It's difficult to accurately, scientifically judge the strength of alcohol until the Early Modern period so we mostly have theories on the average strength. Some Roman wines like Falernian would reputedly catch fire if lit, which has often been taken to assume that it was as strong as brandy, but you can light wine as low as 9% ABV with sufficiently high heat (like dumping wine in a hot pan while cooking). It's generally assumed that this wine was of comparable strength to modern wine.

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One thing that's not discussed quite as often is that wine was generally watered down until recent centuries. Greeks and Romans considered drinking undiluted wine barbaric and public drunkenness shameful, so they would drink large amounts of diluted wine. At parties someone would be declared the "king of the party" and be in charge of determining how diluted the wine was. The typical daily proportion for wine was 3:1 water to wine, which I've tried before; it creates a drink that looks like wine but tastes like water with a little wine aftertaste. Roman taverns sold watered-down wine by the sextarius, roughly equivalent to a pint, so you'd get a refreshing drink on a hot day with the alcohol content of 1 modern glass of wine.

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This continued into medieval times. The Metropolitan Museum of Art's online collection has bowls with metal discs at the bottom; you fill it with wine until the disc is covered, then fill the rest with water.