How accurate are the predictions?

For well-paced races and similar conditions, predictions are often within a few percent. Terrain, weather, fueling, and fitness changes can shift results.

What exponent k should I use?

Start with 1.06 for distances from 3K to marathon. Faster, speed-oriented runners may use 1.03–1.05; endurance-strong runners might use 1.06–1.10.

Does the tool account for hills or heat?

Use the Conditions Adjustment to apply a percentage slowdown or speedup for heat, humidity, hills, altitude, or taper.

Can I enter custom distances in miles or kilometers?

Yes. Choose Custom and set your preferred units. The predictor converts everything internally for consistent calculations.

Who created this tool?

The tool was developed by Ugo Candido and reviewed for accuracy and accessibility to WCAG 2.1 AA.

Running Time Predictor

Q: Which formula does this predictor use?

This predictor uses the Riegel performance model: T2 = T1 × (D2/D1)^k, with k defaulting to 1.06 based on peer‑reviewed research.

Q: Will I get a pace per mile and per kilometer?

Yes. The results include pace per km and per mile, plus speed in km/h and mph.

Running Time Predictor: estimate your finish time, pace, and speed for any race distance using the evidence-based Riegel performance model. Mobile-first, accessible, and fast.

Running Time Predictor

Estimate your finish time, pace, and speed for any race distance using a recent performance. Built on the peer‑reviewed Riegel model and optimized for accuracy, accessibility, and speed.

Units

Kilometers Miles

Recent race distance *

Recent race time (HH:MM:SS) *

Target race distance *

Riegel exponent k (1.00–1.15) *

Conditions adjustment (−20% to +20%)

Results

Predicted finish time

—

Average pace (per km)

—

Average pace (per mile)

—

Average speed

—

Equivalent times across popular races

Based on your recent race and chosen exponent k.

Distance	Predicted Time	Pace/km	Pace/mi
Enter your recent race to see equivalents.

Data Source and Methodology

Authoritative Data Source: Peter S. Riegel (1981), “Athletic Records and Human Endurance,” American Scientist, 69(3), 285–290. Stable reference: JSTOR 27850761. Access summary: https://sporttracks.mobi/labs/race-finish-time-predictor and the original publication where available.
All calculations are derived from the Riegel model: performance scales with distance by a power law using exponent k.

Read the American Scientist reference

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

Riegel model:

$$ T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k $$

Optional conditions adjustment:

$$ T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right) $$

Pace and speed:

$$ \text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]} $$

Glossary of Variables

T1: Your recent race finish time.
D1: Your recent race distance.
D2: Your target race distance.
k: The Riegel exponent (performance decay factor with distance).
A: Conditions adjustment in percent (−20 to +20).
T2: Predicted finish time for the target distance.
T2,adj: Predicted time after applying conditions adjustment.
Pace per km / mile: Average time to cover one kilometer / one mile.
Speed: Average speed expressed in km/h and mph.

How It Works: A Step-by-Step Example

Come Funziona: Un Esempio Passo-Passo

Suppose your recent 10K time is T1 = 00:45:00 and you want to predict a half marathon (D2 = 21.0975 km), using k = 1.06.

Given:
  D1 = 10 km
  T1 = 45 min = 2700 s
  D2 = 21.0975 km
  k  = 1.06

Compute:
  $$ T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05 $$

Pace:
  $$ \text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km $$
  $$ \text{Pace}_{mi} \approx 7:24/mi $$

If race day is hot (+5%), apply A = +5:

  $$ T_{2,adj} = 5825 \times \left(1 + \frac{5}{100}\right) \approx 6116\;s \approx 01:41:56 $$

Frequently Asked Questions (FAQ)

Which formula does this predictor use?

It uses the peer‑reviewed Riegel model: T2 = T1 × (D2/D1)^k. See Data Source and Methodology for the reference.

What exponent k should I choose?

Default to 1.06. Choose 1.03–1.05 if you have strong speed over short distances; choose 1.06–1.10 if you maintain performance well over longer distances.

How do weather and terrain affect predictions?

They can shift results by several percent. Use the Conditions Adjustment to reflect expected race-day factors.

Can I enter custom distances?

Yes. Select Custom for recent and/or target distance and choose your unit (km or miles).

Are the paces per km and per mile both shown?

Yes, results include pace per km and per mile, plus speed in km/h and mph.

