To finish Form 4868, you need to estimate how much tax you owe. The form helps you calculate this number by estimating your total tax liability for the year and subtracting any payments you’ve already made. If you choose to file an extension using TaxAct, our tax software helps calculate your estimated tax bill based on the information you enter. One problem many taxpayers run into is not having all their tax information yet. If you’re still waiting on important tax documents, just try to complete your return as best you can. Rather than leaving something out, try to use estimates for any numbers you don’t have yet. If you are using TaxAct, click Mark as Estimate for any information you need to double-check before officially filing your return. You can also use our separate Income Tax Calculator to help estimate your amount due. When in doubt, it’s better to overpay than underpay. If you overestimated your tax bill and paid too much, you will receive a tax refund for the overpayment. However, if you underpay, you could be responsible for paying interest and penalties. The good news is that the penalty for paying late when you’ve requested an extension is significantly less than if you did not receive an extension.