Facing an income-tax surprise? Now is a good time to update your W-4

After a tumultuous year that has upended traditional employment, millions of Americans could face income-tax surprises. One way to reduce the odds of owing a big bill to Uncle Sam — or paying out too much and thus giving an interest-free loan to the government — is to check your withholding.

More than a year ago, the Internal Revenue Service released a redesigned form W-4 on which taxpayers figure how much money they should withhold from their paychecks. Most taxpayers so far appear to have ignored it.