Why a Marathon Is Exactly 42.195 km

A marathon is 42.195 km because of how one course was laid out in London in 1908 and a rule adopted thirteen years later — not because of anything that happened in ancient Greece. It's a historical accident that got frozen in place, and it now drives how courses are measured and how much fuel you have to get down mid-race. Here's where the number comes from and what it means in practice.

The Pheidippides legend doesn't give you the number

The story everyone knows: a messenger runs from Marathon to Athens, announces the victory over the Persians, and drops dead. The problem is that the oldest source, Herodotus, doesn't describe that run at all — in his account Pheidippides runs from Athens to Sparta to ask for reinforcements, roughly 246 km, arriving the next day. The die-at-the-finish version only shows up in Plutarch and Lucian, centuries later. And the Marathon-to-Athens route itself is somewhere between 34 and 42 km depending on whether you take the coastal road or the mountain pass. The legend gives you no precise figure.

The first 25 years: every marathon a different length

When the marathon was added to the first modern Olympics in Athens in 1896, the course was about 40 km. After that the distance moved around from Games to Games:

• 1896, Athens: about 40 km
• 1900, Paris: about 40.26 km
• 1904, St. Louis: about 40 km
• 1908, London: 42.195 km
• 1912, Stockholm: about 40.2 km
• 1920, Antwerp: about 42.75 km, the longest Olympic marathon ever run

So for two decades, “marathon” meant “roughly 40 km, whatever the organizer measured on the ground.” Comparing times across races was meaningless.

1908: Windsor, the royal box, and 385 yards

The London Games planned a round 26-mile course (41.843 km) from Windsor Castle to White City Stadium. The start was moved onto the lawn in front of the castle so the royal children could watch, and the finish was placed directly in front of the royal box inside the stadium. That extra stretch around the track added 385 yards, or 352 metres.

26 miles + 385 yards = 42,194.99 metres. Rounded: 42.195 km.

The race was also a spectacle: Italy's Dorando Pietri entered the stadium first, collapsed several times, and was disqualified because officials helped him across the line. The press coverage turned the London distance into an informal standard, and the international federation formally adopted it in 1921. From the 1924 Paris Games onward, everyone has raced the same figure.

42.195 km isn't the measurement of a historical route. It's the architecture of one stadium and the position of one seating box.

How that distance is measured today

For a course to be certified, it's measured with a bicycle fitted with a Jones counter, calibrated on a flat stretch of at least 300 metres ridden four times before and four times after the measurement. The rules are specific:

• Measurement follows the shortest possible route: about 30 cm off the curb, cutting the tangents — not the middle of the road.
• A 0.1% short-prevention factor is added (1 metre per kilometre), so the course is actually laid out at about 42,237 m, guaranteeing it is never short.
• For records: net drop no more than 1 m/km, meaning 42 metres total from start to finish.
• The straight-line distance between start and finish can't exceed 50% of the race distance, or 21.1 km, because of tailwind assistance.

That's why Boston, prestigious as it is, isn't record-eligible: it has roughly 136 metres of net drop and a start almost 40 km from the finish.

What the number changes in training

• Weekly volume: realistically 50–80 km for a sub-4-hour marathon, 30–45 km if the goal is simply to finish.
• Long run: cap it at 28–32 km, done 2–3 times in the whole block; almost nobody benefits from 40 km in training because recovery takes 10–14 days.
• Block length: 16–20 weeks, with a down week every fourth week (cut volume 25–30%).
• Taper: in the last 2–3 weeks drop volume by 30–50% but keep the intensity — 4 x 1 km at goal pace instead of a 10 km tempo.

The wall around kilometre 32

A 70 kg runner burns roughly 1 kcal per kilogram per kilometre, so about 70 kcal/km and close to 2,900 kcal for the full race. Your muscles and liver hold 400–500 g of glycogen, which is 1,600–2,000 kcal. The math runs out somewhere around 30–33 km — that's the wall, and it's an arithmetic problem, not a character flaw.

• Carbs during the race: 60–90 g per hour, starting around minute 40, not when you already feel bad.
• Fluid: 400–800 ml per hour, more in heat; don't drink more than you're losing.
• Sodium: 300–600 mg per litre of fluid, especially if you leave salt stains on your shirt.
• Carb loading: 8–10 g of carbohydrate per kilogram of body weight per day for the 36–48 hours before the start.
• Rehearse all of it on long runs at least 4–5 times. Your gut trains like your legs do.

When to see a doctor

• Before a first marathon: a check-up and ECG if you're over 35, or at any age if you have high blood pressure, high cholesterol, diabetes, or you smoke.
• Immediately: chest pain or pressure on exertion, fainting or dizziness while running, an irregular pulse at rest.
• Without delay: sudden cardiac death in the family before age 50.
• After the race: tea-coloured urine plus muscle pain that worsens 24–48 hours later (possible rhabdomyolysis), or headache, nausea and swelling — that can be hyponatremia from drinking too much water.

The short version

• 42.195 km comes from London 1908: 26 miles from Windsor plus 385 yards to the royal box.
• The Pheidippides legend doesn't produce that number — Herodotus' messenger ran about 246 km to Sparta.
• The standard was adopted in 1921, and every race has used it since 1924.
• Courses are measured by bicycle along the shortest possible route, plus a 0.1% safety margin.
• Practically: a 16–20 week block, long runs capped at 32 km, and 60–90 g of carbs per hour on race day.