How many flights of stairs is a mile?
Two answers, because two different questions share the wording. Climbing a mile straight up is 528 flights, since a mile is 5,280 feet and Apple Health scores every ten feet of ascent as one flight. Climbing enough stairs to feel like a mile of walking is usually put at about 20 flights, and that one is a rule of thumb nobody has measured properly.
A vertical mile: 528 flights
This is the only version with a real answer. A mile is 5,280 feet, a flight in Apple Health is ten feet of climbing, and the division comes out even. Nothing about stairs enters into it, because your phone measures air pressure and never counts a step.
No building on earth gets you there. The Burj Khalifa stairwell runs 2,909 steps to the 160th floor, roughly 1,900 feet up, which is about a third of a vertical mile. The longest staircase there is, the service stairs beside the Niesen funicular in Switzerland, is 11,674 steps and locked to the public except for one race day a year.
The Grouse Grind gets you half a vertical mile and takes most people two hours.
A mile of effort: about 20 flights
The number you will see quoted everywhere, usually with no source. It comes from the observation that going up costs several times what going along costs, so a smaller amount of climbing buys the same tiredness as a mile of flat walking. As a way to decide whether the stairwell at work counts as exercise it is fine.
As a measurement it is not. Effort is not a distance, and the exchange rate depends on your weight, your pace and the stairs. Anyone giving you 20 as a fact is giving you a feeling with a number stapled on.
Why the published answers run from 20 to 350
Because the two versions above cannot both be converted through step counts, and most articles try anyway. A mile of walking is around 2,000 steps. A flight is around ten to twelve stairs. Divide one by the other and you get roughly 180 flights, which matches neither answer and describes nothing real, because those 2,000 steps were horizontal and these are not.
Chase it through distance instead and you get a third number. A stair tread is about 11 inches deep with a 7 inch rise, so the line you actually travel is around 13 inches per step, and a mile of that is nearly 4,900 stairs. All of these arrive at real arithmetic. None of them is answering the question you asked.
What your phone will show you
Walk a mile on the flat and Health records zero flights. Walk a mile up a road that climbs 200 feet and it records 20, without a stair involved. The flight count in your Health app is a measure of how high you went, and a mile is a measure of how far. They only line up when you are going straight up, which is why 528 is the honest answer and also the useless one.
What a mile equals
Each one links to where the number came from.