Pace Calculator — Predict Your Race Time

You ran a 10K in 42 minutes and want to know what to expect for a half marathon or marathon? That is exactly what pace prediction formulas are for. With the calculator below you only need to enter your race time once — you immediately get predictions for all standard distances plus target pace per kilometer.

Important upfront: predictions are not guarantees. They show you what would be possible under ideal conditions, provided your preparation is right. More on that below the calculator.

How does the calculator work?

Behind the tool sits the Riegel formula — published by Peter Riegel in 1981 and the scientific standard for race predictions since. The formula:

In words: the new time (T₂) for the target distance (D₂) equals the known time (T₁) for the input distance (D₁), multiplied by the distance ratio raised to the power of 1.06.

The 1.06 exponent is empirically derived. It means: if you double the distance, your target time grows slightly more than double. Specifically: 2^1.06 ≈ 2.08 — you will be about 4% slower per kilometer over double the distance.

An example

You run a 10K in 42:00 minutes (= 2520 seconds). What would Riegel predict for your half marathon?

Where the formula is accurate — and where not

A large study (Vickers & Vertosick 2016) shows that the Riegel formula is very accurate for distances between 1500 m and 21 km in trained runners. In this range the typical deviation is below 2%.

For the marathon distance it begins to underestimate the time — it predicts faster marathon times than most runners actually achieve. The reason: after about 90 minutes of effort, factors come into play that the formula does not capture:

• Carbohydrate stores: Glycogen lasts about 90–120 minutes of hard effort. Once the reserves are empty you get noticeably slower.
• Muscular fatigue: Repetitive impact changes your running form — and costs you pace.
• Heat and hydration: Marathons are often run in suboptimal temperatures, further dragging down pace.
• Mental exhaustion: Three hours of focus is a different thing than 40 minutes.

Rule of thumb: plan your marathon pace 3–5% more conservatively than the formula predicts. If the calculator says 3:30, plan realistically for 3:35–3:40 unless you are world class.

When the formula does not work

Three situations where you should not trust the prediction:

1. Very short distances: Below 1500 m, anaerobic energy systems dominate. A 400 m prediction from your 10K time is usually too fast.
2. Ultra distances: Above 50+ km the real-world exponent is significantly higher (around 1.10–1.15). Specialized models exist for ultras.
3. For untrained runners: The formula assumes sufficient aerobic base. If you have never run a half marathon, do not take the marathon prediction from a 5K time seriously — the endurance base has to be built first.

Which input distance is most accurate?

A simple rule: the closer the input distance is to the target distance, the more accurate the prediction.

• For a marathon prediction, a recent half marathon time is best — ideally a 21K race no older than 6 weeks.
• For a 10K prediction, a 5K time works very well.
• For a 5K prediction, a 3K or 5K tempo session from your training is often more informative than an older race.

Alternative formulas

Riegel is not the only method. Two other widely used approaches:

VDOT (Jack Daniels)

American coach Jack Daniels developed his own system in his book "Daniels' Running Formula": VDOT (a derived VO₂max). Your race time is converted into a VDOT value, from which training paces and race predictions for other distances are derived. More accurate than Riegel in the marathon range, but more complex to use. The VDOT model also powers the training zones calculator if you want to derive your pace zones from it.

Higher exponent for long distances

For long distances you can nudge the Riegel exponent up slightly (about 1.07 instead of 1.06). Often more accurate than the original for marathons, worse for shorter distances.

How to use the calculator in practice

Three useful applications:

1. Set a race goal time: After a recent 10K, you know what is possible at a planned half marathon — and can plan your pacing accordingly.
2. Derive training pace: Your current fitness level gives you appropriate paces for tempo runs, long runs and intervals. To convert that into concrete zones, use the training zones calculator.
3. Reality check for training plans: If a plan is preparing you for a 3:00 marathon but your 10K is 50 minutes, you know that either the goal or the plan is unrealistic.

Conclusion

The Riegel formula is a proven tool for realistic race predictions — especially in the range from 5K to half marathon. For marathons it gives you an ambitious upper limit that you should consciously set more conservatively in your goal-setting.

Important: the calculator is no substitute for training. It does not tell you what you can do, but what would be possible if your preparation is right. A marathon prediction from a 10K time presupposes that you have actually trained for the marathon — with enough long runs, volume and targeted preparation.