W/kg, eFTP, Morton’s 3P and Monod & Scherrer (Intervals.icu) |
| Interval detection | Workout analysis against the zones that applied on the day | Automatic, with spike detection and correction for power and HR (Intervals.icu) |
| Custom charts | Analytics and basic statistics | Custom charts from more than 70 metrics (Custom Charts) and Plotly charts in JavaScript (Custom JavaScript) |
| Custom fields and streams | Not part of this comparison | Computed activity and interval fields, custom streams from FIT data (Custom JavaScript) |
| API and AI connections | MCP connection for ChatGPT, Claude and other assistants | REST API with OAuth 2.0, API keys and webhooks (Open API); 250+ third-party apps (App Integrations) |
Where Intervals.icu wins: This category, clearly. In our view Intervals.icu sets the bar for analytical depth and customization in endurance sport. If you want to define your own metric, build your own chart or script your own analysis, its custom JavaScript gives you that freedom.
Where Coachbox wins: Interpretation inside the coaching workflow. Coachbox AI answers questions about your athletes and writes changes straight into the training plan. We did not find a built-in assistant on Intervals.icu: its integrations page lists AI coaches among the third-party apps built on the API (App Integrations), and in September 2026 the developer wrote that an official MCP server is “at the spec stage” (forum).
Physiological Testing
| Feature | Coachbox | Intervals.icu |
|---|---|---|
| Lactate step-test analysis | Included in all plans | We did not find it in their documentation (Features) |
| Lactate data in activities | Not part of this comparison | Lactate chart and interval fields from Q-LAC sensor data (forum) |
| LT1/LT2 detection | Automatic, under a minute; type values or read a photo of a paper protocol | We did not find it in their documentation (Features) |
| Zones | Zones from test results, dated so each workout is analyzed against the zones that applied that day | Custom zones anchored to FTP, LTHR or custom fields, with zone changes tracked over time (Custom Zones); “completely custom zones” are listed under Supporter (Pricing) |
| Threshold estimation from field data | Not part of this comparison | eFTP, automatic (Fitness Chart) |
| Spirometry | Spirometry data can be added to a lactate test | We did not find it in their documentation (Features) |
| Professional test reports | Branded PDF, client-ready | We did not find it in their documentation (Features) |
Where Coachbox wins: Testing is the clearest structural difference. Intervals.icu estimates thresholds from everyday training data (Fitness Chart), and for self-coached athletes eFTP is often enough. An estimate from field data is a different thing from a measured lactate profile, though. Coachbox runs the chain from test to report: type in the values or photograph the paper protocol, LT1 and LT2 appear in under a minute, the new zones apply from the test date, and a branded PDF report is ready for the client. Every Coachbox plan includes a number of free tests at the start of the license, with per-test pricing after that.
Where Intervals.icu wins: If you never test, eFTP keeps FTP-anchored zones current with no effort and no cost (Fitness Chart), and that is valuable for the self-coached majority. Both products keep a history of zone changes.
Coaching Workflow
| Feature | Coachbox | Intervals.icu |
|---|---|---|
| Athlete management | Profiles with training and testing history | Follow (read only) and coach (write access), tags, activity feed and compliance overview (Athletes); bulk configure athletes with Supporter (Pricing) |
| Structured workouts to devices | Yes, to Garmin, COROS and Wahoo | Yes, with ZWO, FIT, MRC and ERG import and export (Plan) |
| Training plan building | Calendar with annual periodization | Calendar, workout library, training plans linked to multiple athletes (Plan); annual plan builder with Supporter (Pricing) |
| Communication | Messaging, comments on individual workouts, HRV, sleep and resting HR synced from the device | Group messaging with workout, activity and screenshot sharing (Chat) |
| Athlete experience | Web, iOS and Android apps, localized in 7 languages | Web app you can install on your phone, with push notifications (forum); a native iOS and Android app is “in progress” according to the developer in September 2026 (forum) |
| Teams and organisations | Unlimited coaches on every plan; club pricing on request | Teams and coaching organisations with Supporter (Pricing) |
Where Coachbox wins: In our view, the workflow fits the coach-athlete relationship better for athletes who are not data-minded. Athletes get an app in their own language with training and feedback in one place, and coaches respond to individual workouts. With Intervals.icu, the athlete works in the same data platform the coach uses, which we see as a gift for technical athletes and a hurdle for others.
