14,500 t32,000,000 lb
The Leaning Tower of Pisa weighs about 14,500 tonnes (about 32 million lb), the rounded figure engineers use. The Opera della Primaziale Pisana, which looks after the tower, gives a calculated 14,453 tonnes. All of it rests on a ring-shaped foundation about 19.6 m (64 ft) across, set on soft silt and clay.
| Higher published estimateListed in Wikipedia’s technical data for the tower | 14,700 t32,400,000 lb |
|---|---|
| Engineers’ rounded figureThe number used in the stabilisation literature | 14,500 t32,000,000 lb |
| Opera della Primaziale PisanaCalculated weight given by the tower’s custodians | 14,500 t31,900,000 lb |
Nobody has ever weighed the tower. Its weight is calculated from the volume of its masonry and the density of the materials. The Opera della Primaziale Pisana, the body that runs the monuments of the Piazza dei Miracoli, puts it at 14,453 tonnes, and engineers usually round that to 14,500 tonnes. Some reference tables give 14,700 tonnes, which shows how much a calculation like this can shift with the assumptions behind it.
The tower is not solid stone. According to John Burland, Michele Jamiolkowski and Carlo Viggiani, members of the international committee that stabilised it, it is a hollow masonry cylinder ringed by six colonnades. The inner and outer walls are faced with San Giuliano marble, and the space between them is filled with rubble and mortar. It stands 58.36 m (191 ft) from the foundation level and about 56 m (184 ft) above the ground, with an outside diameter of about 15 m (49 ft). The bell chamber at the top holds seven bells weighing about 10.5 tonnes together; the largest, L’Assunta, was cast in the 1650s and weighs 3,620 kg.
The foundation is a ring about 19.58 m (64 ft) across with a 4.5 m (15 ft) hole in the middle, which gives a bearing area of about 285 m² (3,070 sq ft). Spreading 14,453 tonnes over 285 m² works out to about 50 tonnes on every square metre, or roughly 500 kPa (about 73 psi).
Because the tower leans, that load is not even. A computer analysis by Burland and David Potts found that in 1993 the pressure under the south edge was about 1,000 kPa, enough to make the soil there start to yield, while the pressure under the north edge was close to zero.
Construction began in 1173. By 1178, with only the lower floors built, the tower had started to sink and tilt, because its shallow foundation sat on weak ground: about 10 m of soft silts and clayey silts over soft marine clay reaching down to about 40 m. Wars stopped the work for long stretches. Building resumed in 1272, the seventh floor was finished in 1319 and the bell chamber was added in 1372. The builders made the upper floors taller on one side to correct the lean, which is why the tower is slightly curved.
Burland’s committee showed that the tower suffers from what engineers call leaning instability. Above a certain height, a small extra tilt makes the overturning push of the weight grow faster than the ground can resist it. As they put it, the ground is not too weak; it is not stiff enough. By 1990 the tilt had reached about 5.5 degrees and was growing by about 6 arc seconds a year, and the tower was closed to the public on 7 January 1990.
The first fix was temporary. In the second half of 1993, 600 tonnes of lead ingots were stacked on a concrete ring around the north side of the base, cutting the overturning moment by about 10%. In September 1995 the load was raised to 900 tonnes after an attempt to replace the lead with ground anchors set the tower moving south again.
The permanent fix was underexcavation: drilling out small amounts of soil from beneath the north side so the ground there would settle and pull the tower back. Work began on 9 February 1999 and ended on 6 June 2001. In total only about 38 m³ (70 tonnes) of soil was removed. By mid 2002, with a drainage system added on the north side, the foundation had rotated back by about 1,880 arc seconds, roughly half a degree, and the top had moved about 45 cm (17.5 in) north. The tower reopened in December 2001.
About 32 million lb, from the usual figure of 14,500 tonnes. The Opera della Primaziale Pisana’s calculated 14,453 tonnes is about 31.9 million lb, or roughly 16,000 US tons.
Its shallow foundation sits on soft, compressible silt and clay, and the ground under the south side gave way more than the north side. It began to tilt in 1178, only a few years after building started, and the lean kept growing for centuries.
Not any time soon. After the 1999 to 2001 works its northward rotation had slowed to less than 0.2 arc seconds a year by 2008, and the engineers involved estimate that even in their pessimistic scenario it would take at least 200 years to lean as far as it did in 1993.
The tower has seven bells weighing about 10.5 tonnes (23,100 lb) together. The largest, L’Assunta, weighs 3,620 kg (7,981 lb).
About 38 m³, or roughly 70 tonnes of soil, was drilled out from under the north side between 1999 and 2001. That was enough to pull the top of the tower back by about 45 cm.
Last checked October 3, 2026.
