The science behind the prediction
This calculator is built on Riegel's formula, first published by American engineer and runner Peter Riegel in Runner's World in 1977. It has been the gold standard for race time equivalency for nearly 50 years and is used by running coaches, sports scientists, and race time predictors worldwide.
T₂ = predicted finish time · D₂ = target distance · k = fatigue exponent
The exponent k is the key variable — it models how quickly a runner slows as distance increases. This is where the runner profile you select matters. A well-trained runner with a strong aerobic base maintains pace better over long distances (lower k = 1.04). A beginner fades more in the final miles (higher k = 1.08). The default of 1.06 fits most recreational runners.
The training paces are derived from your predicted race pace using well-established zone relationships from exercise physiology. Easy pace sits at roughly 70–75% of race effort. Tempo runs target your lactate threshold — the effort level at which lactate accumulates faster than it clears, typically around 83–88% of VO₂ max. Interval pace targets VO₂ max directly, at around 95–100% of aerobic capacity.
How it works
The calculator walks you through three questions, then gives you a predicted finish time and four specific training paces based on your current fitness.
Enter a recent race result
Use an actual race time — not a training run. A race result represents a true maximum effort, which is what the prediction formula is calibrated for. A training run time will produce an overoptimistic prediction.
Choose your target distance
Select the distance you're training for — the one you haven't raced yet (or want to improve significantly). The bigger the jump in distance, the more important it is that your training specifically prepares you for it.
Tell us your experience level
This adjusts the Riegel fatigue exponent — an experienced runner maintains pace better over longer distances than a beginner. Choosing the right profile gives you a prediction that accounts for how you're likely to pace on race day.
Get your predicted time and training paces
The results show your goal finish time, goal race pace, and four training paces in plain English — easy run pace, tempo pace, and hard effort pace — so you know exactly what each session should feel like.
Understanding your training paces
Most runners either train too hard every day or run everything at the same moderate effort. Neither works well. The four paces the calculator gives you correspond to distinct physiological purposes — here's what each one does.
Goal race pace
The pace you need to hold for your entire target race to hit your predicted finish time. Practice this in tempo workouts so it feels familiar on race day. Never run your long runs at this pace — it's too fast for recovery training.
Easy / long run pace
60–75 seconds per mile slower than race pace. This is the pace for your weekend long run and any recovery runs. The test: you should be able to hold a full conversation without gasping. Most runners go too fast here — resist the urge.
Tempo / comfortably hard
"Comfortably hard" — you can speak in short sentences but wouldn't want to. This is your lactate threshold pace, the single most important training zone for improving endurance. Sustained for 20–40 minutes per session.
Hard effort / intervals
Short repeats of 400m to 1 mile at this pace, with recovery jogs between. Builds VO₂ max and raw speed. One session per week is enough — more than that raises injury risk without adding proportional benefit.
Stepping up to a longer distance
The prediction formula gives you a time based on current fitness. Whether you can actually run that time depends on how well you prepare for the specific demands of the longer distance.
5K to 10K: The jump in distance is manageable — most runners can complete a 10K within a few weeks of running their first 5K. The main adaptation needed is aerobic base, which develops quickly. Follow your predicted pace for the first 5K and adjust in the second half based on feel.
10K to half marathon: This is where long run mileage starts to matter. Your long run should reach at least 11–12 miles before race day. The second half of a half marathon is where underprepared runners fade — the prediction will be accurate if your training included adequate long run volume.
Half marathon to marathon: The biggest and most demanding jump. The prediction formula is reliable here but the margin for error is large. Your long run must reach 18–20 miles. Fueling strategy (carbohydrate intake during the race) is critical and something a time prediction alone can't account for — practice this in training.
Frequently asked questions
Start 30–45 seconds per mile slower than the goal pace this calculator gives you. The most common first-marathon mistake is starting at goal pace — by mile 20 the wheels come off. Run the first 10 miles feeling like you're holding back, then gradually increase effort if you're feeling good after mile 18.
A realistic first marathon goal for most runners is simply to finish strong — not to hit a specific time. Once you have a marathon finish, the next one is where pacing strategy really pays off.
For similar distances (5K to 10K, 10K to half marathon) the prediction is reliable within 2–5 minutes. For larger jumps — especially half marathon to marathon — the formula is a useful starting point but individual variation is much larger.
Key factors that make the actual result slower than predicted: insufficient long run mileage, racing in heat, hilly courses, and poor pacing in the first half. Factors that make it faster: exceptional weather, flat fast course, and stronger-than-expected fitness on race day.
They're the same thing. Your long run should be run at easy pace — comfortably, conversationally, with plenty left in the tank at the end. The purpose of a long run is to build aerobic endurance and practice fueling, not to simulate race effort.
If your long run leaves you exhausted for two days afterward, you ran it too fast. The recovery cost of a hard long run is too high relative to the benefit — you miss quality midweek sessions as a result.
A typical tempo workout is 10 minutes easy warm-up, then 20–30 minutes at tempo pace (continuous), then 10 minutes easy cool-down. Once a week is enough. As fitness improves over a training block, that same tempo pace should feel progressively easier — that's the sign it's working.
Alternatively, tempo intervals: 3–4 × 8 minutes at tempo pace with 2 minutes easy between each. This is slightly easier to manage mentally and produces similar adaptation.
Choose Beginner if: you've been running less than 2 years, this is your first race at this distance, or your training has been inconsistent. The beginner profile (k=1.08) predicts a slower time — it assumes you'll slow more in the final miles, which is typical for less experienced runners.
Choose Regular runner if: you've been running consistently for 2+ years, you have race experience, and you train 3–4 days a week. If in doubt, choose Regular — it's the most commonly used default and sits between the two extremes.