Will taper or training changes impact accuracy?

Yes. The model assumes comparable fitness. If training volume or taper differs, consider a small negative or positive adjustment.

Predictions are estimates, not guarantees. Always pace responsibly and consider your health, course profile, weather, fueling, and race strategy.

Audit: Complete

Formula (LaTeX) + variables + units

This section shows the formulas used by the calculator engine, plus variable definitions and units.

Formula (extracted LaTeX)

\[','\]

','

Formula (extracted LaTeX)

\[T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k\]

T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k

Formula (extracted LaTeX)

\[T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right)\]

T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right)

Formula (extracted LaTeX)

\[\text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]}\]

\text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]}

Formula (extracted LaTeX)

\[T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05\]

T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05

Formula (extracted LaTeX)

\[\text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km\]

\text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km

Formula (extracted text)

Riegel model: $ T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k $ Optional conditions adjustment: $ T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right) $ Pace and speed: $ \text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]} $

Formula (extracted text)

Given: D1 = 10 km T1 = 45 min = 2700 s D2 = 21.0975 km k = 1.06 Compute: $ T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05 $ Pace: $ \text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km $ $ \text{Pace}_{mi} \approx 7:24/mi $

Formula (extracted text)

$ T_{2,adj} = 5825 \times \left(1 + \frac{5}{100}\right) \approx 6116\;s \approx 01:41:56 $

Variables and units

No variables provided in audit spec.

Sources (authoritative):

Health & Fitness — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/health-fitness
Read the American Scientist reference — jstor.org · Accessed 2026-01-19
https://www.jstor.org/stable/27850761

Changelog

Version: 0.1.0-draft
Last code update: 2026-01-19

0.1.0-draft · 2026-01-19

Initial audit spec draft generated from HTML extraction (review required).
Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
Confirm sources are authoritative and relevant to the calculator methodology.

Verified by Ugo Candido on 2026-01-19
Profile · LinkedIn

Full original guide (expanded)

Running Time Predictor

Estimate your finish time, pace, and speed for any race distance using a recent performance. Built on the peer‑reviewed Riegel model and optimized for accuracy, accessibility, and speed.

Units

Kilometers Miles

Recent race distance *

Recent race time (HH:MM:SS) *

Target race distance *

Riegel exponent k (1.00–1.15) *

Conditions adjustment (−20% to +20%)

Results

Predicted finish time

—

Average pace (per km)

—

Average pace (per mile)

—

Average speed

—

Equivalent times across popular races

Based on your recent race and chosen exponent k.

Distance	Predicted Time	Pace/km	Pace/mi
Enter your recent race to see equivalents.

Data Source and Methodology

Read the American Scientist reference

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

Riegel model:

$$ T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k $$

Optional conditions adjustment:

$$ T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right) $$

Pace and speed:

$$ \text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]} $$

Glossary of Variables

T1: Your recent race finish time.
D1: Your recent race distance.
D2: Your target race distance.
k: The Riegel exponent (performance decay factor with distance).
A: Conditions adjustment in percent (−20 to +20).
T2: Predicted finish time for the target distance.
T2,adj: Predicted time after applying conditions adjustment.
Pace per km / mile: Average time to cover one kilometer / one mile.
Speed: Average speed expressed in km/h and mph.

How It Works: A Step-by-Step Example

Come Funziona: Un Esempio Passo-Passo

Suppose your recent 10K time is T1 = 00:45:00 and you want to predict a half marathon (D2 = 21.0975 km), using k = 1.06.

Given:
  D1 = 10 km
  T1 = 45 min = 2700 s
  D2 = 21.0975 km
  k  = 1.06

Compute:
  $$ T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05 $$

Pace:
  $$ \text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km $$
  $$ \text{Pace}_{mi} \approx 7:24/mi $$

If race day is hot (+5%), apply A = +5:

  $$ T_{2,adj} = 5825 \times \left(1 + \frac{5}{100}\right) \approx 6116\;s \approx 01:41:56 $$

Frequently Asked Questions (FAQ)

Which formula does this predictor use?

It uses the peer‑reviewed Riegel model: T2 = T1 × (D2/D1)^k. See Data Source and Methodology for the reference.

What exponent k should I choose?

Default to 1.06. Choose 1.03–1.05 if you have strong speed over short distances; choose 1.06–1.10 if you maintain performance well over longer distances.

