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Race Time Predictor

By Thomas Mortelmans
Race Time Predictor

📋 What it does

This calculator predicts your finish time for a race based on a recent performance. It's like a crystal ball for your racing:

Example:

  • You ran 10K in 45:00
  • The calculator predicts your half marathon time: ~1:40

It uses proven mathematical models that work for most people. However:

  • ✅ More accurate for similar distances (10K → Half)
  • ⚠️ Less accurate for very different distances (5K → Marathon)
  • ⚡ Assumes similar training, weather, terrain

Best Results When:

  • Recent race was within last 4-6 weeks
  • You ran all-out effort
  • Good conditions (not extreme heat/hills)
  • Race-specific fitness maintained

🎯 HOW TO USE IT

  1. Select recent race distance (5K, 10K, Half, Marathon, or custom)
  2. Enter finishing time (HH:MM:SS or MM:SS)
  3. Select target race distance

Example

Recent: 10K in 42:00 → Marathon

Riegel:           3:05:24  (4:23/km)
Fatigue-adjusted: 3:15:07  (4:37/km)

Limitations

  • No terrain, weather, or pacing adjustment
  • Best for 2–3× distance extrapolation
  • Marathon from 1500m = highly unreliable

🔬 Scientific Basis

Key Research:

  • Riegel (1981) - "Athletic records and human endurance"
  • Cameron (2008) - "Ultra-marathon running performance"
  • Vickers & Vertosick (2016) - "Marathon running pace"

The Riegel Formula:

T2 = T1 × (D2 / D1)^1.06

Where:
T1 = Time for recent race
D1 = Recent distance
T2 = Predicted time
D2 = Target distance
1.06 = Fatigue exponent

Why 1.06?

  • Empirically derived from thousands of race results
  • Means: Doubling distance increases time by 2.08×
  • Accounts for cumulative fatigue

Example Math:

10K in 45:00 → Half Marathon?

T2 = 45 minutes × (21.0975 / 10)^1.06
   = 45 × (2.10975)^1.06
   = 45 × 2.228
   = 100.3 minutes
   = 1:40:15

The Cameron Formula:

T2 = T1 × (D2 / D1)^fatigue_factor

Where:
fatigue_factor = 1.06 + (0.0001 × D1)

Why Add D1?

  • Longer starting distance = more fatigue adaptation
  • Marathon runners have more endurance than 5K runners
  • Accounts for training specificity

Example:

Marathon (42.2K) in 3:30 → 100K?

Fatigue = 1.06 + (0.0001 × 42.2) = 1.0642

T2 = 210 min × (100 / 42.2)^1.0642
   = 210 × (2.37)^1.0642
   = 210 × 2.70
   = 567 minutes
   = 9:27

Accuracy Guidelines:

Very Accurate (±2%):

  • 10K → Half Marathon (2:1 ratio)
  • Half → Marathon (2:1 ratio)
  • 5K → 10K (2:1 ratio)

Moderately Accurate (±5%):

  • 5K → Half (4:1 ratio)
  • 10K → Marathon (4:1 ratio)

Less Accurate (±10%):

  • 5K → Marathon (8:1 ratio)
  • Any → Ultra (>5:1 ratio)

Why Predictions Fail:

  1. Different Training:
    • 5K training ≠ Marathon training
    • Speed endurance ≠ aerobic base
  2. Environmental:
    • Hills (marathon courses vary)
    • Heat (affects longer races more)
    • Wind (bigger factor at slower paces)
  3. Nutrition:
    • 5K: No fueling needed
    • Marathon: Critical success factor
  4. Pacing:
    • Even pacing = faster times
    • Most people start too fast