Lactate Threshold Calculator — LTHR From a 30-Minute Test

The Protocol

Joe Friel's field test, and it is thirty uncomfortable minutes:

1. Warm up for 10 to 15 minutes. 2. Go hard and steady for 30 minutes, alone. 3. Press lap at exactly 10 minutes. 4. Enter the average heart rate for the final 20 minutes only.

The first ten minutes are discarded because heart rate lags the effort at the start. Include them and the average is dragged down by the period when your heart had not caught up with what your legs were doing.

Three things that invalidate the test:

Pacing it wrong. Go out too hard and fade, and the average is inflated by the first few minutes. Hold back and never reach threshold, and the number is not your LTHR, it is just a heart rate you held for half an hour.

Company. A group effort becomes a race, and the pacing goes. Solo.

Terrain. Flat and uninterrupted. Hills, junctions and traffic lights all put steps in a measurement that needs to be steady.

Why Not a Percentage of Maximum

Because the percentage route needs a maximum, and a predicted maximum is the weak link in the whole chain.

Friel's own position on 220 minus age is that it is as likely to be wrong as right. The measurement literature agrees: prediction models for maximum heart rate offer limited precision, and the error is individual rather than systematic, so you cannot correct for it.

One study of 165 recreational runners put the mean absolute error of estimating the second lactate threshold at 2.8 to 5.2 bpm from heart rate — and found that estimating the maximum rather than measuring it made those errors larger. Thirty minutes of work is a real cost. It is still cheaper than training in the wrong zones for a season.

This calculator does accept a maximum heart rate, and it does not let it near the zones. It is used for exactly one thing: showing you the LTHR-to-maximum ratio, so you can sanity-check a test result against your training status. Well-trained athletes typically sit at 85 to 92% of maximum, less-trained ones lower. If your test puts you at 97%, something is wrong with one of the two numbers — and this page will say so.

LT1, LT2, and Which One This Is

"Lactate threshold" names two different things, and conflating them is the most common confusion in this topic.

LT1, the aerobic threshold, is where blood lactate first rises above resting. It sits low — around the top of easy running — and it is the boundary that matters for the volume of base training.

LT2, variously called the anaerobic threshold, maximal lactate steady state, or in cycling terms functional threshold, is the highest intensity at which lactate production and clearance stay balanced. Above it, lactate accumulates and the clock starts.

A 30-minute maximal effort lands at LT2. That is what this test measures, that is what Friel's zones are anchored to, and that is what "threshold pace" means in a training plan. If you see a lactate threshold figure that sits around 65 to 75% of maximum heart rate, it is LT1 and it is not comparable to this.

Friel's Zones, and Why Running and Cycling Differ

All percentages are of LTHR, not of maximum heart rate. This is the single most consequential detail on the page. Watches and platforms differ in what they expect: enter LTHR-based percentages into a system built around %HRmax and every zone lands in the wrong place, usually one zone too hard.

The first four zones differ between the sports because heart rate at a given relative intensity differs between them — a cyclist is seated, supported, and using less muscle mass than a runner. The top three zones are identical, which is a reasonable place for the distinction to stop mattering.

Your LTHR Is Sport-Specific Too

Test on the bike, use the cycling zones. Test running, use the running zones. Do not test one and apply it to the other: threshold heart rate is typically several beats lower on the bike than running for the same athlete, and the difference is larger than the width of a zone.

If you do both sports seriously, that means two tests.

Retesting

Threshold heart rate moves less than threshold pace or threshold power. Fitness gains usually show up as more speed at the same heart rate, not as a higher LTHR — which is why pace or power is the better progress metric and heart rate the better intensity control.

Retest every two to three months, or after any block that changes your aerobic base. Also retest if the zones start feeling wrong: an LTHR that no longer matches the effort is usually a sign of accumulated fatigue, illness, or heat acclimation rather than a change in threshold.

One thing that will corrupt a retest: heat. Heart rate rises for the same effort as temperature climbs, so a summer test and a winter test are not comparing the same thing. The heat-adjusted pace calculator covers what heat does to racing; for testing, just do it in similar conditions each time.

Where This Sits Against the Other Tools Here

The heart rate zone calculator covers the %HRmax and Karvonen routes, which are what to use when you have not done a threshold test. The max heart rate calculator covers the prediction equations and their error bars. And maximal aerobic speed is the ceiling rather than the boundary — a different anchor, measured from a different test.