Who built the Pantheon in Rome and how: Agrippa's name, Hadrian's building, the concrete dome that is still the largest, and 1,900 years of use

Architecture · 4 September 2026

The Pantheon in Rome was built by the emperor Hadrian between about AD 118 and 128 on the site of a temple that Marcus Agrippa, Augustus's general and son-in-law, had put up in 27–25 BC and that had twice burned, and Hadrian had Agrippa's inscription, "M. Agrippa, son of Lucius, consul for the third time, made this", set on the new facade, which is why the building bears the wrong name. It is a drum 43.3 m (142 ft) across, roofed by a dome of unreinforced concrete of the same span and height, with a 9 m (30 ft) hole, the oculus, at the top as its only window, fronted by a portico of sixteen granite columns from Egypt; the dome is still the largest of unreinforced concrete in the world, 1,900 years after it was poured, and the building is the best preserved of ancient Rome because it was made a church in 609 and has been in use ever since. Below: who built it and when, who designed it, how the dome was made, the structure that holds it, the portico and its columns, what it was for, its life as a church, the numbers, visiting, and the legends, with its heirs in Roman architecture and the material in Roman roads and concrete.

Agrippa, Hadrian and the name

Agrippa's Pantheon of 27–25 BC was part of his building of the Campus Martius, with the baths and the Saepta beside it; it was, the excavations of the 1890s suggest, a rectangular temple facing south, with a round area in front, and Cassius Dio says it was named for the statues of many gods inside, or for its dome like the heavens, and that Agrippa wanted to name it for Augustus and put his statue in it, which Augustus refused, so Agrippa put statues of Augustus and himself in the porch. It burned in the fire of AD 80 and was restored by Domitian, burned again from lightning in 110, and was rebuilt from the ground by Hadrian, whose bricks are stamped with dates from 115 to 127, mostly 123 to 125; Hadrian, who as a rule put his name on none of the buildings he restored, had Agrippa's inscription recut in bronze letters on the frieze, and held court in the building, which Dio saw him do. The inscription below it, in small letters, records a restoration by Septimius Severus and Caracalla in 202. The building was dedicated about 126–128; Hadrian was in the east for much of the time and the work was run from Rome. Pantheon, Rome.

Who designed it

No ancient source names the architect. Hadrian was an amateur architect of some ambition, and the Historia Augusta says that Apollodorus of Damascus, Trajan's architect, the builder of Trajan's Forum and the bridge over the Danube, was exiled and put to death after mocking the emperor's drawings of domes as "pumpkins", a story that has been read both as evidence that Hadrian designed the Pantheon himself and that Apollodorus did, since the domes of the Pantheon's near relatives, the baths of Trajan, are his. The building's geometry, a sphere of 43.3 m that would touch the floor if completed, inside a cylinder of the same diameter and half the height, and its brick-and-concrete construction are the mature work of the imperial building office that had been raising vaults for fifty years since Nero's Golden House; the most that can be said is that the emperor wanted it and an unknown master of that office made it. The portico was probably by a different hand and does not fit the drum, which suggests a change of plan during construction.

How the dome was made

• Concrete. Roman concrete, opus caementicium, is lime mortar mixed with volcanic ash (pozzolana) from the Bay of Naples and the Alban Hills, which sets chemically and hard, packed by hand in layers around lumps of aggregate; it was not poured liquid, but laid in courses of mortar and stone by men with trowels and rammers.
• Graded aggregate. The Romans lightened the dome as it rose: the foundations use travertine, the lower walls travertine and tufa, the lower dome tufa and brick, and the upper dome, around the oculus, light volcanic pumice, so that the concrete at the top weighs about 1,350 kg per cubic metre against 2,200 at the base; the dome's thickness falls from 6.4 m at the springing to 1.2 m at the oculus.
• Coffers. Five rings of 28 coffers, cut into the underside, remove more weight and were once decorated with bronze rosettes; the number 28 does not divide the circle evenly by the usual Roman methods and was chosen, it seems, for its meaning as a perfect number or a lunar month.
• The oculus. The 9 m opening at the top is not a weakness but the keystone, a ring of brick around which the concrete is in compression; it lights the building, lets the rain in onto a floor that slopes to 22 drains, and lets the hot air out. There was never a cover.
• Formwork. The dome was cast on a timber centering of enormous size, built up from the floor or from the cornice, on which the coffers were formed as wooden boxes; the concrete was laid in horizontal rings, each ring stable when finished so that the work could pause, and the whole took years to cure. The outer surface is stepped in seven rings at the base that add weight where the outward thrust is greatest.
• No reinforcement. There is no iron in the dome; it has cracked, as all such domes do, in the lower part where the ring tension is greatest, and the cracks were found by Terenzio in 1930 to be ancient and stable, the dome behaving as a set of arches meeting at the compression ring of the oculus.

