Issue 005 - Energy supply chains - Unit conversion
How many car fill-ups equal one day of oil through the Strait of Hormuz?
Recent U.S.-Iran tensions renewed attention on the Strait of Hormuz, a major oil-shipping chokepoint. Reuters reported that oil prices rose after fresh U.S. strikes, while AP described renewed anxiety over fuel prices and tanker traffic through the Persian Gulf.
The problem
Estimate how many average U.S. car fill-ups are equivalent to one day of oil that normally passes through the Strait of Hormuz.
When people say "one-fifth of global oil supply passes through Hormuz," is that closer to enough oil for 1 million, 10 million, or 100 million gasoline fill-ups per day?
Because Fermi problems target an order of magnitude, I normally use no more than two significant digits and write most calculations in scientific notation; the Fermi reference explains both conventions.
Before checking sources
Matt's first pass
I had to do some creative estimating because I did not know total worldwide oil use in gallons or barrels per day.
I started from U.S. driving behavior. I assumed oil use in the United States was 50% vehicle fuel and 50% everything else. I guessed the average vehicle is refueled once per week, the average refill is about 8 gallons, and the United States has about 1.5 vehicles per household.
U.S. vehicles ~= (1.5 vehicles/household) x (1.3 x 10^8 households)
~= 2 x 10^8 vehicles
weekly vehicle fuel ~= (2 x 10^8 vehicles) x (8 gallons/week)
~= 1.6 x 10^9 gallons/week
daily vehicle fuel ~= (1.6 x 10^9 gallons/week) / 7
~= 2.3 x 10^8 gallons/dayThen I scaled from vehicle fuel to total U.S. oil use:
U.S. oil use ~= (2.3 x 10^8 gallons/day) / 0.5
~= 4.6 x 10^8 gallons/dayFor the global scale, I remembered the United States is about 1/27 of world population, but much richer and more energy-intensive than average. I guessed U.S. oil consumption might be about 1/20 of world oil consumption.
world oil use ~= (4.6 x 10^8 gallons/day) x 20
~= 9.2 x 10^9 gallons/day
Hormuz oil ~= (9.2 x 10^9 gallons/day) x (1 / 5)
~= 1.8 x 10^9 gallons/dayFinally, I converted that oil volume into 8-gallon fill-ups:
fill-ups ~= (1.8 x 10^9 gallons/day) / (8 gallons/fill-up)
~= 2.3 x 10^8 fill-ups/day
~= 230 million fill-ups/dayThat was about 8 times my own estimate for actual daily U.S. pump visits, because 2 x 10^8 cars refueling once weekly implies roughly 3 x 10^7 fill-ups per day.
Calibration Score
Matt's Calibration Score: 55 / 100
Higher is better: earn points for accurate pegs, sound models, correct math, and a result close to the sourced answer. The image shows percent full of it: 100 minus the Calibration Score.
Pegs: 10/30. U.S. versus world oil-consumption pegs were badly miscalibrated.
Model: 15/30. The crude-to-fill-up model was useful, but the gasoline-yield caveat mattered.
Math: 10/10. The arithmetic was clean.
Result: 20/30. The answer landed on the right answer choice, helped by canceling errors.
Grounding facts
The direct EIA Hormuz figure, 20 million barrels per day, is already one of the best memory anchors here. In crude-volume terms:
20 million barrels/day x 42 gallons/barrel ~= 840 million gallons/dayThat crude-volume comparison is roughly three to four times Matt's estimate for actual daily U.S. pump visits, depending on whether a fill-up is 8, 10, or 12 gallons. If you adjust for gasoline yield, the comparison is closer to U.S. daily pump scale rather than several times larger. Either way, "one-fifth of global oil supply" is not merely a market abstraction; it is visibly connected to everyday fuel behavior.
After checking sources
Check and recalibrate
The EIA's July 2026 Short-Term Energy Outlook puts global petroleum and other liquid fuels consumption near 1 x 10^8 barrels per day. EIA's Hormuz analysis says oil flow through the strait averaged about 20 million barrels per day in 2024, around 20% of global petroleum liquids consumption, and was roughly flat in early 2025.
Hormuz oil ~= 2 x 10^7 barrels/day
crude-volume gallons ~= (2 x 10^7 barrels/day) x (42 gallons/barrel)
~= 8.4 x 10^8 gallons/dayIf an ordinary fill-up is roughly 8 to 12 gallons, then the crude-volume comparison is:
fill-ups ~= (8.4 x 10^8 gallons/day) / (8 to 12 gallons/fill-up)
~= 7 x 10^7 to 1.1 x 10^8 fill-ups/day
~= 70 million to 110 million fill-ups/dayThat makes the answer closer to 100 million fill-ups per day than to 10 million or 1 million.
Matt's first pass landed on the right answer choice, but high. The main issue was the global scaling assumption: the United States is about 1/27 of world population, but for oil consumption it is much closer to one-fifth of the world than one-twentieth. Multiplying the U.S. estimate by 20 therefore overshot global oil use.
The definition caveat matters. A 42-gallon barrel of crude oil does not become 42 gallons of gasoline; EIA says U.S. refineries produce about 19 to 20 gallons of motor gasoline from one barrel, plus diesel and other products. If you adjust to gasoline yield, the comparison drops to roughly several tens of millions of fill-ups per day, but it is still a huge gas-pump-scale number.
Post-check reflection
Matt's reflection
My energy-consumption assumption about Americans was way off, and I already thought I was assuming a high proportional share of world energy use. It was shocking that the United States is such a massive oil user compared with the rest of the world.
I also forgot to correct for gasoline versus crude oil. To be honest, I would not have properly guessed that refineries get roughly one gallon of motor gasoline from every two gallons of crude oil. That correction brought my estimate down and closer to the sourced number, continuing the pattern of canceling errors.
The corrected answer felt about right after seeing the corrections to my assumptions. The scale changes my perspective on the news item somewhat. Using my own first-pass vehicle model, about 2 x 10^8 vehicles refueling once per week means roughly 3 x 10^7 fuel-pump visits per day in the United States, or about 2.3 x 10^8 gallons per day at 8 gallons per fill-up. Comparing those everyday numbers with the Strait of Hormuz estimate further emphasizes how wildly energy-hungry the United States is.
Recommended memory peg
Remember that world oil consumption is roughly 1 x 10^8 barrels per day, and one oil barrel is 42 gallons. For this kind of gas-pump comparison, an average fill-up around 1 x 10^1 gallons is a useful everyday peg.
Reader results
Bars show how submitted estimates sort into the answer choices from the gut-check prompt.