Lactate Threshold Calculator
Estimate LT1 and LT2 from a 20-minute cycling power test, running HRmax, or both.
Returns FTP, threshold heart rate range, and full training-zone breakdown.
Lactate threshold is the most useful single number in endurance training, and one of the most over-mystified. The body produces lactate constantly while exercising; lactate threshold is the intensity at which production starts to outpace clearance, so blood lactate rises sharply. Below it, you can hold the effort for hours. Above it, you can hold it for minutes.
Two thresholds, not one:
- LT1 (first lactate threshold, aerobic threshold) — blood lactate first nudges above resting, around 2 mmol/L. The upper boundary of truly easy aerobic work. Below LT1 you can talk in full sentences.
- LT2 (second lactate threshold, anaerobic threshold, OBLA) — sharp rise begins, around 4 mmol/L. The intensity you can sustain for roughly 30 to 60 minutes flat out. Above LT2 you stop being able to talk; fatigue compounds.
Both are real physiological inflections. A lab test with finger-prick lactate samples taken during a graded exercise test gives the most accurate values. Without a lab, two field tests give estimates good enough for training.
Field test 1, cycling 20-minute power:
FTP (Functional Threshold Power) is the standard cyclist proxy for LT2. The test: warm up thoroughly, then ride as hard as you can sustain for 20 minutes, recording average power. Take 95% of that average and call it FTP. FTP correlates well with LT2 in most riders.
Field test 2, running heart rate from HRmax:
LT2 heart rate sits at roughly 85% to 92% of maximum heart rate for trained athletes. Less-trained runners tend to land near the lower end of that range. The simplest HRmax estimate is 220 minus age, but this method has a standard deviation of about 10 beats, so a tested HRmax is much better when you can get one (run uphill repeats until you cannot go any harder, watch the peak).
LT1 typically falls 10 to 12 beats below LT2 for runners, which corresponds to roughly 70 to 75% of HRmax. This is the upper edge of the “easy zone” where conversation is still possible.
Why the zones matter:
Endurance training works through dose-response across these zones. The polarized model says: spend about 80% of training time below LT1 and about 20% near or above LT2, with very little in the gray zone between. This is what produces consistent long-term improvement; sustained training in the middle zone is the classic recipe for stagnation. Tempo runs, threshold intervals, and sweet-spot rides (just below FTP) all aim to push LT2 higher; long easy runs and Z2 cycling build the aerobic base under LT1.
Coggan power zones (cycling, % of FTP):
| Zone | % FTP | Description |
|---|---|---|
| Z1 Active recovery | < 55% | Coffee-ride pace |
| Z2 Endurance | 56-75% | All-day pace, base building |
| Z3 Tempo | 76-90% | Sustainable for ~2 hours |
| Z4 Threshold | 91-105% | LT2 territory; 20-60 min |
| Z5 VO2max | 106-120% | 3-8 min intervals |
| Z6 Anaerobic | 121-150% | 30 s-2 min max efforts |
| Z7 Neuromuscular | > 150% | Sprints, < 30 s |
5-zone heart-rate model (running, % of HRmax):
| Zone | % HRmax | Description |
|---|---|---|
| Z1 | 50-60% | Recovery walks/jogs |
| Z2 | 60-70% | Easy aerobic, conversational |
| Z3 | 70-80% | Steady-state, includes LT1 |
| Z4 | 80-90% | Threshold, includes LT2 |
| Z5 | 90-100% | Max effort intervals |
Worked example, cycling:
A rider averages 280 W for a 20-minute all-out test. FTP = 0.95 × 280 = 266 W. So Z4 threshold work sits between 0.91 × 266 = 242 W and 1.05 × 266 = 279 W. Sweet-spot rides (88-94% FTP, around 234-250 W) target the most adaptive stimulus for raising FTP without huge fatigue.
Worked example, running:
A 32-year-old runner has a tested HRmax of 188. LT2 HR sits at 0.85 × 188 = 160 to 0.92 × 188 = 173 bpm. So a tempo run target heart rate is roughly 165-170 bpm. LT1 is around 0.72 × 188 = 135, which is the cap on truly easy runs.
What this calculator does not do:
It does not measure your actual lactate (only a finger-prick test can). It does not predict race times (use a race-pace calculator instead). It does not account for heat, altitude, fatigue, or training state, all of which shift thresholds by several beats or watts on any given day. Use the output as a target to train toward, not a number to chase to the decimal.