How big was the Roman Empire at its height?

AD 117: Trajan’s high-water mark

The empire reached its widest under Trajan, a soldier born at Italica, near today’s Seville, and the first emperor from the provinces. Between 101 and 106 he conquered Dacia, north of the lower Danube in what is now Romania, and its gold. His Column in Rome, dedicated in AD 113, tells the story of those wars in a carved band about 190 meters long that spirals up its shaft.

Then he turned east. In 114 he made Armenia a province, and by 116 he had marched down the Tigris to the Persian Gulf, in today’s Iraq. There, Cassius Dio says, he watched a ship leave for India and said he would have gone too, if he were still young. When he died in August 117, Rome governed about 5 million square kilometers, roughly half the area of the United States, and some sixty million people.

How far it reached

In straight lines, the empire ran more than 5,000 kilometers (3,100 miles) from Portugal’s Atlantic coast to the head of the Persian Gulf, and about 4,400 kilometers (2,750 miles) from northern England to southern Egypt. Every shore of the Mediterranean was Roman. Romans called it mare nostrum, “our sea,” and more than 80,000 kilometers of paved road tied the lands around it together.

Which modern countries were Roman

The empire’s land now lies in dozens of countries, among them:

• Europe: Portugal, Spain, France, Italy, Greece, England and Wales, Belgium, Switzerland, Austria, the Balkans and Romania, plus the parts of the Netherlands, Germany and Hungary along the Rhine and the Danube.
• Asia: Turkey, Cyprus, Syria, Lebanon, Israel and the Palestinian territories, Jordan, and briefly Armenia and Iraq.
• Africa: Egypt, Libya, Tunisia, and the coasts of Algeria and Morocco.

Why it stopped growing

Rome had met its limits before. In AD 9 three legions were destroyed in the Teutoburg Forest in Germany, and the Rhine and the Danube became the frontier. Augustus’s last advice to his heirs was to keep the empire within those limits. Trajan ignored it, and his successor Hadrian undid part of his work at once: in 117 he gave up Mesopotamia, and from AD 122 he built his wall across northern England. Rome stopped growing and started guarding.

Not everyone saw the Roman peace as a gift. Tacitus imagines a chieftain in Scotland, Calgacus, speaking to his warriors before they face the legions:

Robbers of the world, having by their universal plunder exhausted the land, they rifle the deep. … To robbery, slaughter, plunder, they give the lying name of empire; they make a solitude and call it peace.

Later empires grew far bigger, the Mongol and the British among them. None has held every shore of the Mediterranean.