Where Intervals.icu wins: For rosters of self-sufficient, data-fluent athletes, the shared platform is a feature: coach and athlete look at the same customizable charts. The text-based workout builder (Plan) is quick, and training plans apply to calendars in bulk.
Business Features
| Feature | Coachbox | Intervals.icu |
|---|---|---|
| Athlete payment automation | Coming soon | We did not find it in their documentation (Features) |
| Branded client reports | Yes, PDF test reports | We did not find it in their documentation (Features) |
| Coach education | Coachbox Academy | Community forum (Forum); we did not find a course program |
| Finding athletes | Matchmaking | Coach directory (Coaches) and Ask A Coach (Ask A Coach) |
| Custom branding | Lactate test PDF in your own brand; the app itself is not brandable | We did not find it in their documentation (Features) |
| Support | Support team by email | Community forum (Forum) and support@intervals.icu (Privacy Policy) |
Common ground: Both products help athletes find a coach. Intervals.icu lists coaches by country and language (Coaches) and connects athletes with coaches inside the platform (Ask A Coach); Coachbox does the same through matchmaking.
Where Coachbox wins: The rest of this category. A coaching business needs client-ready deliverables, education and someone to email when sync breaks in race week. Coachbox provides those; payment automation and a marketplace come later.
Where Intervals.icu wins: The open API. Coaches with development skills can build their own tooling on Intervals.icu for free (Open API), and 250+ third-party apps already have (App Integrations).
Languages and Support
| Coachbox | Intervals.icu | |
|---|---|---|
| Interface languages | 7 (EN, NL, FR, DE, IT, ES, PT) | 21, with translations from the community (Intervals.icu) |
| Support | Support team by email, included in all plans | Community forum (Forum) and support@intervals.icu (Privacy Policy) |
| Education | Coachbox Academy | Forum threads (Forum) |
Intervals.icu’s 21 languages outnumber Coachbox’s seven, and its volunteers translate them (About). We maintain Coachbox’s seven languages ourselves across the coach and athlete experience. On support, the Intervals.icu forum is active, and the developer answers there himself (forum). Its privacy policy gives support@intervals.icu as a contact address (Privacy Policy), but we did not find a support desk or service commitment on their site (About). For professionals, in our view, that difference matters most on the worst day, not the average one.
Pricing
For the full breakdown, see Intervals.icu Pricing. The short version:
| Coachbox | Intervals.icu (Pricing) | |
|---|---|---|
| Entry price | EUR 19/month billed annually, or EUR 25 billed monthly, excl. VAT (Base, 5 active athletes included) | Free |
| Paid tier | EUR 49/month billed annually, or EUR 65 billed monthly, excl. VAT (Build, 25 active athletes included) | Supporter, $4/month, VAT basis not stated |
| Higher tiers | Peak EUR 79, Prime EUR 149 per month billed annually, excl. VAT (50 and 100 active athletes included) | We did not find other tiers on their pricing page |
| Extra athletes | EUR 2.50 to EUR 1.50 per extra active athlete per month, excl. VAT, depending on plan | Not applicable, no per-athlete pricing listed |
| Lactate testing | 5 to 20 free tests at the start of the license, then EUR 6 to EUR 3 per test, excl. VAT | We did not find step-test analysis |
| Free trial | 14 days, no credit card | Core platform free, “no trials or time limits” |
| Athlete cost | Free for athletes of a coach | Free |
| Club pricing | Custom, on request | Teams and coaching organisations with Supporter |
Coachbox prices come from our pricing page and exclude VAT. Intervals.icu lists the Supporter tier at $4 per month (Pricing) and does not state whether that includes VAT, so we show it as listed and do not convert it. The two models differ: Coachbox bills per plan with a number of active athletes included, Intervals.icu charges per Supporter account.
On price, Intervals.icu wins by definition: the core platform is free (Pricing). The question for a professional coach is the cost of the whole workflow: what you pay for testing software, client reports and billing next to Intervals.icu, and the time you spend connecting them, against one subscription that includes testing and reports. For hobbyist coaches, free wins. For coaches whose time bills by the hour, the answer depends on how much of that work you do today.
What Intervals.icu Does Better
Analytical depth and customization. The fitness chart (Fitness Chart), power curve models, automatic interval detection (Intervals.icu), custom zones (Custom Zones) and custom JavaScript (Custom JavaScript) form, in our view, the most capable analysis environment in endurance sport.
Price. “The core platform is free with no trials or time limits,” and a Supporter tier at $4 per month adds features such as the annual plan builder and teams (Pricing).
