A wheelchair ramp meets the sidewalk, a bookshelf meets the floor, a cross street meets the avenue — perpendicular means meeting at a perfect 90° angle, and in coordinate geometry that relationship lives in the slopes. Two lines are perpendicular exactly when their slopes are negative reciprocals: flip the fraction and change the sign. A line climbing with slope 2 is perpendicular to a line falling with slope −1/2, and their product is always −1. Horizontal and vertical lines are the special pair: slope 0 meets the undefined slope of a vertical line.

Carpenters check squareness with the 3-4-5 triangle, but drafters, roofers, and game developers check it with slopes. A roof valley, a perpendicular parking stripe, or a surface normal in 3D graphics all reduce to this flip-and-negate move. This calculator performs the final step: type the known slope in the Variable A box — say 2 — and type −1 in the Variable B box, since the perpendicular slope is −1 ÷ m. The Result box shows −0.5, the slope of every line perpendicular to the original. Verify on graph paper: the two lines should cross forming four right angles.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Perpendicular Lines Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.