What does AXIS mean on your prescription?

AXIS is the one number on your prescription that is not a strength. It is a direction — an angle between 1° and 180° that tells the lab which way round to place the cylinder correction.

A position, not an amount

This is the single most common misreading of a prescription. An axis of 175 is not "worse" than an axis of 12, and going from 90 to 180 between two visits does not mean anything doubled. The axis only answers which way the astigmatism lies; the CYL beside it answers how much.

How the angle is measured

Picture a protractor laid over your eye as someone faces you: 180° runs horizontally, 90° runs vertically. The scale stops at 180 rather than 360 because a cylinder is symmetrical — an axis of 10° and one of 190° describe the same orientation, so only half the circle is needed.

That also explains something that puzzles people comparing prescriptions: 175° and 12° are close together, not opposite. They sit either side of the horizontal, about 17° apart, because the scale wraps around at 180.

Why small axis changes are felt more at high cylinder

The stronger your cylinder, the more a few degrees of misalignment matters. At −0.50 D a 10° error is barely perceptible; at −3.00 D the same error noticeably blurs vision. This is why prescriptions with a large cylinder are re-checked more carefully — and why an axis that drifts across several visits is worth flagging to your optometrist.

Reading it in the wild

You will see the axis written as × 175, x175, Eje 175 (Spanish) or Achse 175 (German), usually straight after the cylinder. The degree symbol is often left off. If you see three numbers in a row, they are almost always SPH, CYL, AXIS in that order.