Planck's constant is:

#h ~~ 6.626 xx 10^(-34) J*s#

It's difficult to make a mistake with this if you have it in front of you on an equation sheet (and it's not something you should feel obligated to remember. But, you will probably use it often if you do use it at all, like to get #E_"photon"#, or in upper-division chemistry classes, for example, so it's better to actually remember it).

Some might have trouble recalling that #J = kg*m^2/s^2# and thus the alternative units for #h# are #kg*m^2/s#.

You may also work with proton/electron charge when using #h#, so if you do, it is #e = pm1.6022 xx 10^(-19) C#, and if your answer is off by a factor of #~~10^19#, that is probably why.

Someone might accidentally remember this wrongly as #6.676 xx10^(-34)#, but that resembles #6.674 xx10^(-11)#, the universal gravitation constant #G#, and #h# isn't it.

And if for some reason you get something ridiculously large (like #10^27#), but you clearly remember #h#, then you probably missed the negative sign on the exponent. Everyone really should check what they type on the calculator, because if you're right, then the calculator is right. ;)