How the calculation works
Lens-days = lenses per box × boxes × days per lens; cost per 30 wearing days (one eye) = price ÷ lens-days × 30; cost per month = that × eyes + solution per month
A worked example
A box of 6 monthly lenses at €30.00 holds 180 lens-days: €5.00 per 30 wearing days for one eye. Both eyes cost €10.00 a month, and with €8.00 of solution the total is €18.00 per month.
Why per wearing day
A box of 90 daily lenses and a box of 6 monthly lenses are not the same amount of lens. Counting the days each box lasts one eye — lenses × days per lens — puts them on one scale, the same lens-day the catalog ranks by.
One box, one eye
Boxes are sold per eye, because the two eyes often need different powers. If both eyes wear the same lens, a month costs twice the one-eye figure; if they need different lenses, work out each eye on its own.
Your prescription comes first
Buy only the brand, base curve, diameter and power on your current prescription; see an eye-care professional for fitting and follow-up. A cheaper lens that is not on your prescription is not a saving: brand, material, base curve and diameter all change how a lens sits on the eye.
Common questions
Do I need to count solution for daily lenses?
No. Daily lenses are thrown away every evening and need no cleaning solution, so leave the solution field at 0. Two-weekly and monthly lenses need solution and a case every night.
Can I wear a lens longer than its schedule to save money?
That is a question for your eye-care professional, not a calculator. The replacement schedule is part of how the lens was tested and approved; this tool only does the arithmetic for the schedule you enter.
How long does a box last?
Lens-days ÷ eyes: a box of 6 monthly lenses lasts one eye 180 days, or both eyes 90 days if you buy one box per eye.
Method: arithmetic from the label values you enter. Reviewed .