Back Vertex Calculator

Back Vertex Power Calculator

Include the sign (+ or -).

Standard distance is usually 12mm to 14mm.

Required Contact Lens Power:

function calculateBVP() { var fs = parseFloat(document.getElementById("spectaclePower").value); var d_mm = parseFloat(document.getElementById("vertexDistance").value); var resultDiv = document.getElementById("bvp-result-container"); var resultValue = document.getElementById("bvp-result-value"); var interpretation = document.getElementById("bvp-interpretation"); if (isNaN(fs) || isNaN(d_mm)) { alert("Please enter valid numbers for both fields."); return; } // Formula: Fc = Fs / (1 – (d * Fs)) // d must be in meters for the formula var d_m = d_mm / 1000; var fc = fs / (1 – (d_m * fs)); // Round to 2 decimal places var finalPower = fc.toFixed(2); var sign = fc > 0 ? "+" : ""; resultValue.innerHTML = sign + finalPower + " D"; resultDiv.style.display = "block"; var diff = Math.abs(fc – fs); if (diff < 0.1) { interpretation.innerHTML = "Minimal change required. Standard rounding to the nearest 0.25D applies."; } else if (fs < 0) { interpretation.innerHTML = "The contact lens requires less minus power than the spectacles."; } else { interpretation.innerHTML = "The contact lens requires more plus power than the spectacles."; } }

Understanding Back Vertex Power (BVP)

In optometry and ophthalmology, Back Vertex Power refers to the effective power of a lens relative to the eye. When you switch from wearing glasses to contact lenses, the distance between the lens and your cornea changes. This distance is known as the Vertex Distance.

Because spectacle lenses sit approximately 12mm to 14mm away from the eye, while contact lenses sit directly on the cornea (0mm vertex distance), the refractive power must be adjusted to ensure the light focuses correctly on the retina.

The Back Vertex Formula

The calculation uses the following mathematical relationship:

Fc = Fs / (1 – (d * Fs))

• Fc: Power of the contact lens (Diopters)
• Fs: Power of the spectacle lens (Diopters)
• d: Change in vertex distance in meters (e.g., 12mm = 0.012m)

When is Vertex Compensation Necessary?

As a general rule in clinical practice, vertex distance compensation is considered necessary when the lens power exceeds ±4.00 Diopters. Below this threshold, the difference is usually negligible (less than 0.12D), which is smaller than the standard 0.25D steps found in lens manufacturing.

Practical Examples

Spectacle Rx	Vertex Dist.	Contact Lens Rx
-6.00 D	12 mm	-5.60 D (Round to -5.50 or -5.75)
+5.00 D	13 mm	+5.35 D (Round to +5.25 or +5.50)
-10.00 D	12 mm	-8.93 D (Round to -9.00)

Key Rules to Remember

• Minus Lenses: When moving from spectacles to contact lenses, a minus lens becomes more effective as it gets closer to the eye. Therefore, you need less minus power in a contact lens.
• Plus Lenses: When moving from spectacles to contact lenses, a plus lens becomes less effective as it gets closer to the eye. Therefore, you need more plus power in a contact lens.

Note: This calculator provides a theoretical value. Final contact lens prescriptions should always be determined by a qualified eye care professional during a contact lens fitting, as factors like tear film and lens material can also affect the final refractive result.