The drum and the hidden arches

The dome sits on a cylinder of concrete faced with brick, 6.4 m thick, but the wall is not solid: it is eight piers connected by arches, with seven great recesses (exedrae) between them on the inside, alternately rectangular and semicircular, screened by columns, and the entrance as the eighth, and behind the recesses a network of relieving arches of brick, visible on the outside, that carry the weight of the dome down to the piers and over the voids. The cylinder is 21.7 m high, half the diameter, so that the cornice at the springing of the dome is at the centre of the sphere, and the interior is the geometry of a sphere in a cylinder that Renaissance architects drew and redrew. The floor is the original of 126, in a grid of squares and circles of granite, porphyry and marble, dished toward the centre; the marble facing of the lower walls is largely original, the attic above it was replaced in 1747 with the present blank panels, of which a section was restored to the Roman design in the 1930s. The bronze doors are Roman, of the 2nd century, and may be the originals; they are 7.5 m high.

The portico and the wrong columns

The portico has sixteen unfluted columns of grey and rose granite, 11.9 m high and 1.5 m thick, each a single stone of about 60 tons, quarried at Mons Claudianus in the Eastern Desert of Egypt and at Aswan, dragged to the Nile, shipped to Ostia and rowed up the Tiber, with Corinthian capitals of white Pentelic marble; the pediment above held a bronze eagle in a wreath, whose fixing holes remain. The portico does not fit: its pediment is too low for the drum, and a second pediment, higher, is outlined on the brick of the transitional block behind it, which shows that the design called for columns of 50 Roman feet and what arrived were columns of 40, the 50-foot monoliths having been lost, delayed or diverted, perhaps to Trajan's Forum; the builders used what they had and left the join. The porch was roofed with bronze trusses and its ceiling was bronze until 1625. The columns are the largest monoliths in Rome after the obelisks.

What it was for, and the church

The name means "of all the gods", and the round plan, the dome as sky and the oculus as sun suggest a temple to the planetary gods, of whom seven had niches, with the sun's beam moving across the coffers through the year and striking the door at noon on 21 April, Rome's birthday; but it was also the emperor's hall, where Hadrian judged cases, and no ancient text describes a cult in it, which is why some scholars call it a dynastic monument to Augustus and Agrippa rather than a temple. In 609 the emperor Phocas gave it to Pope Boniface IV, who consecrated it as Santa Maria ad Martyres, with 28 cartloads of martyrs' bones brought from the catacombs, the first pagan temple in Rome to become a church, which saved it from the quarrying that ate the rest. The Byzantine emperor Constans II stripped the gilded bronze tiles from the dome in 663 and lost them to pirates; the roof was releaded in 735. In 1625 Urban VIII Barberini took the bronze of the portico ceiling, 200 tons, for the baldachin of St Peter's and the cannon of Castel Sant'Angelo, and added two bell towers to the facade, Bernini's "ass's ears", removed in 1883. Raphael was buried there at his own wish in 1520, with Annibale Carracci, Peruzzi and other artists after him, and from 1878 the kings of Italy, Victor Emmanuel II and Umberto I, in the side chapels. It is a state property and a working church, with mass on Sundays.

The numbers and visiting

• Dome. 43.3 m (142 ft) in diameter and 43.3 m from floor to oculus; the largest unreinforced concrete dome ever built, larger than St Peter's (42 m) and Florence (45 m across the octagon but brick, with ribs); the oculus 8.9 m across.
• Weight. About 4,535 tonnes for the dome; the walls 6.4 m thick; the concrete from 2,200 to 1,350 kg/m³ from base to top.
• Portico. 16 columns of 11.9 m and about 60 tonnes; the building 84 m long with the portico; the doors 7.5 m high.
• Dates. Agrippa 27–25 BC; fire 80 and 110; Hadrian 118–128; church 609; bronze removed 1625; towers removed 1883.
• Visitors. About 9 million a year, the most visited monument in Italy; since July 2023 a ticket of 5 euros for non-residents, free under 18 and on the first Sunday of the month, with timed entry and a queue in the Piazza della Rotonda; open daily from 9 to 7, closed during mass; rain falls through the oculus and the floor drains it; the Pentecost mass ends with rose petals dropped through the oculus by firemen, and at noon on 21 April the sun strikes the door.

Versions and theories

• Agrippa built it. He built the first one; the building standing is Hadrian's, as the brick stamps of 115–127 proved in 1892. The inscription is Hadrian's tribute to Agrippa, or his modesty, or his politics.
• Hadrian designed it himself. Possible in outline; the emperor drew buildings and quarrelled with Apollodorus over them, but the construction is the work of professionals and no source credits him.
• Rain does not come through the oculus. It does; the story that the rising warm air keeps it out is false, though a fine drizzle can evaporate on the way down. The floor's 22 drains are Roman.
• The dome is lighter at the top because of secret Roman concrete. Because of the graded aggregate, pumice at the top, and the coffers and the thinning shell; the chemistry of Roman concrete, lime and pozzolana with self-healing lime clasts, is why it has lasted, and is now understood.
• The columns were meant to be bigger. Yes: the second pediment line on the transitional block is the evidence, and the porch as built is too small for the drum.
• The oculus was a sundial. The sun's disc moves across the dome and walls through the day and year, and strikes the entrance at noon on 21 April, the Palilia and the birthday of Rome; whether this was designed or a consequence of geometry is argued, and the alignment is exact enough for design.

The Roman vaults that led to it are in Roman architecture; the domes that copied it, from Brunelleschi's to Jefferson's Rotunda, in Renaissance architecture and Neoclassical architecture.

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