half marathon to marathon calculator

How this half marathon to marathon calculator works

This tool estimates your marathon time from your half marathon performance using a proven race prediction formula (Riegel model). The model assumes your pace slows slightly as distance doubles, and that slowdown depends on your endurance profile.

In short, if you ran a strong half marathon, this marathon pace calculator helps translate that fitness into a realistic goal time for 42.195 km (26.2 miles), along with suggested split checkpoints.

The formula behind the prediction

We calculate marathon time with:

T2 = T1 × (D2 / D1)^exponent

• T1 = your half marathon time
• D1 = half marathon distance (21.0975 km)
• D2 = marathon distance (42.195 km)
• exponent = fatigue factor (usually around 1.06)

A lower exponent is more optimistic and usually fits runners with strong aerobic endurance and high weekly volume. A higher exponent is safer for first-time marathoners or runners with inconsistent long-run training.

How to use the calculator effectively

1) Use a recent half marathon result

For best accuracy, use a race from the last 6–10 weeks on a reasonably flat course and in normal weather.

2) Match the race model to your training

• Aggressive: consistent long runs, high mileage, fueling practiced.
• Balanced: most runners preparing with moderate marathon training.
• Conservative: first marathon, interrupted block, or uncertain race-day fueling.

3) Add conditions adjustment

Heat, humidity, wind, elevation gain, and crowded starts can all slow real race performance. Adjusting by +2% to +6% is common for difficult race days.

What the output means

• Predicted finish time: your estimated marathon result.
• Pace per mile / per km: average pace required.
• Suggested split table: checkpoint times to help pacing strategy.

This is a planning tool, not a guarantee. Marathon outcomes also depend heavily on fueling, hydration, pacing discipline, and weather.

Practical marathon pacing tips

Start controlled

Many runners lose significant time by going out 10–20 seconds per mile too fast. A calm first 10K usually leads to a stronger final 10K.

Fuel early and consistently

Begin carbohydrate intake early (often around 25–35 minutes) and continue regularly. Practice your exact race nutrition strategy during long runs.

Use split bands

Print your predicted split targets and carry them on race day. This helps avoid emotional pacing decisions and improves execution under fatigue.

FAQ

Can I just double my half marathon time?

Doubling is usually too optimistic for most runners. The race predictor accounts for fatigue over longer distance, which is why this method is more realistic.

How accurate is this calculator?

For runners with consistent training and similar race conditions, it can be very useful. But if your long-run endurance is underdeveloped, the prediction may be faster than race-day reality.

Should I use this for a BQ attempt or major PR goal?

Yes—use it as a baseline. Then compare with your long-run workouts, threshold sessions, and fueling rehearsals to finalize your target pace range.