Enter your recent race result and instantly predict your finish times for 5K, 10K, half marathon, and marathon — using the Riegel formula trusted by coaches worldwide.
Your Recent Race
Predicted Race Times
5K
25:00
5:00 /km
10K
52:07
5:13 /km
Half Marathon
1:55:00
5:27 /km
Marathon
3:59:46
5:41 /km
🗺 Have a race course GPX? Scroll down to the Route & Weather Planner — your predicted marathon pace (5:41/km) is already pre-filled as your base pace.
Route & Weather Pace Planner · Beta
Pace your exact race course
Search for your race by name to pull in the official course. Get an elevation- and weather-adjusted pacing strategy that accounts for the specific terrain and race-day conditions.
🔍 Search by race / event name
Searches the Pacecraft event library. Backend database required.
Add-on Calculators
Enter your race details below — or upload a GPX above to auto-fill from your course.
💧 Hydration & Electrolyte Plan
3400 mL
Total fluids
880 mL/hr
Sweat rate
425 mL
Per aid station
every 5 km
Drink interval
1702 mg
Sodium target
🍬 Gel / Fueling Schedule
6 gels recommended · race start: 08:00 · take with water
40m
at 08:40 · ~7.3 km
Regular gel
75m
at 09:15 · ~13.6 km
Regular gel
110m
at 09:50 · ~20 km
Regular gel
145m
at 10:25 · ~26.4 km
☕ Caffeine gel
180m
at 11:00 · ~32.7 km
☕ Caffeine gel
215m
at 11:35 · ~39.1 km
☕ Caffeine gel
🏔️ Altitude-Adjusted Pace
⏱️ Cutoff-Time Checker
Enter official course cutoff times and see whether your adjusted pace clears each one.
✅
📊 Course Difficulty Score
How the Race Time Predictor Works
This tool uses the Riegel formula, published in American Scientist in 1981 by Peter Riegel. The formula is: T2 = T1 × (D2 ÷ D1)^1.06, where T1 is your known race time, D1 is the distance you ran, and T2 is the predicted time for distance D2.
The exponent 1.06 reflects the fact that longer races are proportionally harder — your pace slows as distance increases because of glycogen depletion, fatigue accumulation, and muscle breakdown. The formula works remarkably well for distances from 800m to ultramarathons when your input was run at genuine race effort.
Which Input Race Gives the Best Prediction?
The closer the input race is to the target distance, the more accurate the prediction:
Best for marathon prediction: Half marathon time trial run 4–6 weeks before race day. This is the gold standard.
Good for 10K prediction: 5K parkrun or time trial.
Use with caution: Predicting a marathon from a 5K — the formula will give an optimistic result because the marathon's unique demands aren't expressed in a 5K effort.
Why Your Predicted Marathon Time Might Be Faster Than You Run
The Riegel formula assumes your fitness is proportional across distances. But marathon performance depends on factors that don't scale linearly: glycogen stores, training specificity, and nutrition strategy. A safe rule: for marathon goal-setting, take your Riegel-predicted time and add 3–8 minutes as a buffer, especially for your first marathon.
GPX/KML file upload — the most accurate method. Most race organizers publish a GPX file on their event website. Strava, Garmin Connect, and Wahoo all let you export saved routes as GPX.
Draw on the map — click waypoints on the OpenStreetMap map in order. Note: drawn routes don't have elevation data — import a GPX for elevation-adjusted pacing.
Elevation adjustment — Minetti formula
The elevation penalty uses the full Minetti et al. (2002) formula — the same model used in Strava's Grade Adjusted Pace. A 10% uphill grade costs roughly twice as much as flat running. The result is a single "adjusted pace" that represents the equivalent flat-course effort across the full route.
Weather — live forecast vs historical average
For race dates within 16 days, the planner fetches a live forecast from Open-Meteo. For race dates further away, it falls back to a 5-year historical climate average from the Open-Meteo archive API — shown with an amber "Historical avg" badge.
Frequently asked questions
This calculator uses the Riegel formula, developed by Peter Riegel in 1981 and published in American Scientist: T2 = T1 × (D2÷D1)^1.06. It's the gold standard for race time prediction and is remarkably accurate for distances from 800m to 100 miles when your input race was run at maximum effort.
For most runners, the Riegel formula predicts race times within 2–5% accuracy across similar distances. Accuracy decreases for very large jumps in distance (e.g. predicting a marathon from a 5K) because marathon performance relies more heavily on glycogen stores and fat oxidation capacity that a 5K doesn't stress.
The Riegel formula assumes your physiological limiters scale proportionally with distance — which is approximately true but doesn't fully account for glycogen depletion (the marathon wall), nutrition strategy, or training specificity. A 5K time predicts a marathon aggressively fast for many runners; a half marathon time gives a much more reliable prediction.
A half marathon time gives the most reliable marathon prediction using Riegel — it's long enough to stress your aerobic system but close enough in distance that the formula's assumptions hold well. Run a proper time trial half marathon 4–6 weeks before your goal marathon for the most accurate prediction.
Yes — short-distance predictions (5K to 10K, or 10K to half marathon) are the most accurate applications of the Riegel formula because the physiological demands are most similar. Input your 5K PR and the calculator will give you a very reliable 10K target.
Check the event's official website for a "Course Download" or "Course Map" link — most large races publish a GPX there. Alternatively, search the race on Strava Routes, AllTrails, or community mapping sites like GeeksOnFeet or PlotARoute. You can also export any saved route from Strava, Garmin Connect, or Wahoo as a GPX file.
The Minetti formula is the most physiologically grounded model for grade-adjusted running energy cost and is the same basis used by Strava's Grade Adjusted Pace. Accuracy depends on the quality of the GPX elevation data — GPS altitude from a device can be noisy (±10–20m errors), which affects per-segment grade calculations. For best results, use a GPX from a race organizer or a route-mapping site that uses elevation-API data rather than raw GPS recording.
When your race is more than 16 days away, no weather service can provide a reliable forecast for that date. Instead, we fetch the same calendar date from the last 5 years of Open-Meteo's historical archive and average the temperature, humidity, and wind. This gives a baseline estimate based on what conditions have typically been like at that location in that month. It's a planning tool, not a prediction — check the live forecast once you're within 2 weeks of the race.
The course difficulty score is a single number (1.0 = flat, road, ideal weather) that lets you compare courses side by side. It's computed as: elevation density factor × weather penalty factor × surface factor. For example, a hilly trail race in hot weather might score 1.35, while a flat city marathon in mild weather scores ~1.0. Use it to calibrate your expectations before targeting a goal time on an unfamiliar course.
Yes — select the Trail surface type to apply the effort penalty for technical terrain. The Minetti elevation formula works for any distance and gradient. For ultras with very large total elevation gain (e.g. 4,000m+), the difficulty score and cutoff checker are particularly useful. Note that the cutoff checker requires you to enter the race's official cutoff times from the athlete guide.
Train smarter with an AI coach that knows your race
PacecraftAI uses your predicted race times, route elevation, and weather data to build personalised training paces, tempo targets, and a week-by-week plan that gets you to the start line ready.