You'll generally pay the same amount of estimated tax each quarter. However, if your income is likely to vary significantly during the year or you anticipate any unusual and nonrecurring transactions, that might result in a tax higher than the prior year safe harbor estimate, it may be beneficial to calculate the amount for each quarter individually. Note that the quarters for calculation purposes end in March, May, August and December. 

Underpayment Penalty
If you were late with an estimated tax payment or you underpaid on an annual or quarterly basis, you may be subject to a penalty. The penalty amount is calculated based on the number of days the tax was underpaid. 

In general, you can avoid an annual underpayment penalty if your withholdings and estimated tax payments meet either of the safe harbor amounts: 90 percent of this year's tax liability or 100 percent of last year's tax liability (110 percent for higher income taxpayers). However, you may still be subject to an underpayment penalty imposed on a quarterly basis. 

If your income is higher than was anticipated in your estimated tax calculations, there are things you can do to reduce or possibly even avoid quarterly penalties. For example, because the IRS considers withholdings to have been deducted evenly throughout the year, you can increase withholdings in a subsequent quarter to make up for under-withholding in previous quarters. You can also increase your estimated payments in subsequent quarters and analyze your income on a quarter-by-quarter basis. 

New and Revised Tax Rules That Could Affect Your Estimated Payment Amounts for 2010
The starting point for calculating your estimated tax amount is generally your tax liability for the prior year. However, for 2010 estimated taxes, a number of new, revised or eliminated tax rules may affect your calculations, especially if you are a small business owner. The more significant of these include the following:

  Roth IRAs  Beginning in 2010, you can convert a traditional IRA into a Roth IRA regardless of your income and filing status. You can also convert amounts from your employer's qualified pension or profit-sharing plan―including a 401(k) plan―according to the same rules. When you convert from a traditional IRA or a qualified retirement plan to a Roth IRA it is a taxable event―you will have to pay income tax on all pre-tax contributions and earnings. (If you convert in 2010, a special provision applies: Unless you elect otherwise, you defer recognition of the resulting income by spreading it over the following two years. As a result, you recognize taxable income and pay the related federal income taxes in 2011 and 2012.)

  Small Business Expensing Limits  The recently enacted HIRE Act returns the enhanced expensing rules for small businesses under Section 179 of the tax code to their higher pre-2010 amounts. This increases the maximum amount of qualifying expenditures―such as purchases of machinery and equipment―that businesses can expense for 2010 from $134,000 to $250,000. The expensing election begins to phase out when a business buys more than $800,000 of expensing-eligible assets, up from the pre-Act phase-out of $530,000. For 2011, the dollar limitation is scheduled to decrease to $25,000 and the phase-out begins at $200,000.

  Recapture of First-Time Homebuyer Credit If you bought a home after April 8, 2008 and before July 1, 2009 and you claimed the credit, you'll likely have to begin repaying it in 2010. This credit was calculated as 10 percent of the purchase price of the new home, up to a maximum of $7,500. However, you must pay the credit back over a 15-year period―beginning the second year after your home purchase. And if you sell your home before the end of the 15-year period, or it ceases to be your principal residence, the remainder of the credit becomes due.

  Small-Employer Health Care Credit Recently enacted in the Patient Protection and Affordable Care Act, this federal credit provides a tax incentive for small for-profit and not-for-profit employers that pay at least half the cost of health coverage for their employees. Generally, if your small business qualifies for the maximum credit, your federal tax credit is equal to 35 percent of the amount you pay in health care premiums for your employees. You can apply the credit against your regular tax liability or alternative minimum tax (AMT) liability and can carry the credit back for one year and forward for 20 years.

  Personal Exemption and Itemized Deduction Phase-Outs Unlike prior years, personal exemptions and itemized deductions will not phase out for higher-income taxpayers in 2010. Unless Congress acts, the phase-outs will resume in 2011.

  Alternative Minimum Tax Exemption The alternative minimum tax, or AMT, is a parallel tax system with different rates, deductions, exemptions, credits and calculations than those of its regular counterpart in the federal tax code. Middle- and upper-income taxpayers―those with AMT incomes above the federal AMT exemption amount―calculate their federal income taxes under both sets of rules (traditional and AMT) and then pay the larger of the two tax amounts to the IRS. The AMT exemptions amounts for 2010 have been decreased―for example, from $70,950 in 2009 to $45,000 in 2010 for married couples filing joint returns.