Issue 081 - Sports - Walking metabolic energy
How much energy does it take to walk around the world?
Karl Bushby returned home to Hull after a reported 31,000-mile journey spanning 28 years. How much food energy did physically covering that distance require?
Everyday life and sportsAbout 1 minute
Sources checked November 2, 2026. Figures and circumstances may have changed.
The problem
Estimate the additional metabolic energy needed to walk 31,000 miles. How many years of ordinary food consumption does that walking effort represent?
Choose body mass, carried load, walking cost and a daily food-intake reference. Estimate food Calories first, then convert to equivalent years. Count the walking effort above resting needs, rather than multiplying all 28 years by daily food intake.
The guess unit is years of ordinary food intake equivalent to the walking energy, not years spent walking. Optionally compare gasoline or electricity, or estimate shoes worn out.
Fermi estimates target scale rather than precision. The Fermi reference collects useful facts and equations.
Grounding facts
AP reported Bushby's return on November 2, 2026, describing a 28-year, 31,000-mile journey. We model the rounded 50,000-kilometer distance as walking; actual swimming, terrain, equipment and delays make this an approximation rather than a measured expedition energy budget.
Walking physiology offers a better starting point than adding vertical lifts. The ACSM walking equation's horizontal component is approximately 0.1 milliliters of oxygen per kilogram per meter. At about 5 food Calories per liter of oxygen, that corresponds to 0.5 kcal per kilogram per kilometer above its resting term on level ground. Published validation work cautions that speed, slope and load affect prediction accuracy.
Use Matt's 100 kg body-plus-gear as a simplifying load peg, not Bushby's measured mass. Applying the level-ground coefficient to that combined mass is an approximation; pulling a cart is different from carrying the same load. Hills and difficult footing can raise costs.
A food Calorie is a kilocalorie: about 4,184 joules. The 2,500-Calorie daily reference below is ordinary total food intake, not basal metabolic rate, which excludes most daily activity.
After checking sources
Checked answer and calculation
distance
~= 31,000 miles x 1.6 km/mile
~= 50,000 km
net walking energy, level-ground model
~= 0.5 kcal/(kg km) x 100 kg x 50,000 km
~= 2.5 million food Calories
~= 1 x 10^10 J
ordinary annual food intake reference
~= 2,500 kcal/day x 365 days
~= 910,000 kcal/year
equivalent ordinary food years
~= 2.5 million / 910,000
~= 2.7 years
~= 3 years, roundedThe useful scale is a few million food Calories, or several years of ordinary food intake. A scenario allowance of 0.5 to 1 kcal per kilogram per kilometer gives about 2.5 to 5 million Calories, or roughly 3 to 5 ordinary food years. That range illustrates terrain and load sensitivity; it is not a measured confidence interval for Bushby's journey.
Across all 28 calendar years, the 2,500-Calorie reference totals about 26 million Calories. The modeled walking adds about 10% to that reference, or about 20% at the higher scenario cost. It would be concentrated on travel days, rather than spread evenly over the years. This comparison does not measure how much more he ate than he would have eaten during an ordinary active life.
Matt's model counted a 100-joule mechanical lift per step as though it were 100 joules of food energy. Human walking exchanges potential and kinetic energy, while muscles also spend energy on force, limb motion and step transitions. Mechanical work and metabolic energy are different quantities. A universal efficiency multiplier does not fully repair the model because some motion energy is recovered and other metabolic costs were omitted.
step-count correction
= 50 million meters / (2/3 meter per step)
~= 75 million steps, not 65 million
Matt's lift calculation with corrected steps
~= 75 million x 100 J
~= 7.5 billion J
~= 1.8 million kcal
shoes using Matt's own replacement habit
~= 7 miles/day x 182 days/pair
~= 1,300 miles/pair
pairs for 31,000 miles
~= 31,000 / 1,300
~= 24 pairs, not 31We interpret the two-thirds-meter length as a step; a gait stride conventionally spans two steps. The shoe result follows Matt's habit, not a recommended shoe lifespan or a record of Bushby's actual footwear.
For an energy-content comparison, 2.5 million Calories is about 2,900 kWh or roughly 300 liters of gasoline at an assumed 34 million joules per liter. These compare stored energy, not the efficiency of replacing a human walker with an engine.
Before checking sources
Matt's original estimate
This is Matt's original, unverified estimate, preserved before the source check.
first I converted 31,000 miles to about 50,000 km so the units are easier to work with.
then I assumed the average stride would be about 2/3 of a meter, that the change in vertical position of his center of mass with each stride would be about 10 cm, and then assumed that most of the energy while walking is used raising that center of mass up again after each step, so I could approximate the energy consumption of a step as mgh or about 100 kg (counting average male's bodyweight + gear) * 10 m/s^2 * 0.1 m to get about 100 J per step taken.
then I assumed about 1.3 steps/m * 50,000,000 m distance traveled to get about 6.5 * 10^7 steps, and if the average step costs about 100 J, that would require about 6.5 * 10^9 J for that trek.
a KCal of food energy is about 4 * 10^3 J, so dividing that into the total calculated above gives about 1.6 million KCals required, which would be in excess of the regular energy needed to sustain normal biological functions, and to support the distance walked.
if someone needs to consume about 2500 calories daily to support basal metabolic function, and there's 365 days in a year, that means they need to consume about 900,000 calories annually. That means the excess calorie need for that trek was a bit less than two years worth of regular calorie consumption.
the trek took place over 28 years, so that means the calorie consumption over that time would need to be about 7% higher than the basic consumption to cover the energy cost of the walking.
last, I change my shoes about every 6 months and walk about 7 miles daily, if this guy is changing shoes at the same rate, he would go through about 31 pairs of shoes during that trek.
Reasoning score
Matt's reasoning score: 60 / 100
Useful facts: 20/30. Distance, calorie conversion, mass scale and ordinary food intake were useful pegs. The vertical excursion was unverified, and 2,500 Calories/day was mislabeled as basal metabolism.
Reasoning approach: 15/30. Separating walking effort from 28 years of living was correct. Treating each mechanical lift as food energy omitted muscle efficiency, gait energy exchange and other metabolic costs.
Math: 5/10. Two-thirds of a meter gives 1.5 steps/m rather than 1.3. The shoe replacement assumptions imply about 24 pairs rather than 31; neither changes the overall scale.
Final estimate: 20/30. About 1.6 million Calories and under two food-years are within an order of magnitude of the checked few-million-Calorie, roughly three-year estimate.
Post-check reflection
Matt's reflection
Looks like I was in the right ballpark. Small error converting meters to step count. Bigger error assuming most of the energy from walking comes from returning body mass to the pre-step height, so my per-step energy consumption estimate was off by about 2.5x too low. Still, all my answers were within an order of magnitude of the "authoritative" answer.
Recommended memory peg
For ordinary level walking, start near 0.5 food Calories per kilogram per kilometer above resting needs. Terrain and load can increase this. Keep mechanical joules separate from metabolic food energy, and remember 1 food Calorie ~= 4,200 J.