# Easily Check Your Prescription Strength.

In many respects this calculation is more accurate and relevant than computerized photoropter measurements at the optic shop, and even more detailed than looking at a Snellen chart.

Centimeter Calculator Version 0.1

Instructions:

2.  Use the instructions to find where the text on the screen just barely starts to blur.

3. Measure the distance from your eyes to the screen at this "blur-distance".

4. Enter your current prescription (optional).

5. Enter the 'blur-distance'.

Done!

The calculator will now show you your correct prescription.

You will also want to read this further explanation of how to do centimeter measurements, correctly:  Centimeter How To

## How Does It Work?

It simply calculates diopters (diopter being nothing more than inverse meters).  The variance is how you look at the 'edge of blur'.  Important to note here is that you want to be where things are still clear, but another half centimeter further away, you notice a loss of sharpness.  This requires a little bit of experimenting.

You can also look at your current prescription strength, and the related centimeters - so now you can get that exact distance to the screen and see how sharp the image is (without wearing glasses).  How much can you move backward from your centimeter equivalent of diopters?  If you can move back and still see sharply, you're overprescribed.

Be sure to keep a log of your measurements.  Especially if you are participating in my Vision Improvement Course, you will want to know where you started, and how much you are improving.  Read this topic about keeping a log.  Also, for a better understanding of why your prescription is quite damaging when you use it for close-up work, take a look at an explanation of the blur horizon.

We discuss the topic of overprescription and it's contributing effects to worsening myopia in various parts of the site and as part of vision rehabilitation.

Take a look for example at 'Snellen is not a fixed number" or the (somewhat lengthy) explanation of myopia:  Understanding Myopia.

Click the link below to be taken to the centimeter calculator.

Take a moment to read the instructions. It takes a small measure of patience to find your values the first time - quite worth it! You can easily check your prescription value, and the degree of changes throughout the day.

Superseded! You can now skip the step of converting centimeters to diopters. Find my Myopia Ruler here.

Try it out now: Centimeter Calculator

If you have questions or comments about the calculator, there is a forum topic discussing it here.

Android calculator available as well.  Check it out here.

Enjoy!