How does Base estimate my threshold pace?

From your best sustained effort of at least twenty minutes in the window, projected to a one-hour equivalent using the Riegel formula with an exponent of 1.06. That projection matters: a twenty-minute effort is genuinely faster than hour pace, and using its raw pace would set every zone too fast.

How is FTP estimated without a test?

The classic field test generalised. Average power for the effort is scaled by how long the effort already lasted — 95 percent at twenty minutes, rising linearly to 100 percent at fifty minutes and holding there. A flat 95 percent discount is correct for a twenty-minute test and underestimates a forty-five-minute climb.

What does the confidence label mean?

How much weight the estimate will bear, based on how recent the effort was and how long it lasted. Low means the estimate is old or the effort was short, and every band built on it should be read accordingly. A manual entry from a recent race or a proper test always overrides an inferred number.

How often should I retest?

Every four to six weeks during a build, or when several sessions in a row grade as systematically wrong. If the bands feel off across multiple sessions the threshold behind them is wrong, and fixing that beats fighting the zones.

Why does Base keep run and ride separate?

They are different units measuring different capacities. Threshold pace is seconds per mile and FTP is watts, and no meaningful conversion exists between them for an individual. A combined endurance score could not answer either question, which is the same reason strength and endurance are never pooled.

Thresholds: One Number Per Discipline, With Provenance

What threshold is

Roughly the hardest effort you could hold for about an hour. Physiologically it tracks the intensity at which blood lactate accumulates faster than it is cleared; practically it is the dividing line between work you can sustain and work that builds fatigue faster than you clear it.

It is the only endurance number worth building a plan on, for three reasons: it is trainable, unlike VO2 max which is largely a ceiling; it is measurable from an ordinary hard session, requiring no lab and no blood; and every prescription is a fraction of it, so one estimate produces the whole plan.

The last property is also why it needs provenance. A wrong threshold does not produce one wrong session. It produces every session wrong, in the same direction, for as long as it stands.

The running estimate

Base takes your best sustained effort of at least twenty minutes and projects it to a one-hour equivalent using Riegel's formula:

t2 = t1 * (d2 / d1) ^ 1.06

Solved for the distance you would cover in an hour at the same relative effort, then converted back to a pace.

The projection is not optional. If you run hard for twenty minutes you average a pace you could not hold for sixty. Using it raw would set every zone too fast — and the damage lands hardest on the easy band, which is precisely where being too fast is most expensive. The correction is roughly 4 to 6% for a twenty-minute effort: small enough to sound ignorable, large enough that ignoring it puts your easy runs a full zone too hard.

Twenty minutes is a hard floor. Below it the projection extrapolates so far that the result describes the formula rather than you.

The cycling estimate

The classic field test, generalised:

factor = min(1, 0.95 + (minutes - 20) * (0.05 / 30))
ftp    = avgWatts * factor

95% at twenty minutes, rising linearly to 100% at fifty minutes and holding there.

The generalisation matters because the familiar "take 95% of your twenty-minute power" rule was written for a twenty-minute test specifically, and people apply it to everything. A forty-five-minute climb is much closer to FTP than a twenty-minute effort, and discounting it by 5% throws away a usable data point.

The confidence label

Every estimate carries high, medium or low, based on how recent the effort was and how long it lasted.

Confidence	Typically means	How to read the bands
High	Recent, and 30+ minutes	Trust them
Medium	Several weeks old, or a 20–30 minute effort	Usable; retest soon
Low	Old, or barely over the 20-minute floor	Directional only. Get a real effort in

A number without provenance is a number the athlete has no way to check, and every band on the endurance screen is downstream of this one estimate. Showing it bare would be asking for trust that has not been earned.

Manual override always wins

If you have a recent race result or a properly executed test, enter it. That number is better than anything inferred from training data, and Base does not argue with it.

The reason inference exists at all is that most people do not test. An estimate from a hard Tuesday is much better than no zones, and much better than zones from a threshold you guessed at in 2023.

Run and ride never mix

Threshold pace is seconds per mile. FTP is watts. There is no meaningful conversion between them for an individual — the relationship depends on running economy, cycling efficiency, body composition and training history, and it differs enough between people that any general formula is a fiction.

More importantly, a combined "endurance score" could not answer either question. If it moves, was your running better or your cycling? The same reasoning that keeps strength and endurance from being pooled applies one level down.

A triathlete gets two separate numbers, and that is the correct shape of the answer.

Heart rate as the cross-check

Easy and long runs also carry a heart-rate band at 60 to 70% of your age-estimated maximum.

Pace drifts several percent with heat, hills, wind and accumulated fatigue. Heart rate reflects the actual physiological cost. On easy days, obey heart rate; on interval days, obey pace or power — heart rate lags the effort by thirty to sixty seconds and is useless for a three-minute rep.

If there is no birth year on file there is no honest maximum to take a percentage of, so no heart-rate zones are shown at all rather than an age being assumed. That is the same rule that governs every missing input in the app.

When to suspect the threshold rather than yourself

If sessions grade systematically wrong across several weeks — repeatedly "under" on intervals, or repeatedly "over" on easy days despite genuine effort to hold back — the threshold behind the bands is wrong.

Fixing the estimate beats fighting the zones. The usual causes:

The estimate came from an effort that was not representative — a race-pace parkrun when you were fresh, or a tempo run in 90°F heat.
Fitness has moved. Threshold changes in weeks.
The effort was on a course that flattered or punished you.

Retest every four to six weeks during a build, and stop testing between times. A test day is not a training day, and testing every three weeks means a sixth of your training is not training.

What to do next

The endurance screen is where this number becomes a prescription and a grade. The threshold pace calculator runs the same arithmetic on the web if you want to check it.