How tall is a fire hydrant?
The part you can see is usually two to three feet. The part you cannot see is often taller than the part you can.
Short answer: the part you can see is usually somewhere between two and three feet tall. The part you cannot see is often taller than the part you can.
Above ground
Most municipal dry-barrel hydrants stand roughly two to three feet above grade at the top of the bonnet. That is a range rather than a figure because it genuinely varies. The common manufacturers in the American market are Mueller, Clow, Waterous and American Darling, and each runs its own dimensions across its own model lines.
What is actually regulated is not the total height. It is the outlet.
The dimension that is in the code
NFPA 24, the standard for private fire service mains, requires the center of the lowest outlet to sit at least 18 inches above final grade. On a typical hydrant that is the large steamer connection, which sits below the two smaller hose outlets.
Local fire codes frequently add to this. A common additional requirement is clear space around the hydrant, often cited as three feet in every direction, though the figure varies by jurisdiction and some require five. Anything planted, parked or stacked inside that circle is usually a violation.
If you are specifying or inspecting rather than curious, check your own jurisdiction rather than this page. Adoption of the model standards is not uniform.
Below ground
The buried portion is set by the local frost line, because the main valve has to sit below the depth at which the ground freezes. That is why an identical hydrant model ships in different bury depths depending on where it is going, and why the spec sheet asks for a depth rather than assuming one.
A hydrant in Missouri is buried deeper than one on the Gulf Coast and shallower than one in northern Minnesota. Total length from bonnet to valve can pass six feet.
Why any of it is regulated
A hydrant is useless if a firefighter cannot get a wrench on it and a hose into it in the dark, in a hurry, possibly in snow.
The outlet clearance exists so the connection can be made without anybody digging first. The clear space around it exists for the same reason. Both numbers are written for somebody working fast in bad conditions, which is a good thing to remember before planting something decorative next to one.
Replica hydrants for dog parks
Sizes here run smaller, because they are furniture rather than plumbing. The common range is 19 to 33 inches, with novelty versions up to six feet.
They are not connected to anything and they are not regulated by any of the above. The only dimension that matters is whether a dog recognizes it, and dogs are not fussy.
