All you need is a 5K or 10K time to instantly calculate your projected finish time for a half marathon or full marathon. By applying the Riegel formula, this tool accounts for the natural pace fade that comes with longer distances — giving you a far more realistic prediction than simply multiplying your current pace. Use it as a reference whether you’re toeing the line at your first full marathon or locking in your next time goal.
| 5K (5km) | — | — |
| 10K (10km) | — | — |
| Half Marathon (21.1km) | — | — |
| Full Marathon (42.2km) | — | — |
Riegel formula — predicts longer-distance times based on a shorter-distance result.
The Riegel Formula — How This Predictor Works
This predictor uses the formula proposed by Pete Riegel in 1981: T₂ = T₁ × (D₂ / D₁)^1.06. In other words, if you know a race distance (D₁) and your finishing time for it (T₁), you can calculate your predicted time (T₂) for any other distance (D₂).
The exponent 1.06 is the key. If your pace stayed perfectly constant, the exponent would simply be 1 — but in reality, pace naturally fades as distance increases. That 1.06 captures that natural slowdown. Take a runner who finishes a 5K in 25 minutes (5:00/km pace): running a full marathon at that same pace would arithmetically give 3 hours 31 minutes, but the Riegel formula predicts roughly 4 hours 1 minute — that 30-minute gap is exactly the pace degradation that comes with distance.
The formula is most accurate when the two distances are roughly 2–3 times apart. Predicting a 10K from a 5K PR is highly reliable, and predicting a half marathon from a 10K is solid as well. However, predicting a full marathon from just a 5K carries significant variability — without the proper long-run training base to back it up, your actual race result could come in well slower than the prediction.
Real-World Use Cases
Beginner — “Can I run a sub-4 marathon if my 5K is 30 minutes?”
Enter a 5K time of 30 minutes (6:00/km pace) → full marathon prediction: approximately 4 hours 49 minutes. To break 4 hours, you’d need to bring your 5K down to around 24 minutes. One important caveat with the Riegel formula is that it assumes adequate long-run training — to actually finish in 4 hours, you’ll need at least 6 weeks of 25–30 km long runs under your belt.
Intermediate — “What half marathon time can I expect off a 45-minute 10K?”
Enter a 10K time of 45 minutes (4:30/km) → half marathon prediction: approximately 1 hour 39 minutes (4:42/km). A natural pace drop of about 12 seconds/km is typical. To hold that pace comfortably on race day, your easy runs should be around 5:00/km or faster, with tempo runs around 4:20/km.
Advanced — “Half marathon 1:30 — can I BQ (3:00)?”
Enter a half marathon time of 1:30 → full marathon prediction: approximately 3 hours 8 minutes. That puts you 8 minutes away from a Boston qualifier. To prevent the classic late-race blow-up after the 30 km mark, you’ll want at least 3 completed 30 km long runs at your goal marathon pace (4:16/km).
Frequently Asked Questions
How accurate is a marathon time prediction?
The closer the two distances are, the more accurate the prediction. Predicting a half marathon from a 10K typically lands within ±2–3% (about 2–3 minutes), while predicting a full marathon from a 5K can be off by ±10% or more (roughly 20–30 minutes). For the most accurate prediction, try a 30 km pace run at your goal marathon pace plus 5 sec/km about 6 weeks out from your race and use that result as your input.
Can I predict a full marathon from just my 5K time?
Technically yes, but the reliability is low. The Riegel formula assumes comparable training volume and distance adaptation — a runner who excels at 5Ks but jumps straight into a marathon will often finish 20–40 minutes slower than predicted. At a minimum, having at least one half marathon finish makes a 5K → full marathon prediction meaningful.
How does the Riegel formula compare to other formulas?
Two common alternatives are the Cameron formula (optimized for elite runners) and the McMillan pace calculator (which provides training paces across distances). Riegel is the simplest and the most suitable for everyday runners. Cameron is slightly more accurate for shorter distances (5K–10K), but for the full marathon there’s little practical difference between the two. For general purposes, Riegel gets the job done.
Why does my actual race finish slower than the prediction?
There are four main culprits: ① Insufficient long-run training — for the full marathon, the number of runs over 25 km is critical; ② Course difficulty — hilly or technical courses can run 5–10% slower than flat ones; ③ Weather — temperatures above 25°C can cost you 10+ seconds/km; ④ Pacing strategy — going out too fast leads to a hard crash in the back half. Think of the prediction not as a guaranteed finish time, but as a goal that reflects your current fitness ceiling.
How should a first-time marathoner set their race pace?
We recommend taking your predicted time and adding a 5–10% buffer. For example, if the predictor says 4 hours, set your actual goal at 4 hours 15 minutes — and run the first 5 km at 5–10 seconds/km slower than goal pace to bank energy for the back half. The biggest enemy in your first marathon is going out too hot. Whether you can hold your pace through the 30 km mark is what ultimately decides your finishing time.
Related Running Tools
- Pace Calculator — Instantly calculate distance, time, and pace
- Running Calorie Calculator (coming soon) — Calories burned based on time, distance, and body weight
- Pace Chart (coming soon) — Per-kilometer pace comparison by goal finish time
- Training Plan Generator (coming soon) — Auto-generated plans based on your target race and timeline
Once you have your predicted time, the real challenge is actually hitting it on race day. A full marathon training cycle typically takes 16–20 weeks, and consistently nailing your weekly long runs and pace workouts throughout that stretch is where the true test lies. If you struggle to stay consistent without accountability, a commitment-based running app — like geowill — can give you that extra push to stay on track.