Free AeT Calculator & Heart Rate Zone Builder for Runners

Stop Guessing Your Training Zones

Most runners train in zones based on “220 minus your age.” That formula is wrong for most people — your actual thresholds could be 20 beats higher or lower. Our free tools analyze your real heart rate data to find your actual aerobic and anaerobic thresholds, then calculate personalized training zones.

Aerobic Threshold Calculator (Cardiac Drift Test)

What it does: This cardiac drift calculator analyzes Pa:HR decoupling during a steady-state run to find your aerobic threshold (AeT) — the ceiling of Zone 2 and the most important number in endurance training.

How it works:

Connect your Strava account and sync your activities
Select a 40+ minute steady-effort run
The tool splits your effort into two halves and calculates heart rate drift percentage
If drift is under 5%, you’ve found your AeT

Why it matters: Your aerobic threshold determines how fast you can run while still burning primarily fat. For ultra and marathon runners, 80-90% of training should be at or below this heart rate. Unlike the MAF 180 formula or age-based calculators, the cardiac drift test uses your actual physiological data.

Try the Aerobic Threshold Calculator

Lactate Threshold Heart Rate (LTHR) Calculator

What it does: This LTHR calculator determines your anaerobic threshold from a hard time trial effort — the intensity above which lactate accumulates faster than you can clear it.

How it works:

Select a 20+ minute all-out effort (race or hard tempo run)
Trim the segment to isolate the hard effort
The tool discards the first 10 minutes of warm-up and averages the remaining heart rate
That average is your LTHR / anaerobic threshold

Why it matters: Your anaerobic threshold sets the upper boundary of sustainable intensity. It defines your tempo and threshold training zones and helps predict race performance.

Try the Anaerobic Threshold Calculator

Heart Rate Training Zone Calculator

What it does: This heart rate zone calculator takes your aerobic and anaerobic thresholds and generates a personalized 4-zone training model based on the Uphill Athlete methodology.

The zones:

Zone	Name	Purpose
Z1	Recovery	Active recovery, very easy effort
Z2	Aerobic	Base building, fat burning, race pace for ultras
Z3	Tempo	Threshold development, comfortably hard
Z4	Above Threshold	High intensity, race-specific for shorter events

Bonus: The tool calculates your AeT-to-AnT gap percentage — a diagnostic that tells you how developed your aerobic base is and what kind of training you should prioritize.

Try the Zone Calculator

How to Get Started

Create a free account at app.readyforultra.com
Connect your Strava to sync your activities
Run your tests — select activities from your history and analyze them
Calculate your zones — the zone calculator auto-fills from your test results

No formulas. No guesswork. Just your real data.

Frequently Asked Questions

What data do I need to use these threshold calculators?

You need activities recorded with a heart rate monitor and synced to Strava. A chest strap or arm band monitor gives the most accurate results. Wrist-based optical sensors can be unreliable during hard efforts.

Are these heart rate zone calculators free?

Yes. The aerobic threshold calculator, anaerobic threshold calculator, and zone builder are completely free for all users. Create an account, connect Strava, and start testing.

How accurate is the cardiac drift test compared to a lab test?

Field tests are highly accurate when done correctly. The cardiac drift test for aerobic threshold is the same protocol used by coaches worldwide. The 20-minute time trial for LTHR is an established method from Joe Friel’s training methodology. Lab tests offer more precision, but field tests get you close enough for effective training.

What is a good cardiac drift percentage?

A cardiac drift (Pa:HR decoupling) of less than 5% indicates that your heart rate is at or below your aerobic threshold. Between 5-10% means you were slightly above AeT. Over 10% means you were well above AeT and should retest at a lower heart rate.

What is the difference between aerobic threshold and lactate threshold?

Aerobic threshold (AeT) is the lower intensity where lactate first begins to rise above baseline — the top of Zone 2. Lactate threshold (AnT/LTHR) is the higher intensity where lactate accumulates faster than it can be cleared. The gap between them indicates aerobic fitness: a smaller gap means better aerobic development.

How often should I retest my aerobic threshold?

Retest your aerobic threshold every 6-8 weeks during base building to track progress. Retest your anaerobic threshold every 8-12 weeks or after a focused intensity block. As your fitness improves, your thresholds should shift.

What is the AeT-to-AnT gap and what does it mean?

The gap between your aerobic and anaerobic thresholds, expressed as a percentage. A gap under 10% indicates a strong aerobic base. A gap over 20% suggests Aerobic Deficiency Syndrome — your aerobic system hasn’t kept up with your anaerobic fitness, and you need more Zone 1-2 training.

How do you calculate heart rate zones from lactate threshold?

Using the Uphill Athlete 4-zone model: Zone 1 (Recovery) is well below your aerobic threshold, Zone 2 (Aerobic) is up to your AeT, Zone 3 (Tempo) is between AeT and AnT, and Zone 4 (Above Threshold) is above your AnT. You need both your AeT and AnT to calculate all four zones accurately.

Heart Rate Zone Testing for Mountain Runners — The full protocol for running your own threshold tests
Mountain Running Training Principles — How to apply your zones to a structured training plan
Race Time Predictor — Estimate your finish time with segment-based pacing