Rather than ascend to 600 feet as Mills had intended in the original plan, Casey was persuaded to make the height of the structure ten times the width of the base, meaning the optimal height for the Washington Monument was 555 feet.

The Washington Monument stands at 555 feet and 5 1/8 inches tall primarily because it was designed to be a massive Egyptian-style obelisk that followed a specific geometric ratio. The architect, Robert Mills, originally envisioned a more ornate structure, but the final design was simplified into a classic obelisk where the height is roughly ten times the width of the base. When it was completed in 1884, it was the tallest structure in the world, a title it held until the Eiffel Tower surpassed it in 1889. The specific height was also a result of engineering adjustments; during the second phase of construction, Lt. Col. Thomas Lincoln Casey had to reinforce the foundations to support the immense weight of the stone. The "555" figure became an iconic part of the D.C. skyline, and local building codes (the Height of Buildings Act of 1910) ensure that the monument remains the tallest structure in the city. Its height serves as a soaring tribute to George Washington's stature in American history, designed to be visible from miles away as a permanent landmark of the nation's capital.