About how long was the front of the Waterloo battlefield?

The front of the Waterloo battlefield was about 2.5 miles long.

The relatively short frontage was one of the strengths of Wellington’s position. His army occupied a ridge running broadly east to west, with the main road from Brussels crossing the position. Because the line was compact, Wellington could arrange troops in depth rather than spreading them thinly across a much longer front.

Wellington also used the reverse slope of the ridge to conceal much of his infantry from Napoleon’s view. French commanders could see only some forward positions, skirmishers, and artillery, which made it harder to judge the Allied army’s actual strength. Strongpoints such as Hougoumont, La Haye Sainte, and Papelotte anchored the position and forced French attackers to deal with defended obstacles before reaching the main line.

The 2.5-mile figure describes the battlefield frontage in the tactical sense, not the total area over which every movement occurred. Fighting also extended around Plancenoit and the approaches from Wavre as Prussian forces joined the battle.