I
Calpurnia in the sources⁽¹⁾

There are very few references to Calpurnia in the ancient sources. Cicero never mentions her, although he has much to say of her father (see my article The Art of Defamation in Cicero's In Pisonem).

Says Suetonius: At about the same time he took to wife Calpurnia, daughter of Lucius Piso, who was to succeed him in the consulship (Divus Iulius, 21), and and his wife Calpurnia thought that the pediment of their house fell, and that her husband was stabbed in her arms (D.I., 81)

Appian says: He designated his friend Aulus Gabinius as consul, with Lucius Piso as his colleague, whose daughter, Calpurnia, Caesar married (Civil Wars, II, 14); his wife, Calpurnia, had a dream, in which she saw him streaming with blood, for which reason she tried to prevent him from going out in the morning (C.W., II, 115); That same night Caesar's money and his official papers were transferred to Antony's house, either because Calpurnia thought that they would be safer there or because Antony ordered it (C.W., II, 125)

Cassius Dio: [Caesar] attached to himself both Pompey and the other consul, Lucius Piso, by ties of kinship: upon the former he bestowed his daughter, in spite of having betrothed her to another man, while he himself married Piso's daughter (Roman History, XXXVIII, 9), For the night before [Caesar] was slain his wife dreamed that their house had fallen in ruins and that her husband had been wounded by some men and had taken refuge in her bosom (RH, XLIV, 17)

Plutarch: and a little while afterwards Caesar took Calpurnia to wife, a daughter of Piso, and got Piso made consul for the coming year (Caesar, XIV); After this, while he was sleeping as usual by the side of his wife, all the windows and doors of the chamber flew open at once, and Caesar, confounded by the noise and the light of the moon shining down upon him, noticed that Calpurnia was in a deep slumber, but was uttering indistinct words and inarticulate groans in her sleep; for she dreamed, as it proved, that she was holding her murdered husband in her arms and bewailing him. (…) when day came, she begged Caesar, if it was possible, not to go out, but to postpone the meeting of the senate; if, however, he had no concern at all for her dreams, she besought him to inquire by other modes of divination and by sacrifices concerning the future. And Caesar also, as it would appear, was in some suspicion and fear. For never before had he perceived in Calpurnia any womanish superstition, but now he saw that she was in great distress. (Caesar, LXIII)

This is practically all we get from literary sources, and we can safely assume all of it comes from the same original; Plutarch however, who is the more detailed, leads us to think that Calpurnia may not have been a nonentity tossed about by political expediency but instead, a strong, educated woman, at whose side Caesar still slept as usual after fifteen years of a peculiar marriage, and that the relationship was strong enough that she would worry about his safety and he would heed her advice.
A few authors have remarked on Calpurnia's lack of womanish superstition as probably reflecting her Epicurean education at her father's house.

And there is (or there was, it seems it disappeared in the sixteenth century!) a funeral inscription for Calpurnius Ikadion, either a freedman of Calpurnia's or the son of a freedwoman of hers, quoted by U. of Texas' David Armstrong⁽²⁾:

My fame and fortune were good, my patroness
the wife of Caesar, the great and magnificent god,
who kept me well and safe, and dear to dear friends
who also spent much care on me. Anthis
was the authoress of my being, who laid my dear bones
in her own sepulchre, and my name is Ikadion.

This is obviously an Epicurean epigram, as Armstrong explains: the reference of Ikadion's name to Epicurus' birthday, the importance of friendship, the joyful tone of the epitaph point clearly to the doctrine which was adhered to by L. Piso Caesoninus and his household, on the strength of Philodemus of Gadara's teachings.

II
So what else do we know about Calpurnia's family?

The Calpurnii were a plebeian gens⁽³⁾, who claimed descent from Calpus, the third of the four sons of King Numa. They produced several consuls of whom the first was C. Calpurnius Piso in 180 BCE. Other cognomina used in the Calpurnia gens were Bibulus, Flamma and Bestia. Relations were not necessarily friends; Calpurnia's father and Marcus Calpurnius Bibulus were political opposites.
Family ties were as convoluted as any in Republican Rome. A Calpurnia Bestia was the first mother-in-law of Pompey the Great. A Calpurnius Piso Frugi was the first son-in-law of Cicero. Calpurnius Bibulus was Cato the Younger's son-in-law. Loyalties were obviously divided. When Cicero was under attack from Clodius in early 58 BCE, his son-in-law tried to enlist the support and protection of Calpurnia's father who was then consul; the latter however gave him no help, being bound to Caesar's strategy.

On her mother's side Calpurnia was closely related to the Aurelii Cottae and, through them, to Caesar himself⁽⁴⁾. It is a pity we don’t know much about the female lines other than her paternal grandmother, who was alive during Piso's consulship, was the daughter of a Claventius whose family came from Placentia in Cisalpine Gaul. Of this Claventius' business we don’t know anything, but while the Calpurnii Pisones were traditional estate owners (the name Piso is related to the grinding of grain or grapes), there was also a large investment in arms manufacture, which considering the militaristic nature of the Roman nation shows very good financial sense.

Piso was very rich. The size and characteristics of the Villa of the papyri near Ercolano in the bay of Naples speaks of it. It is a huge seaside mansion, first excavated in the eighteenth century, where a magnificent art collection and library have been found⁽⁵⁾ and are still under study. Wealthy Romans often patronized greek philosophers (Cato the Younger was a good example), and Piso was host to Philodemus of Gadara, a respected Epicurean teacher and author of several important treatises. Both the library and the art seem to have been collected according to Epicurean principles and interests.⁽⁶⁾

III
A good match

We may therefore safely consider that from Caesar's point of view Calpurnia was a good match: she came from an illustrious, consular family, was educated in what may be considered state-of-the-art scientific and philosophic theories of the time, was rich, and probably, if she inherited her father's good looks, attractive. She would be very young, as usually were most Roman first time brides.

The poet Catullus accused Caesar of liking his women very young⁽⁷⁾ – would he be thinking of Calpurnia? For in fact most of Caesar's known mistresses were married, and probably not so young as to warrant such an accusation. Catullus wrote before Caesar met Cleopatra, by the way.

Indeed, if Piso was about Caesar's age, as we can safely assume from his having become consul the year after Caesar, it is equally probable that Calpurnia was about the age of Julia, Caesar's daughter, possibly younger, if we consider that Caesar was first married at an uncharacteristic young age for a male⁽⁸⁾.

Actually, we don't know Julia's birthyear. That her parents were married in 85 BCE does not mean she was born immediately afterwards; but her mother died in 69 BCE so she was born prior to this date. Most Roman girls were married when aged between fourteen and sixteen, so we may assume both Julia and Calpurnia were born sometime around 76 BCE⁽⁹⁾. So when Caesar died in 44 BCE, Calpurnia would have been around thirty – not an older woman, as she is often portrayed in plays and movies.

After Caesar's death and funeral the only reference we have to Calpurnia is the inscription mentioned above; she probably lived quietly as the widow of the deified Caesar and as such would have commanded respect and would have yielded some influence; she is not mentioned in what we know of Caesar's will but she must have at least have had a fortune of her own. She had a much younger (half) brother, Lucius Calpurnius Piso Frugi Pontifex, who was consul under Augustus, lived to about eighty, and had two sons to whom the poet Horace addressed his Ars Poetica. The line was continued, and a Piso was one of the main characters in a conspiracy against the emperor Nero, and was killed for it.