Diopter Calculator

How to Use This Calculator

1. Select direction: Focal Length → Diopters or Diopters → Focal Length.
2. Enter the known value.
3. Use Stacked Lenses to add up to 3 lens powers.
4. Professional mode handles full eyeglass prescriptions with sphere, cylinder, axis, and vertex correction.

Formula

Example

Frequently Asked Questions

• What is a diopter? ▼
  A diopter (D) is the unit of optical power = 1/focal length in meters. P = +5 D → f = 20 cm (converging). P = −2 D → f = −50 cm (diverging). Positive = farsighted correction; negative = nearsighted correction.
• How do I combine two lenses in contact? ▼
  For thin lenses in contact: P_total = P₁ + P₂. Example: +3 D and +2 D touching → +5 D total, f = 20 cm. This is why optometrists simply add sphere and add-power values.
• What is vertex distance? ▼
  Vertex distance is how far the lens sits from the cornea (typically 12 mm). A contact lens correction differs from spectacle lens correction because d = 0 for contacts. Vertex-corrected power: P_contact = P_spectacle / (1 − d × P_spectacle).
• How does presbyopia change with age? ▼
  Presbyopia (loss of near-focus ability) progresses with age. Typical additions: 40s: +1.0 D, 50s: +1.5–2.0 D, 60s: +2.0–2.5 D, 70s: +2.5–3.0 D. These are added to the distance prescription for reading glasses.
• What does sphere and cylinder mean in a prescription? ▼
  Sphere = main power for myopia/hyperopia. Cylinder = astigmatism correction (power in one meridian). Axis = orientation of the cylinder (0–180°). A prescription of −2.50 sphere, −0.50 cylinder, axis 90° means −2.50 D in the horizontal meridian and −3.00 D in the vertical.

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