Lactate Threshold Heart Rate Calculator

About this calculator

Lactate threshold heart rate (LTHR) is the heart rate at which lactate starts accumulating in the blood faster than the body can clear it — roughly the highest intensity you could sustain for about an hour. Coach Joe Friel popularized estimating it from a field test rather than a lab: a 30-minute all-out time trial (averaging the heart rate over the final 20 minutes) or a 60-minute race effort, and from that single number Friel derives five training zones, from easy recovery through threshold to anaerobic-capacity work.

Runners and cyclists use LTHR-based zones as the backbone of structured training plans because, unlike percent-of-max-heart-rate zones, they are anchored to a physiological marker that shifts as fitness improves, making them more precise for prescribing easy, tempo, threshold and interval efforts. It is a staple of TrainingPeaks-style periodization and of cycling and running plans built around Friel's methodology.

This calculator takes your time-trial or race heart rate data (or your max heart rate as a fallback) and returns your estimated LTHR along with Friel's five heart-rate training zones built on top of it.