Test your vision by using the University of Buffalo's IVAC Tool, or completing the vision test at Thinkquest.org. Each of these tests incorporates the Snellen eye chart used during official vision tests. It is possible to use the chart to directly measure vision without using other online tools. Note that such at home vision tests are no substitute for having an eye doctor check your eyes.
Use the University of Buffalo's IVAC Tool by measuring the length of the line on the screen with a physical ruler and the distance from the screen to your eyes. Next, read the different letters and click Smaller to make the letters smaller each time. While reading these letters, cover one eye and read from top to bottom. Then, cover the other eye and repeat the process, stopping when you can no longer read the letters.
Use the Flash-based vision tool on thinkquest.org if your computer monitor ranges in size between 15 and 19 inches. Simply stand four feet away from the screen and follow the onscreen instructions. Click Stop when you can no longer read the letters and the program estimates your visual acuity.
A final option is to open the Snellen eye chart on the computer by measuring the size of the letter E using a ruler. Next, multiply that number by exactly 2.838 and then stand that far from the screen and read the letters from top to bottom to determine visual accuracy.