Ultra running calorie calculator: burn and fuelling plan

Written by James Frost, founder of Flaming Phoenix and the maker of Phoenix Bars. Updated November 2026.

What you'll burn and what to eat

Quick answer: A 70 kg runner burns about 4,500 calories in an eight-hour trail 50k and about 14,550 in a 26-hour hundred-miler, but the gut can only take in about 240 to 440 calories an hour. So every ultra is run in a planned calorie deficit, and fuelling is the art of keeping that deficit survivable.

The calculator below gives you both figures in one go: your total burn and your eating plan in grams, calories, gels and high-calorie bars, plus the deficit between them, so you can plan the race you'll actually run rather than the one a generic running calculator imagines.

I built this because the standard tools answer the wrong question. Knowing you'll burn 14,000 calories is trivia; knowing you need 90 grams of carbohydrate an hour for 26 hours, and that as gels alone that's nearly a hundred sachets, is a race plan, and it's the plan ultra runners from UK trail hundreds to the Marathon des Sables actually build. Strategy, hydration and gut training live in the full ultra running nutrition guide; this page is the arithmetic.

How the maths works

Two calculations, both shown openly. The burn side uses MET values, the standard exercise-science measure where one MET costs roughly one calorie per kilogram of body weight per hour: a runnable fast ultra sits around 9.5 METs, the mixed run-hike of a typical trail ultra about 8, and a mountainous grind heavy with hiking around 6.5, multiplied by your weight and hours. The fuelling side runs the other direction entirely, from your gut's trained capacity: 60 grams of carbohydrate an hour is the reliable starting point, 90 a well-trained gut, and up to around 110 the seriously rehearsed competitive end, converted at four calories per gram. The burn number scales with your body and the course; the fuelling number scales only with training and hours, which is why big engines in untrained guts blow up.

Worked example for a 72 kg runner on an eight-hour trail 50k at 60 grams an hour: 8 × 72 × 8 = 4,608, which the calculator rounds to about 4,600 calories burned. 60 × 8 = 480 grams of carbs, or 1,920 calories eaten, leaving a deficit of about 2,700.

How many calories does an ultra marathon burn?

Most ultras burn roughly 3,000 to 20,000 calories, depending on distance, body weight and how much of the course you hike. These figures use the same formula as the calculator, with example finishing times; enter your own time above for a personal figure. They're approximate and rounded to the nearest 50 calories.

The last column shows what a practised 75 grams of carbs an hour puts back in: roughly half to two-thirds of a 70 kg runner's burn. The rest is the planned deficit.

Why is every ultra run in a calorie deficit?

Subtract your fuelling from your burn and the gap looks alarming: a 70 kg runner on a 14-hour mixed 100k burns about 7,850 calories, fuels perhaps 4,200 at 75 grams an hour, and runs the day about 3,650 calories in the red. That gap is normal in ultra running, and your body bridges it from stored fat and glycogen provided the incoming stream never stops. The real danger is letting intake fall away for hours at a time, so a steady hourly floor of around 200 calories, even through bad patches, is the number to protect, whatever the size of the overall deficit. Fuel from the first hour, hold your rate through the rough patches in smaller pieces if you must, and let recovery eating repay the balance; a proper 2,000 calorie refuel plate the evening after is part of the plan.

Reading your fuelling number

The calculator gives your total in gels and in bars deliberately, because the comparison teaches the strategy. At 100k distances the pure-gel figure lands in the forties and beyond, and at a hundred miles it reaches roughly a hundred sachets, a number no stomach or race vest can manage, which is why experienced runners fuel in stages instead: fast carbs early while the gut is fresh, real solid food through the long middle, soft, salty and drinkable late when chewing and sweetness both fail. That full staging, food by food, is the best foods for ultra running timeline, with more detail in our guides to energy gel alternatives and ultra high carb solid foods. The bar figure is there for the middle stage. I make Phoenix Bars, with up to 557 calories and up to 66g of carbs in a compact, 120g bar, so one Phoenix Bar covers most of an hour at 75 to 90 grams of carbs an hour.

Train the number before you race it

One rule turns this page from arithmetic into a finish: never race a fuelling rate you haven't rehearsed. The gut trains like a muscle, taking weeks of practising your target intake on long runs to hold it under race stress, so pick your rate in the calculator honestly, based on what you've actually practised, not what the elites absorb. Moving from 60 toward 90 grams an hour takes weeks of long runs at the higher rate; the nutrition guide covers how. Rehearse with the exact products you will race on; our guide to the best energy bars for ultra running compares the main options.

Frequently asked questions

How many calories do you burn in an ultra marathon? Roughly 3,000 to 6,000 for a 50k, 6,500 to 12,000 for a 100k and 12,500 to 20,000 for a hundred miles, for runners between 60 and 90 kg, driven by body weight, terrain and hours on feet. The calculator above personalises it.

How many calories should I eat per hour in an ultra? Aim for 60 to 90 grams of carbohydrate an hour, about 240 to 360 calories, trained upward toward 110 grams if you're competitive. On a bad patch, try not to let intake fall much below about 200 calories an hour for long.

Can you eat back what you burn during an ultra? No, and you're not supposed to. At 60 to 90 grams of carbs an hour you take in about 240 to 360 calories, against a burn of roughly 500 to 650 an hour for most runners on a mixed trail ultra, so ultras are run in a planned deficit that steady fuelling manages and post-race eating repays.

How many gels do you need for a 100k? At 75 grams an hour for 14 hours you'd need around 42 standard gels, which is why nobody fuels a 100k on gels alone: the working answer is gels early, solids through the middle, soft and liquid late.

Does hiking the climbs burn fewer calories than running them? Per minute yes, per race barely, because you're out there longer. The effort setting in the calculator handles the mix; the fuelling requirement stays relentless either way.

Written by James Frost, founder of Flaming Phoenix and the maker of Phoenix Bars, which are used by ultra runners on UK trails and at the Marathon des Sables. This calculator exists to answer "how many gels is that?" Last updated November 2026.

Sources: 2024 Adult Compendium of Physical Activities; Jeukendrup, carbohydrate intake during exercise (Sports Medicine, 2014); Jeukendrup, Training the gut for athletes (Sports Medicine, 2017); Phoenix Bar values from our own pack labels.