Why is my contact lens prescription different from my glasses prescription?

The difference is called vertex distance compensation. A glasses lens sits roughly 12 mm in front of your eye, while a contact lens sits directly on the cornea. The same optical correction therefore needs a slightly different power.

Rule of thumb:

• Up to ±4.00 D: glasses and contact lens powers are basically the same.
• ±4.00 to ±8.00 D: contacts are typically 0.25–0.50 D weaker for myopia, slightly stronger for hyperopia.
• Above ±8.00 D: an optician must recalculate — never just convert.

Other differences to expect:

• Glasses prescriptions don't carry BC or DIA. Those come from a separate contact-lens fitting.
• Glasses use prism notation sometimes; contacts can't correct prism the same way.
• Cylinder on contacts is always negative, even if your glasses show it as positive — the math is equivalent, just the convention differs.

Bottom line: you cannot order contact lenses based on a glasses prescription alone. You need a contact-lens-specific prescription from an optician.