Free lactate threshold heart rate (LTHR) calculator. Find your threshold heart rate from a 30-minute time trial or max HR and build Friel-style running training zones.
How runners use this tool
A lactate threshold heart rate calculator anchors your training zones to the effort you can hold for roughly an hour, which is more useful than age-based percentages once you can field-test it.
If you have done a 30-minute solo time trial, enter your average heart rate from the final 20 minutes as your lactate threshold heart rate.
If you have not tested yet, enter your maximum heart rate (or use an age estimate) and the calculator will approximate LTHR for you.
Use the Friel-style zone table to anchor recovery, endurance, tempo, sub-threshold, and threshold sessions.
How to use the result
Run most easy mileage below 90% of LTHR so hard days stay genuinely hard and easy days stay easy.
Use the threshold and sub-threshold bands for tempo and cruise-interval work, then convert effort into pace with the training pace calculator.
Re-test your LTHR every few months, because it drifts as fitness changes and stale zones drift with it.
Formula and assumptions
Field test: LTHR = average heart rate over the final 20 minutes of an all-out 30-minute time trial. Estimate: LTHR ≈ 88% of maximum heart rate. Zones are percentages of LTHR (Joe Friel running zones).
A field-tested LTHR is far more reliable than an estimate derived from maximum heart rate.
Heart rate is affected by heat, fatigue, caffeine, and altitude, so treat zone edges as ranges rather than hard cutoffs.
Worked examples
Beginner: Estimate from max HR of 190 → LTHR around 167 bpm with Friel zones. Gives a usable threshold anchor without a hard test.
Race prep: 30-minute time trial, last-20-min average 172 → LTHR 172 bpm and seven training zones. Anchors tempo and threshold sessions to real data.
Advanced: Re-test LTHR after a training block → Updated zones as fitness improves. Keeps zones honest as threshold heart rate drifts.