This depends on how strong your power is.
A contact lens prescription can be the same as a prescription for glasses when:
- the prescription is a lower power, ie: -4.00 to +4.00 and
- there is absolutely no astigmatism present.
If you have any astigmatism or require a lens power greater than +/-4.00, the parameters of the contact lens can change.
- if you have astigmatism under 0.50, a spherical lens should be prescribed.
- astigmatism of 0.50 to 0.75 can be effectively corrected with an aspheric lens.
- astigmatism greater than 1.00 should be managed with a toric lens.
If your prescription power for glasses is greater than +/-4.00, you should see a difference in the power of your contact lens prescription. Generally, every time your glasses prescription increases or decreases by a power of 2, your contact lens prescription will increase by an additional power of +0.25. For every power change of 2 in a glasses prescription, the difference of +0.25 power is needed to compensate for the location of the contact lens. Your glasses are much further from the front surface of the cornea than contact lenses.
Sources: http://www.contactlenses.co.uk/education/public/spectacle_vs_contact_lens_rx.htm
