How Wide Is the Strait of Hormuz? It Depends Who Is Measuring

People ask me this more than anything else about the strait, usually with a finger already resting on the map. How wide is it, really? The honest answer comes in layers, and peeling them back is as good a way as any into the whole history of the place.

The number on the chart

The figure you will meet most often is about twenty one nautical miles at the narrowest point, a little under forty kilometres, between the Musandam peninsula on the Omani side and the Iranian coast to the north. Some references give a larger number because they measure between different points, and along its length the strait opens out to more than fifty nautical miles.

Either way, this is not a crack in the rock. Twenty one nautical miles is open sea. A small boat with a modest engine could spend most of a morning crossing it, and the far coast would sit on the horizon as a low brown line the whole time.

Two miles each way

A large ship, though, does not get twenty one miles. Big commercial traffic follows a traffic separation scheme: one lane in, one lane out, each about two nautical miles wide, with a buffer of roughly the same width between them. The lanes run north of the Musandam peninsula, in Omani waters.

So the strait that matters to a laden tanker is a corridor of about two miles, shared with everything else heading the same way. The rest of the width is still there. Shallows, islands, fishing grounds, the approaches to other people's harbours. It is simply not the road.

This is the first lesson of the place, and the one I find hardest to explain over dinner: a passage is the part of the water people have agreed to use. The agreement is as real as the rock.

A passage is the part of the water people have agreed to use.

Fingers and watches

Five centuries ago the question would have been asked in different units altogether. Arab navigators of the Indian Ocean measured the height of a star above the horizon in fingers, the isba. They measured distance in the zam, which began as a watch of three hours and came to mean the stretch of sea a ship covered in that time. Eight zam made a day and a night. Ahmad ibn Majid set much of this craft down in his Kitab al-Fawa'id in 1490, and he was writing up knowledge that pilots had carried for generations before him.

What I love about the zam is its honesty. It never pretends that distance is fixed. A watch with a fair wind covers more water than a watch in a flat calm, and scholars who study these texts point out how much the length of a zam could vary. A sailor leaving the old island market of Hormuz did not ask how wide the water was. He asked how long, with this wind, in this month, in this hull.

The width you cannot measure

So there are at least three answers already: the width between the coasts, the width of the lane, and the width of a watch. I have come to believe there is a fourth, and across the centuries it has mattered more than the other three together. A strait is as wide as people's confidence in crossing it.

When merchants trusted the passage, a lane of two miles was plenty, and the island traders who grew rich at its mouth understood that what they really sold was that trust. When merchants lost it, all twenty one miles of open water could not persuade a single captain to weigh anchor.

That fourth width is the one I spent most of my time chasing while writing Strait of Hormuz, because no chart records it. You find it instead in what people chose to do: who sailed, who waited, who wrote a promise into a ledger and who kept it.

The next time someone puts a finger on the map and asks me how wide it is, I will give them the number. Then I will ask which width they mean.