What is a good fatigue index for a runner?

There is no universally "good" value. A balanced runner (0.98–1.03) scales evenly. Elite marathoners often have fatigue indices around 0.96–0.99, reflecting years of endurance-focused training.

How is the fatigue index calculated?

The calculator uses the Riegel formula (T₂ = T₁ × (D₂/D₁)^1.06) to predict your long-race time from your short-race result. Your fatigue index is simply your actual time divided by this predicted time.

Can I use training runs instead of races?

The formula assumes maximal efforts. A hard time trial works, but easy training runs will produce misleading results. Both efforts should be recent and at full effort.

What does speed-biased mean?

A speed-biased profile (FI > 1.03) means you slow down more than the population average as distance increases. You likely have strong neuromuscular speed but need more aerobic development for longer races.

What does endurance-biased mean?

An endurance-biased profile (FI < 0.98) means you outperform predictions at longer distances. You have excellent aerobic endurance and fatigue resistance, but may benefit from speed work at shorter distances.

How often should I re-test my fatigue index?

Every 8–12 weeks, or whenever you race at two different distances. Your fatigue profile shifts as training emphasis changes between endurance blocks and speed phases.

Does the calculator work for distances shorter than 1500m?

The Riegel model is validated for distances from 1500m to marathon. Shorter distances introduce neuromuscular and anaerobic factors that the power-law model does not capture well.

Running Fatigue Calculator — Fatigue Index & Bias

Input Field Guide

Short Race Distance

select

The distance of your shorter race. Choose from standard distances (1500m to marathon) via the dropdown.

Options

1500m1 Mile3K5K10KHalf MarathonMarathon

Pick the distance where you ran your fastest recent effort. It must be shorter than the long race.

Short Race Time

time

Your finish time for the short race in HH:MM:SS format. Must be a recent maximal effort.

Use a race result from the last 4–6 weeks for best accuracy.

Long Race Distance

select

The distance of your longer race. Must be greater than the short race distance.

Options

1500m1 Mile3K5K10KHalf MarathonMarathon

The bigger the gap between short and long distances, the more revealing the fatigue index becomes.

Long Race Time

time

Your finish time for the long race in HH:MM:SS format. Should be from a similar fitness period as the short race.

Ideally from the same training block as the short race — within 6–8 weeks.

Discover Your Running Fatigue Profile

The Running Fatigue Calculator compares your actual race performance to Riegel-predicted times to reveal whether you are speed-biased, endurance-biased, or balanced. Enter two race results at different distances and instantly see your fatigue index — a single number that guides smarter training decisions.

What Is a Running Fatigue Index?

The fatigue index is the ratio of your actual long-race time to the time predicted by the Riegel power-law model (T₂ = T₁ × (D₂/D₁)^1.06). A value below 0.98 means you outperform predictions at longer distances (endurance-biased). A value above 1.03 means you slow more than expected as distance increases (speed-biased). Between 0.98 and 1.03 is balanced. This metric was derived from Riegel's 1981 paper "Athletic Records and Human Endurance" and is used by coaches worldwide to identify training weaknesses.

Why Calculate Your Fatigue Index?

Identify whether you need more endurance work or more speed training
Understand why your race results may not scale linearly across distances
Set realistic race goals based on your individual fatigue profile
Track changes over a training cycle — a moving fatigue index means your training is working
Choose the right race distance for your current fitness profile
Free, instant, private — all calculations run in your browser

Who Uses the Running Fatigue Calculator?

5K specialists targeting a marathon

A speed-biased runner (FI > 1.03) with a 19:00 5K may be disappointed by a 3:45 marathon. The fatigue calculator shows the gap early so training can focus on long runs and tempo work.

Ultrarunners assessing their strengths

An endurance-biased runner (FI < 0.98) beats Riegel predictions at longer distances. They can be confident their marathon will exceed expectations based on shorter PRs.

Coaches designing training blocks

By profiling each athlete's fatigue index, a coach can allocate more volume for speed-biased runners and more intensity for endurance-biased runners.

Runners choosing their next race

If your fatigue index is 0.95, you likely perform better at longer distances. This data point can guide race selection toward half-marathon or marathon events.

Under the Hood

Riegel Power-Law

Uses the exponent k = 1.06 from Riegel (1981) to predict long-race time from short-race time. The formula is T₂ = T₁ × (D₂/D₁)^1.06.

Three-Tier Classification

Fatigue index < 0.98 → endurance-biased. 0.98–1.03 → balanced. > 1.03 → speed-biased. Thresholds based on population distributions from published research.

Browser-Only Computation

All calculations run in your browser using pure TypeScript. No race data is sent to any server.

Privacy

We do not store, transmit, or log any of your race results. The calculator operates entirely client-side.

Example Scenarios

Speed-Biased 5K/Marathon Runner

5K: 22:00, Marathon: 3:55:00. Expected marathon via Riegel: 3:40:23. Fatigue index: 3:55:00 / 3:40:23 = 1.066 → Speed-biased. This runner needs more long-run volume.

FI = 1.066 · Bias: Speed

Balanced 10K/Half Runner

10K: 45:00, Half Marathon: 1:38:30. Expected HM via Riegel: 1:37:48. Fatigue index: 5910 / 5868 = 1.007 → Balanced. Training is well-rounded.

FI = 1.007 · Bias: Balanced

Endurance-Biased Ultrarunner

Half Marathon: 1:35:00, Marathon: 3:15:00. Expected marathon via Riegel: 3:21:12. Fatigue index: 11700 / 12072 = 0.969 → Endurance-biased. Strong aerobic engine.

FI = 0.969 · Bias: Endurance

Research & References

Riegel, P.S. (1981). Athletic Records and Human Endurance. American Scientist, 69(3), 285-290.
Jones, A.M. & Vanhatalo, A. (2017). The Critical Power Concept: Applications to Sports Medicine. Sports Medicine, 47(Suppl 1), 65-78.
Billat, V. et al. (1999). Interval Training at VO₂max: Effects on Aerobic Performance. European Journal of Applied Physiology, 80(5), 465-473.
Noakes, T.D. (2003). Lore of Running (4th ed.). Human Kinetics.
Burnley, M. & Jones, A.M. (2007). Oxygen Uptake Kinetics and the Power-Duration Relationship. European Journal of Sport Science, 7(2), 63-79.

Running Fatigue Calculator

Short Race

Long Race