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Contact Lens Vertex Calculator
Your glasses prescription
Enter the sphere power from your glasses prescription — negative for nearsightedness, positive for farsightedness. The effect is negligible below ±4.00 D. The standard vertex distance is 12 mm.
Estimate only, not a prescription. A real contact lens prescription comes from an eye exam and a fitting by an eye care professional.
Example: −6.00 D glasses at a 12 mm vertex distance convert to about −5.60 D in contact lenses — 0.40 D weaker.
Behind the numbers
How the vertex distance changes the power
A lens bends light differently depending on how far it sits from your eye. Glasses sit about 12 mm in front of the cornea, while a contact lens sits right on it, so the contact needs a slightly weaker power for the same correction. The vertex formula adjusts the glasses power for that distance — use the distance in metres, so 12 mm is 0.012 m.
−6.00 ÷ (1 − 0.012 × −6.00) = −6.00 ÷ 1.072 = −5.60 D. For plus powers the effect reverses: +6.00 D glasses become +6.47 D in contacts, because the denominator gets smaller.
A few comparisons
Glasses power vs contact power at 12 mm
| Glasses power | Vertex distance | Contact power | Difference |
|---|---|---|---|
| -2.00 D | 12 mm | -1.95 D | +0.05 D |
| -4.00 D | 12 mm | -3.82 D | +0.18 D |
| -6.00 D | 12 mm | -5.60 D | +0.40 D |
| -8.00 D | 12 mm | -7.30 D | +0.70 D |
| +4.00 D | 12 mm | +4.20 D | +0.20 D |
| +6.00 D | 12 mm | +6.47 D | +0.47 D |
Small shift, sharper vision.
Below ±4.00 D the change is under a quarter dioptre, so glasses and contact powers are often identical — do not be surprised if your prescription shows no change.
For minus powers the contact is always weaker than the glasses; for plus powers it is always stronger. If your result goes the other way, check the sign you entered.
Contact lenses are sold in 0.25 D steps, so a result like −5.60 D would usually be rounded to the nearest available power — −5.50 D or −5.75 D — by your eye care professional.
Power is only part of a contact lens prescription. Base curve, diameter, cylinder and axis all need an eye exam and a fitting — never order lenses from the sphere power alone.
Good to know
Your vertex questions, answered.
Why do contact lenses have a different power than glasses?
Glasses sit about 12 mm in front of your eye, while a contact lens sits directly on it. Moving the lens closer changes how strongly it bends light, so the power must be adjusted to give the same correction. The further the lens moves, the bigger the change.
When does vertex distance actually matter?
Below about \u00b14.00 dioptres the change is tiny — usually less than 0.25 D, which is one lens step — so glasses and contact powers are often the same. Above \u00b14.00 D the difference grows: a \u22126.00 D glasses prescription becomes about \u22125.60 D in contacts.
Can I order contacts using the result on this page?
No. This calculator is an estimate only, not a prescription. A real contact lens prescription comes from an eye exam and includes more than sphere power — such as cylinder, axis, base curve and diameter — and must be fitted by an eye care professional.
What vertex distance should I use?
12 mm is the standard distance used for most glasses fittings, and it is the default here. Your optician measures the real distance when fitting your glasses; if your glasses sit further from or closer to your eyes, enter that number instead.