A square degree (denoted deg²) is a non-SI unit measure of solid angle. Just as degrees are used to measure parts of a circle, square degrees are used to measure parts of a sphere. Analogous to one degree being equal to π / 180 radians, a square degree is equal to (π / 180)² or about 1/3283 steradian. The number of square degrees in a whole sphere is 4π(180 / π)² = 129600 / π or approximately 41,253 deg².

For example, observed from the surface of the Earth, the Moon has a diameter of approximately 0.5°, so it covers an solid angle of approximately 0.20 deg², which is 4.8×10^-6 of the total sky sphere.