What Marathon Can I Run Off My Half Marathon Time?

Here is the short answer. Your half marathon is the best clue to your marathon time. But a marathon is not just two half marathons added together. Most runners slow down a lot in the second half. A good guess is that your marathon will take about 2.1 to 2.3 times your half marathon time. Where you land depends on your training. A 1:45 half can mean a 3:30 marathon on a great day. The same half can mean 3:45 or even 4:00 if you have not trained your endurance.

The table below shows what different half times can mean. The middle column is your best case if you run an even pace. The last column is the honest range to plan for.

Recent half	Best-case marathon	Realistic range
1:30	3:09	3:01 – 3:16
1:40	3:30	3:21 – 3:38
1:50	3:51	3:41 – 4:00
2:00	4:12	4:02 – 4:22
2:10	4:33	4:21 – 4:44

Why isn't the marathon just 2× my half marathon?

If you doubled your half time, you would have to hold the same speed for twice as long. Almost no one can do that. The longer you run, the more you slow down. And the slowdown gets much worse after about 25 to 30 km. Your half marathon cannot tell you what happens that far out.

A big study showed this clearly. Researchers looked at 2,303 runners. They tested the math that most free calculators use. It worked well up to the half marathon. But it guessed marathon times too fast. For half of the runners, it was off by at least 10 minutes (Vickers & Vertosick, 2016).

This is why a good tool shows a range, not one exact time. The 1:45 half above gives a 3:30 best case. But many runners will really finish closer to 3:40. A fair projection should tell you that up front.

What changes the time between your half and your marathon?

Three things decide if you finish fast or slow.

1. How much you run each week. The same study found big differences here. Running 50 miles a week instead of 30 made runners about 25 to 32 minutes faster (Vickers & Vertosick, 2016). Your half shows your fitness today. Your weekly miles show if that fitness will last for the whole marathon.

2. Fuel and "the wall." Your body stores a kind of sugar called glycogen. It is your main fuel. You only store enough to run hard for about 30 km. After that, you run low. Then you slow down a lot. Runners call this "hitting the wall." It usually happens after the 20-mile mark (Rapoport, 2010).

This is very common. One study looked at more than 4 million races. About 28% of men and 17% of women hit the wall. Starting too fast made it much more likely (Smyth, 2021). Your half marathon time cannot warn you about any of this.

3. Your long runs. The last 10 km of a marathon is the hardest part. Long training runs teach your body to keep going when it is tired. If you have done many long runs of 30 to 35 km, you will finish near the fast end of the range. If your longest run was short, you will likely finish near the slow end, no matter how fast your half was.

How do I check if my marathon goal is realistic?

Use the projection as a reality check, not a promise.

• If your goal is inside the range, it is realistic — but only if your training backs it up. Plan to race near the middle, not the fast edge.
• If your goal is faster than the range, you are betting on fitness you have not shown yet. That can be a stretch goal. But run the first 10 km at the realistic pace. Going out too fast is the top cause of hitting the wall.
• If you have not done the long runs, expect a slow-end finish. The smart move is to fix your training first, then check again.

Two facts are worth remembering. Even pacing leads to faster marathon times. And fast starts hurt the most, because they raise your chance of hitting the wall (Smyth, 2021). The marathon rewards patience that the half never needed.

How does PIRX project your marathon?

Most free calculators use one simple formula on one race time. PIRX works differently. PIRX's proprietary machine-learning prediction engine learns your own endurance style. It checks several methods against each other. When they do not agree, it makes the range wider. So you get an honest range, not a fake-exact number. Add more than one race — like a 10K and a half — and the range gets tighter.

This is what makes PIRX different. Strava tells you what you did. Garmin shows your data. PIRX tells you why it happened and what's next — with validated prediction accuracy of 98% across PIRX users.

Want to see how shorter races feed your projection? Try the Half Marathon Time Predictor and the 10K Time Predictor. When you are ready, project your own marathon below.

Sources

• Vickers, A.J. & Vertosick, E.A. (2016). An empirical study of race times in recreational endurance runners. BMC Sports Science, Medicine and Rehabilitation. (2,303 recreational runners.)
• Smyth, B. (2021). How recreational marathon runners hit the wall: a large-scale data analysis of late-race pacing collapse in the marathon. PLOS ONE. (More than 4 million race records.)
• Rapoport, B.I. (2010). Metabolic factors limiting performance in marathon runners. PLOS Computational Biology.