Guide
Intervals.icu for Runners — Free Training Analytics
Intervals.icu is the best free training analytics platform for runners. It tracks your fitness, fatigue, and form — and with STAS, it becomes the data layer for your AI running coach.
Connect for freeWhat is Intervals.icu?
Intervals.icu is a free training analysis platform created by David Tinker. It connects directly to Garmin, COROS, Polar, and Suunto — syncing every workout automatically with full detail: heart rate, pace, intervals, power, and GPS data.
Think of it as a free alternative to TrainingPeaks. You get the Performance Management Chart (CTL/ATL/TSB), detailed workout analysis, HR and pace zone breakdowns, a training calendar, and an open API that lets third-party tools (like STAS) build on top of your data.
- ✓Syncs with Garmin, COROS, Polar, Suunto, and Strava — automatically
- ✓Performance Management Chart: CTL (fitness), ATL (fatigue), TSB (form)
- ✓Heart rate and pace zone analysis for every workout
- ✓Training calendar with planned workouts and notes
- ✓Open API — the foundation that lets STAS connect your data to ChatGPT
Why runners love Intervals.icu
No subscription, no premium tier, no feature gates. All analytics features are available to every user. David Tinker runs it as a community project.
Strava shows you a map and pace splits. Intervals.icu shows CTL/ATL/TSB curves, interval structure, HR drift analysis, zone breakdowns, and workout classification — the metrics a coach would look at.
The PMC chart tracks your Chronic Training Load (fitness), Acute Training Load (fatigue), and Training Stress Balance (form). You can see at a glance whether you're building fitness, overreaching, or peaking.
Plan workouts directly in the calendar, set descriptions and training load targets. Intervals.icu pushes planned workouts to your Garmin so you can execute them on the watch.
Unlike closed platforms, Intervals.icu has a full REST API. This is what makes STAS possible — it reads your training data through the API and feeds it to ChatGPT for AI coaching.
Getting started with Intervals.icu
Create a free account
Go to intervals.icu and sign up with your Google account or email. No credit card, no trial period — the platform is free for personal use.
Connect your watch or Strava
In Settings, link your Garmin, COROS, Polar, or Suunto account. Intervals.icu will import your entire workout history and sync new workouts automatically. You can also connect Strava, but direct watch sync preserves more data.
Understand your dashboard
The main dashboard shows the Performance Management Chart. The blue line is CTL (fitness — how trained you are), the pink line is ATL (fatigue — recent stress), and the yellow line is TSB (form — readiness to perform). When TSB is positive, you're fresh; when negative, you're carrying fatigue.
Set up HR and pace zones
Go to your athlete profile and configure heart rate zones (based on max HR or lactate threshold HR) and pace zones. This enables accurate zone analysis for every workout — so you can see if your Easy runs are actually easy.
Key metrics explained
Intervals.icu tracks several metrics that matter for endurance training. Here are the ones you'll see most often and what they mean for your running.
Chronic Training Load (Fitness)
A rolling 42-day weighted average of your daily training stress. Higher CTL means more accumulated fitness. It rises slowly when you train consistently and drops when you rest. Think of it as your "training bank account."
Acute Training Load (Fatigue)
A rolling 7-day weighted average of training stress. It responds quickly to hard training blocks and drops fast during rest. High ATL means you're carrying recent fatigue.
Training Stress Balance (Form)
TSB = CTL minus ATL. When positive, you're relatively fresh and ready to perform. When negative, fatigue exceeds fitness — you're in a loading phase. Most runners race best with TSB between +10 and +25.
Training Load
A single-number score for each workout that combines duration and intensity (based on HR or pace). Higher values mean a harder session. Used to calculate CTL and ATL.
TRIMP (Training Impulse)
A heart rate-based training load metric. It weights time spent at higher heart rates more heavily. Useful when pace data isn't available (treadmill, trail) or when you want a purely effort-based measure.
How STAS extends Intervals.icu
Intervals.icu gives you data. STAS gives you a coach. STAS connects to your Intervals.icu account and builds an AI layer on top — so ChatGPT can read your training history, understand your fitness trends, and give coaching that's actually personalized.
Intervals.icu alone
- -Charts and tables — you interpret them yourself
- -No AI-driven workout classification
- -No automatic VDOT / pace zone calculation
- -No coaching recommendations based on your data
Intervals.icu + STAS
- +ChatGPT reads your full training history automatically
- +Every run classified: Easy, Threshold, Interval, Long, Recovery
- +VDOT and pace zones calculated from your race results
- +AI condition assessment: fitness trends, risks, recommendations
Frequently asked questions
Is Intervals.icu free?
Yes, completely free for personal use. There are no paid tiers or premium features behind a paywall. Creator David Tinker maintains it as a community project for athletes.
How does Intervals.icu compare to TrainingPeaks?
TrainingPeaks Premium costs $120/year. Intervals.icu offers comparable analytics for free: PMC chart, HR/pace zones, interval analysis, training calendar. For most recreational and competitive amateur runners, Intervals.icu is more than sufficient.
Can I use Intervals.icu without Strava?
Yes. Intervals.icu connects directly to Garmin, COROS, Polar, and Suunto — no Strava needed. In fact, direct sync preserves more data (lap structure, interval details, HR zone breakdowns) than the Strava route, which strips some fields.
Does Intervals.icu work with all watches?
Direct sync is available for Garmin, COROS, Polar, and Suunto. Apple Watch works through HealthFit or RunGap apps. Other watches (Amazfit, etc.) can connect via Strava or manual FIT/GPX file upload.
Related guides
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