Openness. A REST API with OAuth 2.0, API keys and webhooks, activity downloads in FIT, TCX and GPX (Open API), and 250+ third-party integrations (App Integrations).
Community and development pace. Feature requests happen in public on the forum (Forum), and the developer announces new features there, such as lactate sensor support (forum). A small team builds the platform, backed by volunteer moderators and translators (About).
Virtual training platforms. Intervals.icu lists Zwift, MyWhoosh and Rouvy among the platforms it connects with (Intervals.icu). If indoor platforms carry your training, check how each one connects to Coachbox during the trial.
What Coachbox Does Better
Lactate testing is a workflow. From the test values to LT1 and LT2, to zones that apply from the test date, to a branded report for the client. You can register, start the free trial and enter a first test the same day. For coaches who test, or want testing to become a paid service, this changes the comparison.
An athlete app built for clients. A localized mobile app for iOS and Android that non-technical athletes open to see their training and your feedback.
AI on your athletes’ data. Coachbox AI answers questions about your athletes and writes changes straight into the training plan. You can also connect ChatGPT, Claude or another assistant to Coachbox through MCP.
The professional layer. Branded test reports, the Academy, matchmaking, unlimited coaches on every plan and a support team you can email. In our view, this is the difference between software for training and software for a coaching business.
Seven languages we maintain ourselves, across the coach and athlete experience, which matters when your clients span the Benelux and beyond.
Who Should Choose Intervals.icu
Intervals.icu is the better fit if you:
- Coach yourself and enjoy owning your own analysis
- Love data: custom charts, custom fields and scripting reward curiosity (Custom JavaScript)
- Coach a few technical athletes who want the same platform you use
- Have no testing workflow and are happy with eFTP field estimates
- Want to spend nothing: the core platform is free (Pricing)
- Build integrations: in our view the open API is the best in the category (Open API)
- Train indoors on Zwift, MyWhoosh or Rouvy: Intervals.icu connects with all three (Intervals.icu)
Ideal Intervals.icu user: A self-coached cyclist, runner or triathlete who reads their own power curve and enjoys the analysis as part of the sport. If that is you, use it, support it, and enjoy it.
Who Should Choose Coachbox
Coachbox is the right choice if you:
- Coach professionally: your athletes are clients and your platform is your workplace
- Test athletes, or want testing to become part of your paid service, with results flowing into zones and reports
- Coach across languages: athletes get the app in Dutch, French, German, Italian, Spanish, Portuguese or English
- Want AI assistance that works from your athletes’ actual data
- Need client-ready output: branded test reports and an athlete app
- Value a support team and a company behind your daily workflow
Ideal Coachbox user: An endurance coach managing 10 to 50+ athletes as a business, who uses lactate testing to set zones and wants planning, testing, communication and client deliverables in one subscription.
Switching from Intervals.icu to Coachbox
You can keep your Intervals.icu account for personal use when you move your coaching practice, since the core platform has no trials or time limits (Pricing). The practical picture:
What transfers easily: Device connections. Coachbox syncs with Garmin, Polar, Wahoo, Suunto and COROS, so athlete data flows from day one. Zone structures can be recreated, and once you test, zones come from measured thresholds rather than field estimates.
What needs rebuilding: Workout templates and plan libraries need recreating in the Coachbox builder. Historical analysis stays in Intervals.icu, which is fine: the core platform is free (Pricing).
What you gain immediately: The testing workflow, branded reports, Coachbox AI, and a mobile app for your athletes.
What you give up: The chart customization and the free price. If your coaching identity is built on custom analytics, use the 14-day trial to check that Coachbox’s analytics cover your needs.
The Bottom Line
Intervals.icu deserves its reputation. In our view it is the best free product in endurance sport, and for self-coached athletes we recommend it. Read our full Intervals.icu Review for the complete picture, or see the wider field in Intervals.icu Alternatives.
Coachbox is built for a different job: the professional coaching practice. Both products let you plan training and talk to your athletes. Lactate testing with LT1/LT2 detection, Coachbox AI on athlete data, branded reports, the Academy and a support team are what Coachbox adds on top, and we did not find those in Intervals.icu’s documentation.
Choose by the job. If the job is understanding your own training, Intervals.icu is free and, in our view, superb. If the job is running a coaching practice on physiological data, Coachbox is built for that, and the 14-day trial with no credit card is the fastest way to feel the difference.
