What’s the ideal temperature for a marathon?

There’s a reason so many marathons take place in April and October. These are the “Goldilocks” months, before it gets too cold and you risk ice or snow putting a stop to proceedings and, with it, months of training squandered. It’s never truly lost, but it can be a massive disappointment. And summer? Way too hot.

While the spectators might love a hot, sunny day, it's far from ideal for the runners. Once temperatures climb above around 15°C, your chances of a PB begin to dwindle. That’s because your body now has to divide its energy between maintaining pace and keeping itself cool.

The physiological demands are significant. Your core temperature rises. To prevent overheating, blood flow is diverted away from muscles to the skin to help release heat. Sweat production increases to promote cooling, but this comes at the cost of fluid and electrolyte loss. Heart rate rises, perceived effort skyrockets, and pace almost always drops.

A case in point is the London Marathon 2025, where race-day temperatures reached 22°C – a full 9°C warmer than the year before. Strava data, published in Runner’s World, found that men ran an average of eight minutes slower than in 2024. Women were more than ten minutes slower. That’s not a marginal effect.

I was at both, and the difference was palpable. As things started to heat up around 10 or 11am in 2025, you could feel the sun sapping your energy. Sweat was pouring. Runners were ditching hats and clothing. Strong athletes were pulling to the side, defeated by conditions. Any stretch of shade – a tunnel, a shadow cast by buildings – gave you a temporary reprieve, and your pace would briefly pick up, almost magically.

For me, the ideal marathon temperature is around 7 to 8°C. I like it cold. At 45 years old, my heart rate only goes so high, and my body can only multi-task so well. Once the mercury tips beyond 14°C, I’ve pretty much written off a PB.

That gut instinct is backed up by research. A 2012 study in PLOS One, which analysed nearly 2 million marathon finish times, found that performance declines begin above 10°C. Another study in Medicine & Science in Sports & Exercise concluded that the optimal marathon temperature for peak performance is around 7°C for elite athletes, and closer to 10°C for recreational runners. Beyond that, for every 5°C rise, the average finish time slows by roughly 1.5 to 3 percen.

Can you choose your conditions?

Of course, you can't control the weather, but you can factor temperature into your marathon strategy. If you’re targeting a sub-3 finish, course selection matters. Here are a few tips to help you assess conditions before committing:

• Check historical weather data: Look up average race-day temperatures for the location and time of year. Websites like TimeAndDate or WeatherSpark offer historical trends.
• Watch for time of day: Early morning starts are preferable, especially in warmer regions. A 7am gun time buys you precious cooler miles.
• Look at previous years: Marathons often publish finishing stats, and forums will give anecdotal reports on heat, wind and other conditions. Be wary of outliers – like London 2025 – but do your homework.
• Avoid risky months: Late spring and summer marathons are tempting, especially scenic ones, but heat is often a performance killer. Consider autumn or early spring instead.

Bake temperature into your strategy

Getting a sub-3 isn’t just about training hard. It’s also about making intelligent choices. Temperature can be the difference between a breakthrough and a blow-up. The same runner who finishes in 3:07 on a warm day (such as London 2025) might have gone sub-3 in cooler conditions the year before. It’s easy to fall into the trap of blaming external factors for a poor performance. So don’t. Instead, plan for them.

You wouldn’t wear the wrong shoes or run without fuel, but even with the best preparation, sometimes the weather just doesn’t play ball. When that happens, your options are simple: adapt your strategy, or adjust your expectations. Either way, don’t ignore the forecast. Make temperature part of your marathon planning from the outset – not as an excuse, but as a factor that can make or break a sub-3 attempt.