How big was the Roman economy?

We know as far empires go Rome was a biggie (sorry, couldn't help making a Life of Brian quote)...but not all empires are made equal - the Zulus and Persians had big empires too - but their economies fell far behind the Romans. In the end, how much money you have is what counts in making and sustaining a great empire, and if you have any doubts about the Roman-era here's a snapshot of Imperial annual metal production during the 1st-century AD.

Pig iron; 82,500-tons. At the same time the Han Empire in China was producing about 5,000-tons and in 1759, Britain was producing 35,000-tons

Copper; 15,000-tons. In 1860 the United States was producing 8,000-tons

Lead; 80,000-tons. Lead production in the United States in 1845 was 33,750-tons

Silver; 200-tons. Total Roman-era silver stock was estimated at around 10,000-tons, ten times more than the combined holdings of medieval Europe. In 1995 the United States produced 1640-tons.

Gold; 9-tons. This is from just the two Spanish provinces. World-wide production between 1800 and 1850 averaged just 24-tons per annum.

The up-shot? The British Empire's economy probably didn't overtake that of 1st-century Rome until the late 18th-century, and the United States during the 1860s. Yep, the Roman Empire was a biggie. For more on the Roman economy, you can read 'A Body of Doubt' - available from Amazon, just follow the links