What is the required altimeter accuracy for IFR flight?

If there is an altimeter error of more than 50 feet while on the ground, you have a problem.
To check the accuracy, apply the present altimeter setting to the altimeter and compare the altitude indicated with the runway or aerodrome elevation. The altimeter should be within 50 feet +/- of the elevation of the runway. If the deviation is more, have the instrument checked by maintenance.