How do I Calibrate a volt meter?

[path1]

I have couple cheap volt meters. They don't give show the same numbers. Any ideas how to calibrate?

I'm not sure of a cheap and good way. Maybe something that plugs into 120 and then inverts to a known 12 volt number?

Thanks

(Elec is not one of my better known quality's)

[2oldman]

The only answer I can see where that's possible is to compare it to a known good one. Then you'd have to buy or borrow a good one. They are inexpensive.