What is Critical Power in running?

Critical Power (CP) is the highest power output you can sustain for a long duration without fatiguing — roughly your one-hour power. It is the running equivalent of a cyclist's FTP and anchors all five power zones as percentages of itself.

Why use power instead of pace?

Power responds instantly to hills, wind, and surface, while pace lags and is distorted by terrain. Holding a steady power target keeps your physiological effort constant whether you are climbing or descending, which pace alone cannot do.

What percentages define the zones?

This tool uses a common five-zone model: Zone 1 below 80 percent of CP, Zone 2 from 80 to 90, Zone 3 from 90 to 100, Zone 4 from 100 to 115, and Zone 5 above 115 percent. These bands mirror the widely used Stryd power zone framework.

Is running power comparable to cycling power?

No. Running power meters estimate the metabolic cost of running and read lower than your cycling power for the same effort. Never compare the raw watt numbers across sports — only the zones relative to each sport's own threshold are meaningful.

How often should I retest my CP?

Retest every four to six weeks of consistent training, or after a clear fitness change. As your CP rises, all five zones shift up with it, so stale CP values will make your zones too easy.

What is the Running Power Zone Calculator?

Enter Critical Power (CP) in watts from a Stryd or Garmin run to generate all 5 running power zones — an alternative to pace zones that adjusts automatically for hills and wind. It runs free in your browser on Gera Tools, with nothing uploaded.

Running Power Zone Calculator

Name: Running Power Zone Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Running with power instead of pace lets your training intensity stay honest on hills, into headwinds, and on soft ground. This calculator takes your Critical Power and splits it into the five standard training zones so you can run by watts without second-guessing the terrain.

How it works

Every zone is a percentage band of your Critical Power (CP):

Zone 1  Recovery / easy      < 80% of CP
Zone 2  Endurance            80% – 90% of CP
Zone 3  Tempo                90% – 100% of CP
Zone 4  Threshold / interval 100% – 115% of CP
Zone 5  Anaerobic / sprint   > 115% of CP

CP itself sits right at the top of Zone 3, the boundary between sustainable aerobic work and efforts that accumulate fatigue. Because power reflects the mechanical work you are actually doing, a zone target holds the same physiological demand regardless of gradient.

Worked example

With a CP of 280 watts:

Zone	Purpose	Watt range
Zone 1	Recovery / easy	below 224 W
Zone 2	Endurance base	224 – 252 W
Zone 3	Tempo / marathon pace	252 – 280 W
Zone 4	Threshold / intervals	280 – 322 W
Zone 5	Anaerobic / sprints	above 322 W

Zone 2 endurance runs (224–252 W) represent the aerobic base where most training volume should accumulate. Threshold intervals at 280–322 W are short, structured efforts — typically 5–20 minute repeats — that specifically raise CP over time.

Why power outperforms pace for training control

Pace responds to terrain with a lag. On a 10% incline, maintaining your flat-road marathon pace is a vastly harder physiological effort; maintaining your marathon- equivalent power is the same effort. A power meter captures this instantly because it measures the mechanical cost of movement directly.

The practical result:

Hills: power holds the intended zone automatically. Pace drops on climbs; that is fine — the effort is equivalent.
Headwinds: a 20 kph headwind can cost the same effort as a moderate hill. Pace would tell you nothing; power shows the cost.
Soft surfaces: sand, mud, and trail add rolling resistance that pace ignores. Power accounts for the extra work.

Finding your Critical Power

Your CP comes from a maximal test or from your Stryd or Garmin running power app’s auto-detected CP after a period of structured training. A rough estimate is your best sustainable power over 30–60 minutes of hard running. Retest every 4–6 weeks of consistent training, or after any meaningful fitness change, because all five zones scale with CP — stale values make easy runs too easy and hard runs not hard enough.

How to use the zones

Zone 1–2: 80% or more of weekly training volume; easy enough to speak in sentences; where aerobic adaptation happens over months.
Zone 3: marathon-specific training runs; comfortably hard, not conversational.
Zone 4: interval sessions, 5 km race effort; accumulate fatigue quickly.
Zone 5: sprint repeats and strides; short duration, full recovery between.

Power readings over short efforts (under 30 seconds) are noisy due to measurement lag. Average over at least 30 seconds before judging which zone an interval sits in.