Running-time estimator (predict across distances)

What it does

The running-time estimator takes one known race or training result (a distance + time) and projects times across all the other standard race distances, using three fatigue tiers based on the Riegel exponent:

• Perfect (exponent 1.03) — well-trained, fully tapered, flat course, friendly conditions.
• Normal (exponent 1.06) — typical Riegel default.
• Poor (exponent 1.12) — under-prepared, hot day, hilly, or stepping up dramatically in distance.

URL: /calculators/ then open Running time. Direct path: /static/calculators/running_time_estimator.html.

Inputs the form asks for

• Known distance (select). Preset distances only — 5K, 10K, half marathon, marathon, and several intermediate options. There is no custom-distance input.
• Known time (HH:MM:SS).

Pace fields auto-derive from distance + time and display in both MM:SS per km and MM:SS per mile for cross-check.

Outputs

The output is a matrix — every standard distance across the columns, the three fatigue tiers (Perfect / Normal / Poor) down the rows. Each cell shows the predicted time at that distance under that fatigue regime.

Click any cell to lock that target:

• The selected distance, time, and pace populate the Race target fields (rpDistance, rpFinishTime, rpPace) at the bottom of the form.
• The pace becomes the suggested goal pace for race day.

Save-back option:

• Use selected pace as run threshold checkbox — when ticked, saving the result writes the locked pace back to the athlete's run_threshold_pace in MetricHistory. Useful when a recent race result is the cleanest read on threshold and you want zones to update accordingly.

How to use it

1. Enter your most recent honest race or all-out training result. (Don't use a workout that wasn't all-out — the exponents calibrate against maximal efforts.)
2. Scan the matrix. The Normal row is the default prediction for the next race at a different distance.
3. If your training has been disrupted (illness, low volume, heat), shift one row down toward Poor; if you've peaked perfectly, shift up toward Perfect.
4. Click the cell at your target race distance to lock it as the goal pace.
5. If you want zones to update to this result, tick the "use selected pace as run threshold" save-back.

When NOT to use it

• The Riegel formula is least accurate at extreme distance ratios. Predicting marathon time from a 5K result has ±10% noise even at the right fatigue tier.
• Predictions assume you're running on a flat course in reasonable conditions. Hilly courses or extreme heat shift the realistic tier down by one row.

See also: Critical Power calculator — analogous power-duration model for the bike, Finish-time estimator.