Angle type
Straight Angle — Exactly 180°
A straight angle measures exactly 180° — a straight line. It’s half a full circle (π radians).
Updated Online Protractor
Measure: 180° · π ≈ 3.1416 rad.
Think: a flat edge, a book opened flat, or the horizon line.
In 180° mode the protractor’s flat edge is the diameter — in 360° mode the arc still reads 180° at the opposite point.
How to confirm
- Set the center on the vertex and baseline on one side of the line.
- Drag the second ray to sit exactly on the extension — if the points are collinear you’ll see 180.0° labeled Straight.
- Tiny offsets show 179°/181° — use snap “Off” and arrow keys for fine alignment, plus grid/crosshair.
Also often confused with a “flat angle” — same thing. See obtuse (just under 180°) vs reflex (just over 180°).