Can You Predict Your Marathon Time from a 10K?

“You ran a 45-minute 10K? You should be able to hit 3:20 in a marathon.” You hear predictions like that everywhere – at your running club, on forums, on running websites. But where exactly do these formulas come from? Can you really trust them? And most importantly, what are their limitations? Personally, I think they’re super useful… as long as you don't take them as gospel.

The Prediction Formulas

Riegel's Formula

This is the most well-known, published in 1977. The basic idea is that your time for a given distance is proportional to that distance raised to a power: T₂ = T₁ × (D₂/D₁)^1.06. The 1.06 factor reflects the "natural" slowdown as the distance increases. Our race time predictor uses this type of formula.

Daniels' Tables

Jack Daniels developed correlation tables based on VDOT — an indicator that combines VO₂max and running economy. Essentially, if you run a 45:00 10K, your VDOT is 44.7, which corresponds to a marathon potential of around 3:24. It's a pretty cool tool.

Cameron's Approach

Dave Cameron proposed a non-linear model that applies a different correction factor depending on the pair of distances being compared. From my readings, it gives more conservative marathon predictions than Riegel's — and that's often more realistic for amateur runners.

When Prediction Works

Prediction formulas tend to work quite well in the following scenarios:

• Your reference race distance is close to the target distance (half marathon → marathon is more reliable than 5K → marathon)
• You train in a balanced way (speed AND endurance)
• Your reference race is recent (within 2-3 months)
• Race conditions are comparable (similar terrain, weather)

A recent half marathon remains the best predictor of a marathon time, in my opinion. The margin of error is typically around ±3-5% under good conditions.

Why 10K → Marathon Prediction is Risky

The Endurance Factor

A 10K typically takes 35 to 60 minutes for most of us. A marathon is 3 to 5 hours. The transition from one to the other demands very different qualities: the ability to use fat as fuel, muscular endurance over time, managing nutrition, and mental fortitude. If you're fast at a 10K but have never run for more than 1.5 hours, your "endurance factor" will be low — and the formula will overestimate your marathon performance.

The Impact of Training Volume

The formulas assume you're training consistently with the target distance in mind. If you run 50 km/week (30 miles/week) with a lot of interval training, you might excel at a 10K. But a marathon ideally requires 60 to 80 km/week (37 to 50 miles/week) with regular long runs. Without that volume, the prediction will be too optimistic.

Metabolic Profile

Some runners are naturally "endurance-oriented" (good fat metabolism, high lactate threshold) and overperform in marathons relative to their 10K times. Others are more "explosive" (high VMA/VO2max, good anaerobic efficiency) and underperform over long distances. Formulas don't capture this individual difference — and personally, I think that's their biggest limitation.

How to Use These Predictions Smartly

1. Prioritize the half marathon as a reference — if you're training for a marathon, run a half marathon 4 to 6 weeks beforehand. It's the best possible calibration.
2. Use a reliable predictor — our prediction tool allows you to calculate a target time by taking your reference distance into account.
3. Add a safety margin — for a first marathon, take the prediction and add 5 to 10%. Trust me, it's better to finish with something left in the tank than to hit the wall at mile 22.
4. Assess your "endurance factor" — if you've regularly done runs of 2+ hours and weeks of 60 km+ (37+ miles), the prediction will be more reliable than if your longest run is only 1 hour and 15 minutes.
5. Adjust for weather — temperature heavily impacts a marathon (2-5% slowdown above 68°F/20°C).