Diagonals

The Player's Handbook presents a simple method for counting movement and measuring range on a grid: count every square as 5 feet, even if you're moving diagonally. Though this is fast in play, it breaks the laws of geometry and is inaccurate over long distances. This optional rule provides more realism, but it requires more effort during combat.

When measuring range or moving diagonally on a grid, the first diagonal square counts as 5 feet, but the second diagonal square counts as 10 feet. This pattern of 5 feet and then 10 feet continues whenever you're counting diagonally, even if you move horizontally or vertically between different bits of diagonal movement.

For example, a character might move one square diagonally (5 feet), then three squares straight (15 feet), and then another square diagonally (10 feet) for a total movement of 30 feet.

Source: DMG, page 252