The KD 7000 has a built-in calibration feature as well as 30 year guarantee. Below is a link to the user manual, in case you can't find yours. The calibration instructions are on page 3. If you don't have the calibration weights, you can buy them at Amazon (
here are the ones I have).
http://myweigh.com/resources/manuals/kd7000.pdf
Edit: How frequently one needs to calibrate a scale depends on how frequently it is used and I believe how much movement of the scale might occur (such as when I travel with it.)