Assuming it's Traditional (not Roth) 401(k), that depends upon your marginal tax rate. Also note that any increase in income can make (for example) more of your Social Security income taxable, affect Medicare premiums two years down the road, lower or eliminate some deductions/credits, etc. Try using this tool for estimating:
https://turbotax.intuit.com/tax-tools/calculators/taxcaster
And remember State taxes.
