Most runners set their marathon goal by one of two methods:
1. "My coach said so." 2. "I ran a half at X, so I'll double it and add a bit."
Both are less accurate than a formula published in *Runner's World* in 1977 — and the difference has measurable consequences on race day.
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The Riegel Formula
In 1977, researcher Peter Riegel published a mathematical relationship between race times at different distances:
T2 = T1 × (D2 / D1)^1.06
Where:
- T1 = your known race time
- D1 = the distance you ran
- D2 = the distance you're predicting
- T2 = the predicted time
The formula predicts that a runner who completes a half marathon in 1:45:00 should target a marathon in approximately 3:41–3:42, not 3:30 (which is what simple doubling would suggest).
The gap between those two targets — 11–12 minutes — is the difference between a well-paced marathon and hitting the wall at kilometer 32.
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Why the "Double + Add" Method Fails
The intuitive "double your half and add 10–15 minutes" heuristic originated from average marathon outcomes — but it systematically overestimates performance for most recreational runners for two reasons:
1. Training specificity Most recreational runners are well-conditioned for the half-marathon distance but undertrain for the specific demands of marathon distance — specifically the 3–4 hour sustained effort and the glycogen depletion that occurs past 28–30km at race pace.
2. Physiological non-linearity The Riegel exponent 1.06 reflects the real, measured fatigue cost of longer distances. This exponent has been validated across thousands of race results. Simple doubling uses an implicit exponent of 1.0 — effectively assuming fatigue is perfectly linear, which the data does not support.
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How to Use the Formula Correctly
Step 1: Choose your reference race strategically
The most accurate predictions use reference races within 2–3x the target distance. For marathon prediction:
- A recent 10K → good input (within the aerobic system's operating parameters)
- A recent half marathon → better input (closer to marathon-specific demands)
- A 5K → possible, but the fatigue exponent compounds over 8× the distance, reducing accuracy
The formula requires an actual race performance, not a training run estimate. A training 10K effort will underestimate your race capability because pacing strategy, competition effect, and taper all raise performance by 2–5% on race day.
Step 3: Apply a training-specificity adjustment
The Riegel formula assumes equivalent fitness and training for both distances. If you've raced a recent half marathon but have not exceeded 32km in your long run training, apply a conservative 3–5% addition to the formula output as a practical buffer.
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Cross-Distance Application
The same formula applies bidirectionally across all running distances:
| Known Result | Predicted Distance | Riegel Projection |
| 10K in 45:00 | Half Marathon | 1:39:30 |
|---|---|---|
| 10K in 45:00 | Marathon | 3:28:15 |
| Half in 1:50:00 | Marathon | 3:51:45 |
| Half in 2:00:00 | Marathon | 4:12:30 |
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The Pacing Implication
The value of an accurate race time prediction is not the finish time — it is the starting pace.
Every marathon that ends in a death-march walk from kilometer 35 to the finish was almost always caused by a starting pace 5–10% too fast. The athlete "felt great" at kilometer 10 because they were still in glycogen surplus. The pain at kilometer 35 was a direct consequence of the first 20km.
The Riegel formula gives you the finish time that your current fitness — if correctly paced — can actually deliver. Use it to set your first-kilometer target, not your final-kilometer aspiration.
What method do you currently use to set your race time targets?