How do weather and terrain affect predictions?

They can shift results by several percent. Use the Conditions Adjustment to reflect expected race-day factors.

Can I enter custom distances?

Yes. Select Custom for recent and/or target distance and choose your unit (km or miles).

Are the paces per km and per mile both shown?

Yes, results include pace per km and per mile, plus speed in km/h and mph.

Will taper or training changes impact accuracy?

Yes. The model assumes comparable fitness. If training volume or taper differs, consider a small negative or positive adjustment.

Predictions are estimates, not guarantees. Always pace responsibly and consider your health, course profile, weather, fueling, and race strategy.

Audit: Complete

Formula (LaTeX) + variables + units

This section shows the formulas used by the calculator engine, plus variable definitions and units.

Formula (extracted LaTeX)

\[','\]

','

Formula (extracted LaTeX)

\[T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k\]

T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k

Formula (extracted LaTeX)

\[T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right)\]

T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right)

Formula (extracted LaTeX)

\[\text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]}\]

\text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]}

Formula (extracted LaTeX)

\[T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05\]

T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05

Formula (extracted LaTeX)

\[\text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km\]

\text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km

Formula (extracted text)

Riegel model: $ T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k $ Optional conditions adjustment: $ T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right) $ Pace and speed: $ \text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]} $

Formula (extracted text)

Given: D1 = 10 km T1 = 45 min = 2700 s D2 = 21.0975 km k = 1.06 Compute: $ T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05 $ Pace: $ \text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km $ $ \text{Pace}_{mi} \approx 7:24/mi $

Formula (extracted text)

$ T_{2,adj} = 5825 \times \left(1 + \frac{5}{100}\right) \approx 6116\;s \approx 01:41:56 $

Variables and units

No variables provided in audit spec.

Sources (authoritative):

Health & Fitness — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/health-fitness
Read the American Scientist reference — jstor.org · Accessed 2026-01-19
https://www.jstor.org/stable/27850761

Changelog

Version: 0.1.0-draft
Last code update: 2026-01-19

0.1.0-draft · 2026-01-19

Initial audit spec draft generated from HTML extraction (review required).
Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
Confirm sources are authoritative and relevant to the calculator methodology.

Verified by Ugo Candido on 2026-01-19
Profile · LinkedIn

Running Time Predictor

Estimate your finish time, pace, and speed for any race distance using a recent performance. Built on the peer‑reviewed Riegel model and optimized for accuracy, accessibility, and speed.

Units

Kilometers Miles

Recent race distance *

Recent race time (HH:MM:SS) *

Target race distance *

Riegel exponent k (1.00–1.15) *

Conditions adjustment (−20% to +20%)

Results

Predicted finish time

—

Average pace (per km)

—

Average pace (per mile)

—

Average speed

—

Equivalent times across popular races

Based on your recent race and chosen exponent k.

Distance	Predicted Time	Pace/km	Pace/mi
Enter your recent race to see equivalents.

Data Source and Methodology

Read the American Scientist reference

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

Riegel model:

$$ T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k $$

Optional conditions adjustment:

$$ T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right) $$

Pace and speed:

$$ \text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]} $$

Glossary of Variables

T1: Your recent race finish time.
D1: Your recent race distance.
D2: Your target race distance.
k: The Riegel exponent (performance decay factor with distance).
A: Conditions adjustment in percent (−20 to +20).
T2: Predicted finish time for the target distance.
T2,adj: Predicted time after applying conditions adjustment.
Pace per km / mile: Average time to cover one kilometer / one mile.
Speed: Average speed expressed in km/h and mph.

How It Works: A Step-by-Step Example

Come Funziona: Un Esempio Passo-Passo

Suppose your recent 10K time is T1 = 00:45:00 and you want to predict a half marathon (D2 = 21.0975 km), using k = 1.06.

Given:
  D1 = 10 km
  T1 = 45 min = 2700 s
  D2 = 21.0975 km
  k  = 1.06

Compute:
  $$ T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05 $$

Pace:
  $$ \text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km $$
  $$ \text{Pace}_{mi} \approx 7:24/mi $$

If race day is hot (+5%), apply A = +5:

  $$ T_{2,adj} = 5825 \times \left(1 + \frac{5}{100}\right) \approx 6116\;s \approx 01:41:56 $$

Frequently Asked Questions (FAQ)

Which formula does this predictor use?

