How to predict your race times from 5K to marathon

If you just finished a strong 5K or 10K, you might be wondering what you could do in a half marathon or a full marathon. The math seems simple at first glance. A marathon is roughly 8.44 times longer than a 5K. If you run a 25-minute 5K, multiplying by 8.44 gives you a marathon time of about 3 hours and 31 minutes.

But human bodies do not scale linearly. As distances stretch out, muscle fatigue compounds, core temperature rises, and your body burns through its glycogen stores—the stored carbohydrates your muscles use for fuel. A typical race time predictor relies on mathematical formulas to account for this drop in efficiency. Instead of a straight line, it gives you a curve that reflects human physiology.

The math behind the predictions

Most standard running calculators rely on a formula published in 1981 by engineer Peter Riegel. It is straightforward but highly accurate for standard race distances.

The formula is T2 = T1 × (D2 / D1)^1.06.

Here, T1 and D1 are your known time and distance. T2 and D2 are your target time and distance. The heavy lifting is done by that 1.06 exponent. This number represents the physical toll of sustaining an effort over time. It mathematically guarantees that every time your race distance doubles, your pace will slow down slightly.

Going back to that 25-minute 5K (5,000 meters) and the marathon (42,195 meters). If we apply the Riegel formula with the 1.06 fatigue exponent, the prediction shifts from that impossible 3:31 to a much more realistic 3 hours, 52 minutes, and 12 seconds. That extra 21 minutes is the mathematical cost of cumulative fatigue.

What typical predictions look like

To get a feel for how the math plays out, it helps to look at a few benchmarks. The table below shows how three different 5K finish times—20 minutes, 25 minutes, and 30 minutes—scale up to the 10K, half marathon, and marathon using the Riegel formula.

Look at the actual paces behind these finish times, and the slowdown makes sense. A 25-minute 5K requires running roughly 8:03 per mile. By the time that same runner scales up to the predicted 3:52:12 marathon, their average pace has slowed to 8:51 per mile. A slowdown of 48 seconds per mile is a completely normal adjustment when moving from a short, high-intensity aerobic effort to a pure endurance event.

Context for judging your own times

The Riegel formula is a well-validated approximation, but it assumes you train optimally for the target distance. These numbers are what you are theoretically capable of, not guarantees on race day.

How much you can trust the prediction depends on how far you are jumping up in distance.

Predictions between a 5K and a 10K are highly accurate for most runners. Both events rely heavily on your cardiovascular system. If you can run a 25-minute 5K, you almost certainly have the physical endurance to finish a 52-minute 10K without drastically altering your weekly training routine.

Moving from a 10K to a half marathon is also generally reliable, assuming you are putting in consistent weekend long runs. However, the half marathon introduces new demands on your leg muscles. If your weekly mileage is low, your legs will likely fatigue before your lungs do, pushing your finish time a few minutes past the prediction.

The jump from a 5K or 10K to a full marathon is where things get tricky. The formula often over-predicts marathon times for beginners because it assumes you have the deep endurance base required to avoid severe glycogen depletion. This depletion is what runners call “hitting the wall,” and it usually strikes around mile 18 or 20.

A runner with a 25-minute 5K might see a predicted marathon time of 3:52:12. But if they lack high-mileage training, they are far more likely to finish between 4:00 and 4:30. For the tightest estimates, stick to the 2x rule: predictions work best when you jump to a distance no more than twice as long as your known race. Predicting a marathon off a half marathon result will always be more accurate than predicting it off a 5K.

Training efforts vs. race efforts

Always use a real race result in a predictor if you have one.

It is tempting to plug in a recent hard training run or tempo effort to see where you stand, but this usually spits out an overly optimistic prediction. A race environment gives you advantages that are hard to replicate on a solo Tuesday morning run. You are usually tapering (resting your legs in the days prior), feeding off crowd energy, and running on adrenaline. You are also more willing to push yourself into deep discomfort with a finish line in sight.

If a training run is all you have, treat the resulting prediction as a best-case scenario. You can also look at a VO2 max calculator to estimate your current aerobic fitness using your resting heart rate or standard walk tests. This gives you a secondary data point to verify your endurance capacity.

Putting the numbers into practice

Once you have a realistic target time, you can build your training plan around it.

This does not mean you should try to run your predicted race pace every time you step outside. Instead, use a running pace calculator to break your predicted finish time down into a specific per-mile or per-kilometer target. Use that exact pace for your interval workouts and tempo runs. Keep your standard everyday training runs significantly slower to safely build your aerobic base.

Understanding how race math works helps you set realistic expectations. It makes pacing your longer runs easier, and most importantly, it keeps you from falling into the trap of trying to hold your 5K pace for 26.2 miles.

Ready to see your own projected finish times? Calculate your targets with the Race Time Predictor.