Although intended to stand vertically, the tower began leaning to the southeast soon after the onset of construction in 1173 due to a poorly laid foundation and loose substrate that has allowed the foundation to shift direction. The tower currently leans to the southwest.
The height of the tower is 55.86 m (183.27 ft) from the ground on the lowest side and 56.70 m (186.02 ft) on the highest side. The width of the walls at the base is 4.09 m (13.42 ft) and at the top 2.48 m (8.14 ft). Its weight is estimated at 14,500 metric tons (16,000 short tons). The tower has 296 or 294 steps; the seventh floor has two fewer steps on the north-facing staircase. Prior to restoration work performed between 1990 and 2001, the tower leaned at an angle of 5.5 degrees, but the tower now leans at about 3.99 degrees.This means that the top of the tower is 3.9 metres (12 ft 10 in) from where it would stand if the tower were perfectly vertical.
* On January 5, 1172, Donna Berta di Bernardo, a widow and resident of the house of dell'Opera di Santa Maria, bequeathed sessanta soldi or "sixty coins" to the Opera Campanilis petrarum Sancte Marie. This money was to be used toward the purchase of a few stones which still form the base of the bell tower today.
* On August 9, 1173, the foundations of the Tower were laid.
* Nearly four centuries later Giorgio Vasari wrote : "Guglielmo, according to what is being said, in [this] year 1174 with Bonanno as sculptor, laid the foundations of the belltower of the cathedral in Pisa."
* Another possible builder is Gerardo di Gerardo. His name appears as a witness to the above legacy of Berta di Bernardo as "Master Gerardo", and as a worker whose name was Gerardo.
* A more probable builder is Diotisalvi, because of the construction period and the structure's affinities with other buildings in Pisa. But he usually signed his works, and there is no signature by him in the belltower.
* Giovanni di Simone was heavily involved in the work of completing the tower, under the direction of Giovanni Pisano, who at the time was master builder of the Opera di Santa Maria Maggiore. He could be the same Giovanni Pisano who completed the belfry tower.
* Giorgio Vasari indicates that Tommaso di Andrea Pisano was the designer of the belfry between 1360 and 1370.
* On December 27, 1233 the worker Benenato, son of Gerardo Bottici, oversaw the continuation of the construction of the belltower.
* On February 23, 1260 Guido Speziale, son of Giovanni, a worker on the cathedral Santa Maria Maggiore, was elected to oversee the building of the Tower.
* On April 12, 1264 the master builder Giovanni di Simone and 23 workers went to the mountains close to Pisa to cut marble. The cut stones were given to Rainaldo Speziale, worker of St. Francesco.