It uses the peer‑reviewed Riegel model: T2 = T1 × (D2/D1)^k. See Data Source and Methodology for the reference.

What exponent k should I choose?

Default to 1.06. Choose 1.03–1.05 if you have strong speed over short distances; choose 1.06–1.10 if you maintain performance well over longer distances.

How do weather and terrain affect predictions?

They can shift results by several percent. Use the Conditions Adjustment to reflect expected race-day factors.

Can I enter custom distances?

Yes. Select Custom for recent and/or target distance and choose your unit (km or miles).

Are the paces per km and per mile both shown?

Yes, results include pace per km and per mile, plus speed in km/h and mph.

Will taper or training changes impact accuracy?

Yes. The model assumes comparable fitness. If training volume or taper differs, consider a small negative or positive adjustment.

Predictions are estimates, not guarantees. Always pace responsibly and consider your health, course profile, weather, fueling, and race strategy.

Audit: Complete

Formula (LaTeX) + variables + units

This section shows the formulas used by the calculator engine, plus variable definitions and units.

Formula (extracted LaTeX)

\[','\]

','

Formula (extracted LaTeX)

\[T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k\]

T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k

Formula (extracted LaTeX)

\[T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right)\]

T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right)

Formula (extracted LaTeX)

\[\text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]}\]

\text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]}

Formula (extracted LaTeX)

\[T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05\]

T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05

Formula (extracted LaTeX)

\[\text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km\]

\text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km

Formula (extracted text)

Riegel model: $ T_2 = T_1 \times \left(\frac{D_2}{D_1}\right)^k $ Optional conditions adjustment: $ T_{2,adj} = T_2 \times \left(1 + \frac{A}{100}\right) $ Pace and speed: $ \text{Pace}_{km} = \frac{T_{2,adj}}{D_2\,[km]} \quad;\quad \text{Speed}_{km/h} = \frac{D_2\,[km]}{T_{2,adj}\,[h]} $

Formula (extracted text)

Given: D1 = 10 km T1 = 45 min = 2700 s D2 = 21.0975 km k = 1.06 Compute: $ T_2 = 2700 \times \left(\frac{21.0975}{10}\right)^{1.06} \approx 5825\;s \approx 01:37:05 $ Pace: $ \text{Pace}_{km} = \frac{5825}{21.0975} \approx 276\;s/km \approx 4:36/km $ $ \text{Pace}_{mi} \approx 7:24/mi $

Formula (extracted text)

$ T_{2,adj} = 5825 \times \left(1 + \frac{5}{100}\right) \approx 6116\;s \approx 01:41:56 $

Variables and units

No variables provided in audit spec.

Sources (authoritative):

Health & Fitness — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/health-fitness
Read the American Scientist reference — jstor.org · Accessed 2026-01-19
https://www.jstor.org/stable/27850761

Changelog

Version: 0.1.0-draft
Last code update: 2026-01-19

0.1.0-draft · 2026-01-19

Initial audit spec draft generated from HTML extraction (review required).
Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
Confirm sources are authoritative and relevant to the calculator methodology.

Verified by Ugo Candido on 2026-01-19
Profile · LinkedIn

Formulas

(Formulas preserved from original page content, if present.)

Version 0.1.0-draft

Citations

Add authoritative sources relevant to this calculator (standards bodies, manuals, official docs).

Changelog

0.1.0-draft — 2026-01-19: Initial draft (review required).

Running Time Predictor

Running Time Predictor

Interactive Calculator

Results

Equivalent times across popular races

Data Source and Methodology

The Formula Explained

Glossary of Variables

How It Works: A Step-by-Step Example

Come Funziona: Un Esempio Passo-Passo

Frequently Asked Questions (FAQ)

Full original guide (expanded)

Running Time Predictor

Interactive Calculator

Results

Equivalent times across popular races

Data Source and Methodology

The Formula Explained

Glossary of Variables

How It Works: A Step-by-Step Example

Come Funziona: Un Esempio Passo-Passo

Frequently Asked Questions (FAQ)

Running Time Predictor

Interactive Calculator

Results

Equivalent times across popular races

Data Source and Methodology

The Formula Explained

Glossary of Variables

How It Works: A Step-by-Step Example

Come Funziona: Un Esempio Passo-Passo

Frequently Asked Questions (FAQ)