AeT Test: Find Your Aerobic Threshold Heart Rate
Free AeT test protocol and calculator. Find your aerobic threshold with the heart rate drift test, calculate your AnT with a time trial, and build personalized training zones.
Want to skip the math? Our free threshold calculators analyze your Strava activities automatically. Try the Aerobic Threshold Calculator or the Anaerobic Threshold Calculator — no spreadsheet required.
Why Bother Testing?
Most training plans tell you to run in “Zone 2.” But Zone 2 of what? If your zones are based on 220-minus-your-age, they’re probably wrong. Your actual aerobic threshold could be 20 beats higher or lower than what that formula predicts.
Two numbers anchor your entire training:
- Aerobic Threshold (AeT) — the top of Zone 2. The highest intensity where you’re still burning mostly fat and can sustain the effort for hours.
- Anaerobic Threshold (AnT) — the top of Zone 3. The point where lactate builds faster than you can clear it. You can hold this intensity for maybe 30-60 minutes before it catches up with you.
Everything else—your zones, your training plan, which days should be easy and which should be hard—falls out of knowing these two numbers.
The Zone System
| Zone | What’s Happening | How It Feels |
|---|---|---|
| 1 | Recovery | Very easy. Could talk on the phone and no one would know you’re running. |
| 2 | Aerobic Base | Easy. Full sentences. This is where most of your training lives. |
| 3 | Tempo | Comfortably hard. Sentences get shorter. |
| 4 | Threshold | Hard. You’re counting minutes until it’s over. |
| 5 | Anaerobic | Very hard. Seconds to minutes only. |
For ultra training, you’ll spend 80-90% of your time in Zones 1-2. The rest is strategic.
Finding Your Aerobic Threshold
The Heart Rate Drift Test is free, simple, and you can do it yourself.
What You Need
- A heart rate monitor — chest strap is best for accuracy. (I’ve personally been scarred from chest straps and use one that goes on my bicep with good results.)
- A GPS watch or app that records heart rate over time
- Flat terrain or a treadmill
The Test
- Warm up 15 minutes at an easy jog.
- Run 60 minutes at a steady pace. Pick a heart rate you think might be near your AeT and try to hold it constant. Start conservative.
- Split the data. Average heart rate for minutes 0-30, then for minutes 31-60.
- Calculate the drift: (Second Half HR - First Half HR) / First Half HR × 100
Reading the Results
| Drift | What It Means |
|---|---|
| Under 5% | You found it. That heart rate is at or below your AeT. |
| 5-10% | Slightly above AeT. Test again a few beats lower. |
| Over 10% | Well above AeT. Drop your target HR and retest. |
Example: First half average 142 bpm, second half 149 bpm. Drift = (149-142)/142 × 100 = 4.9%. Your AeT is around 142.
Do it automatically: Our free Aerobic Threshold Calculator does this analysis for you. Connect Strava, pick the activity, and get your AeT in seconds.
Get a Clean Test
- Be well-rested. Don’t test after hard training days.
- Stay hydrated. Dehydration inflates heart rate.
- Control temperature. Heat adds drift that has nothing to do with fitness.
- Test in the same conditions each time so you can compare results.
Finding Your Anaerobic Threshold
A 30-minute time trial gives you a solid estimate.
- Warm up 15-20 minutes with a few short pickups.
- Run the hardest pace you can sustain for 30 minutes. This should hurt.
- Your average heart rate from the last 20 minutes is approximately your AnT. (Ignore the first 10—HR is still climbing to steady state.)
If you want precision, a blood lactate test at a sports performance lab will give you exact numbers. But the time trial gets you close enough for training purposes.
Automate it: Use our free Anaerobic Threshold Calculator to analyze any 20+ minute hard effort from your Strava history.
The Gap That Tells You Everything
Once you have both numbers, calculate the gap:
Gap = (AnT - AeT) / AeT × 100
| Gap | What It Means | What to Do |
|---|---|---|
| Under 10% | Strong aerobic base | You can handle more intensity |
| 10-20% | Room to improve | Prioritize easy running |
| Over 20% | Aerobic Deficiency Syndrome | Almost all training should be Zone 1-2 |
Example: AeT of 138, AnT of 168. Gap = (168-138)/138 = 21.7%. This runner needs to spend months building their aerobic base below 138 bpm before adding any hard work.
Aerobic Deficiency Syndrome (ADS)
A big gap means your aerobic system hasn’t kept up with your anaerobic fitness. Common in people coming from CrossFit, HIIT, or team sports. Also common in runners who do too many hard workouts and not enough easy volume.
The fix is boring but it works: run below your AeT for months. Your easy pace will feel painfully slow at first. Over time, your AeT rises, the gap closes, and you’ll be running faster at the same heart rate. There’s no shortcut.
Retest Regularly
| Test | How Often |
|---|---|
| AeT (drift test) | Every 6-8 weeks during base building |
| AnT (time trial) | Every 8-12 weeks, or after an intensity block |
You’re looking for your AeT to rise over time. That means your aerobic engine is getting bigger.
Applying Zones to Your Training
During Base Building
- 85-90% below AeT (Zones 1-2)
- 10-15% Zone 3 (only if your gap is under 10%)
- 0-5% Zone 4-5
During Build/Peak
- 75-80% below AeT
- 15-20% Zone 3
- 5% Zone 4
Why so much easy running?
Because your Zone 2 is your 100-mile race pace. Every easy mile is race-specific training. Every easy mile builds mitochondria, capillaries, fat-burning capacity, and cardiac efficiency. You can’t replace this with intervals.
Common Mistakes
Using formula-based zones. “220 minus age” was never meant to set individual training zones. Test yourself.
Running “easy” too fast. If you can’t hold a conversation, slow down. This is harder than it sounds.
Adding intensity before you’re ready. Close your AeT-AnT gap before adding Zone 3 work. Intensity on a weak aerobic base is a waste.
Testing in bad conditions. Heat, dehydration, altitude, and fatigue all inflate heart rate. Control what you can.
Ignoring the data. If the test says your AeT is 135, train at 135—even when your ego says you should be faster.
Calculate Your Zones Automatically
Done testing? Plug your AeT and AnT into our free Heart Rate Zone Calculator to generate your personalized 4-zone training model. If you’ve already run the threshold tests in the app, your values are pre-filled automatically.
Related Pages
- Free Threshold Calculator & Zone Builder — All three tools in one place
- Mountain Running Training Principles
- Crazy Mountain 100: 20-Week Training Plan
- Sleep Optimization for Ultramarathon Training
Recommended Reading
- Training for the Uphill Athlete by Steve House, Scott Johnston, and Kilian Jornet
- 80/20 Running by Matt Fitzgerald
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