[CALCULATOR]
One number, and every training zone falls out of it.
[WHAT THIS DOES]
Threshold pace is roughly the fastest pace you could hold for an hour, and every other training zone is a percentage of it. Enter a recent sustained hard effort of at least twenty minutes and this projects it to a one-hour equivalent using the Riegel formula, then prints the five bands Base prescribes sessions in.
What threshold actually is
Threshold is the dividing line between work you can sustain and work that accumulates fatigue faster than you clear it. Physiologically it tracks the intensity at which blood lactate starts accumulating faster than it is cleared. Practically it is close enough to "the fastest pace you could hold for about an hour" that the two are used interchangeably in training plans.
It is the only endurance number worth building a plan on, for one reason: it is trainable, it is measurable from an ordinary hard session, and every prescription is a percentage of it. VO2 max is a ceiling you mostly cannot move and that a wrist device estimates with wide error. Threshold moves in weeks, and you can measure it by running hard for twenty minutes.
Why a 20-minute effort is not your threshold pace
If you run hard for twenty minutes you will average a pace you could not hold for sixty. Using it directly would set every zone too fast, and the specific failure mode is that your easy days stop being easy — which is the single most common reason a training block stops working.
The correction is the Riegel formula, which relates performance at two distances with an exponent of about 1.06:
t2 = t1 * (d2 / d1) ^ 1.06Solved for the distance you would cover in an hour at the same relative effort, then converted back to a pace. For a 20-minute effort the correction is roughly 4–6% — small enough to sound ignorable, large enough that ignoring it puts your easy runs a full zone too fast.
The bands
| Zone | Multiple of threshold pace | What it is for |
|---|---|---|
| Easy | 1.20–1.35× | Aerobic base. Deliberately wide, because the only error that matters is going too hard |
| Long | 1.15–1.30× | The same adaptation, accumulated for longer |
| Tempo | 1.03–1.09× | Comfortably hard, sustainable for 40–60 minutes |
| Threshold | 0.98–1.03× | On the line, in reps or a continuous 20–40 minutes |
| Intervals | 0.88–0.96× | 3k–5k pace, in reps with full-ish recovery |
A higher multiple of a pace is a slower pace, which is the one counterintuitive thing about working in pace rather than power. Easy at 1.35× threshold is much slower than threshold, not faster.
Why hybrid athletes get the easy zone wrong more than anyone
A lifter coming to endurance work brings a useful instinct — that hard work is what produces adaptation — and applies it to a domain where it is actively counterproductive. Easy running produces its adaptation through volume, and running it at tempo pace converts a session that would have cost you almost nothing in recovery into one that competes directly with tomorrow's squats.
This is the specific mechanism by which the interference effect shows up in practice for people who insist it is not real. The research says the effect is small when sessions are separated and volume is moderate. It does not say that running every easy day at threshold is free.
What to do next
Run the effort, get the number, then check the next four easy runs against the easy band. If they are consistently faster than the slow end of it, the problem is not the calculator.
Questions
How do I find my threshold pace without a lab test?
Run as hard as you can sustain for a flat 20 to 30 minutes, on a track or a flat road, and record the distance. Project that to a one-hour equivalent with the Riegel formula. The result is within a few percent of what a lactate test would give you, and unlike a lactate test you can repeat it every six weeks.
How fast should my easy runs be?
Between 1.20 and 1.35 times your threshold pace — so if your threshold is 7:00 per mile, easy is 8:24 to 9:27. That range is deliberately wide, because the only error that matters on an easy day is running it too hard. Most runners run their easy days 20 to 40 seconds per mile too fast.
Should I use pace or heart rate?
On easy and long days, heart rate. Pace drifts several percent with heat, hills and fatigue while heart rate reflects the actual physiological cost. On interval days, pace or power — heart rate lags the effort by 30 to 60 seconds and is useless for a three-minute rep.
How often should I retest my threshold?
Every four to six weeks during a build, or whenever several sessions in a row grade as systematically too easy or too hard. If the bands feel wrong across multiple sessions the threshold behind them is wrong, and fixing that is better than fighting the zones.