Calculator
Race Time Predictor
Estimate equivalent race times at other distances from a recent result using the Riegel model.
This extrapolates performance across distances — different from the Pace Calculator, which does known pace/time arithmetic.
Time
Predicted Half Marathon
1:47:01
5:04 /km · 8:10 /mile
Based on: 10K in 48:30
Closer estimate · Estimated equivalent — not a guaranteed race outcome.
Your equivalent race times
| Distance | Estimated time |
|---|---|
| 1 mile | 7:00 |
| 5K | 23:16 |
| 10K (known) | 48:30 |
| Half Marathon | 1:47:01 |
| Marathon | 3:43:07 |
Race predictors estimate equivalent fitness, not guaranteed race outcomes. Endurance, terrain, weather, pacing, fueling and course profile can materially change actual results.
Know your target time? Calculate your race pace →
Training for this race? Build your running shoe rotation →
How predictions work
- The prediction estimates equivalent performance at another distance based on your supplied result.
- It assumes broadly comparable fitness and race conditions — not a guarantee of race-day outcome.
- We use the Riegel formula, a widely used race-equivalency model for road running.
T₂ = T₁ × (D₂ / D₁)^1.06
- T₁
- — Known finish time
- D₁
- — Known distance
- D₂
- — Target distance
- T₂
- — Predicted time
Limitations
- Less reliable when source and target distances differ a lot
- Training that is highly distance-specific can change outcomes
- Terrain, weather, pacing and fueling can materially change actual results
Related tools
- Running Pace Calculator →
Calculate pace, finish time and distance — with race splits and unit conversion.
- Running Shoe Finder →
Answer questions about terrain, training and goals to get explainable shoe matches.
- Shoe Rotation Planner →
Build a running shoe rotation around your training — see coverage, gaps and the best shoe to add next.
Related guides
Training for a race?
Find running shoes matched to your training.
Running Shoe Finder →FAQ
How accurate are race time predictors?
They estimate equivalent fitness, not guaranteed outcomes. Closer distances are usually more reliable than large jumps (e.g. 5K → marathon).
Can a 5K predict my marathon time?
It can give a broader estimate only. Marathon performance depends heavily on endurance training, fueling and pacing — treat long-range predictions cautiously.
What is the Riegel formula?
T₂ = T₁ × (D₂ / D₁)^1.06. It models how finish times typically scale with distance for runners of similar fitness.
Why is my predicted marathon slower than simply doubling my half-marathon time?
Doubling ignores the extra fatigue of a longer race. The Riegel exponent (>1) builds in that performance decay.