Marathon time calculator: predict your finish

Q: How do I use Riegel's formula to predict my marathon time?

T_marathon = T_known × (42.195 ÷ D_known)^1.06. Example: half marathon in 1:45 (105 min) → 105 × 2.112 = 221.8 min = 3:41:48. Most accurate when predicted from a half marathon with equivalent training.

Q: What is the wall and how do I avoid hitting it?

The wall is sudden fatigue from glycogen depletion, usually between mile 18–22. Avoid it by starting conservatively, fueling with 30–60g carbs per hour from mile 6, staying hydrated, and completing multiple 18–20 mile training runs.

Q: How many weeks of training do I need for a marathon?

18 to 24 weeks with a structured plan is standard for a first marathon. Enter training able to run 45–60 minutes comfortably. Peak weekly mileage reaches 40–55 miles for intermediate runners.

Q: What qualifying time do I need for the Boston Marathon?

BQ times vary by age and sex. Men 18–34 need approximately sub-3:00; women 18–34 need approximately sub-3:30. Times become more lenient with age. Meeting the BQ doesn't guarantee entry — cutoff times apply.

Q: Is it better to run negative or positive splits?

Negative splits (second half faster than first) consistently produce faster finishing times. Start 5–10 seconds per mile slower than goal pace for the first 13 miles to protect glycogen stores and set up a strong finish.

Running a marathon is on the bucket list of millions of people worldwide — and for good reason. The 26.2 miles (42.195 km) of a marathon represent not just a physical feat but a months-long mental and physical journey of preparation. Whether you're running your first marathon with the simple goal of crossing the finish line or targeting a specific time like the symbolic sub-4 or coveted sub-3 hour, knowing your target pace is the foundation of every sound training plan and race strategy.

This marathon time calculator works in multiple directions: enter your target finish time to get pace, enter your pace to get finish time, or use a recent race result from a 5K, 10K, or half marathon with Riegel's prediction formula to estimate what you can realistically expect over 26.2 miles. All calculations support both metric (km/min/km) and imperial (miles/min/mile) units, with automatic conversion.

How to calculate your marathon time

The three core relationships are:

Finish time (min): time = pace (min/mile) × 26.2 — or — pace (min/km) × 42.195
Target pace (min/mile): pace = target time (min) ÷ 26.2
Riegel's prediction formula: T_marathon = T_known × (42.195 ÷ D_known)^1.06

Riegel's formula uses an exponent of 1.06 to account for the increasing fatigue factor as distance grows beyond any previous effort. The longer the gap between known distance and the marathon, the less precise the prediction — it works best when predicting from a half marathon result, and still well from a 10K if you have appropriate long-run training.

Pace reference table and the wall at mile 20

Here are finish times for common marathon paces:

9:00/mile (5:35/km, 6.67 mph) → 9 × 26.2 = 235.8 min = 3:55:48
10:00/mile (6:13/km, 6.0 mph) → 10 × 26.2 = 262 min = 4:22:00
11:00/mile (6:50/km, 5.45 mph) → 11 × 26.2 = 288.2 min = 4:48:12
12:00/mile (7:27/km, 5.0 mph) → 12 × 26.2 = 314.4 min = 5:14:24

For the magic sub-4 hour (240 min) target: required pace = 240 ÷ 26.2 = 9:09/mile (5:41/km). Speed = 6.55 mph = 10.55 km/h.

The notorious wall at mile 20 (km 32) occurs when muscle glycogen stores are depleted — typically after 2 to 2.5 hours of sustained aerobic effort. Without adequate glycogen, the body shifts to fat metabolism, which produces energy far more slowly, causing the dramatic slowdown many runners experience in the final 10K. Strategies to avoid the wall: run the first 13 miles 10–15 seconds per mile slower than goal pace; consume 30–60 grams of carbohydrates per hour starting at mile 6–7 (every 30–45 minutes); complete at least two or three long runs of 20 miles or more in training; and keep well hydrated in the days before the race.

Using Riegel's formula from a half marathon: if you ran a half in 1:50:00 (110 min), your predicted marathon is 110 × (42.195 ÷ 21.0975)^1.06 = 110 × 2.112 = 232.3 min ≈ 3:52:18. This prediction assumes comparable fitness and appropriate marathon-specific training, including high long-run mileage. Runners who have never run beyond 16 miles in training will often finish slower than predicted regardless of their half-marathon speed.

Frequently asked questions

How do I use Riegel's formula to predict my marathon time?

T_marathon = T_known × (42.195 ÷ D_known)^1.06. Examples: 10K in 50 min → 50 × (42.195/10)^1.06 = 50 × 4.489 = 224.5 min = 3:44:30. Half marathon in 1:45 (105 min) → 105 × 2.112 = 221.8 min = 3:41:48. The formula is most accurate when predicted from a half marathon with equivalent training.

What is the wall and how do I avoid hitting it?

The wall is the sudden fatigue caused by glycogen depletion, usually hitting between mile 18–22. To avoid it: start conservatively (10–15 seconds slower than goal pace for the first half), fuel with 30–60g carbs per hour from mile 6 onward, stay hydrated, and build long-run endurance with multiple 18–20 mile training runs before race day.

How many weeks of training do I need for a marathon?

For a first marathon, 18 to 24 weeks with a structured plan is the standard recommendation. You should enter training already able to run comfortably for at least 45–60 minutes. Peak weekly mileage in most plans reaches 40–55 miles (65–88 km) for intermediate runners, with the longest single run being 20–22 miles.

What qualifying time do I need for the Boston Marathon?

Boston Qualifying (BQ) times depend on age and sex. For the 2025 race, men aged 18–34 needed sub-3:00:00 to have a strong chance of acceptance; women aged 18–34 needed sub-3:30:00. Qualifying times become progressively more lenient with age. Note: meeting the BQ standard doesn't guarantee entry — cutoff times apply based on how oversubscribed the race is.

Is it better to run negative or positive splits?

Research consistently shows that negative splits (running the second half faster than the first) produce the fastest and most consistent finishing times. It's psychologically difficult to execute because of race-day excitement at the start, but starting just 5–10 seconds per mile slower than goal pace for the first 13 miles protects your glycogen stores and sets up a strong finish.

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