Rates are set by fiscal year, effective October 1 each year. Find current rates in the continental United States ("CONUS Rates").
If a C corporation retains earnings (doesn't distribute them to shareholders) above a certain amount, an amount which the IRS concludes is beyond the reasonable needs of the business, the corporation may be assessed tax penalty called the accumulated earnings tax ( IRC section 531) equal to 20 percent (15% prior to 2013) of accumulated taxable income.
The purpose of the accumulated earnings tax is to discourage the accumulation of earnings if the reason for such accumulation is to allow shareholders to avoid paying taxes on such earnings by not paying them dividends. Keep in mind, this is not a self-assessed tax, it can be imposed via IRS review of a corporation.
C corporations may accumulate earnings up to $250,000 without incurring an accumulated earnings tax.
A personal service corporation (PSC) may accumulate earnings up to $150,000 without having to pay this tax.
If a corporation accumulates earnings that exceed the exemption amounts an accumulated earnings tax of 20% (15% prior to 2013) of the excess earnings may be assessed. In addition, interest applies to the tax from the date the corporate return was due, without extensions.
Even if the exemption amount is exceeded, regardless by how much, if the earnings are being accumulated for what the IRS considers to be the reasonable needs of the business, then the accumulated earnings tax won't be imposed.
The question is, what does the IRS consider reasonable needs of the business?
Here's what the IRS says the reasonable needs of a business are:
Business expansion, constructing a new facility, and investing in newer and more productive equipment are reasonable business needs.
S corporations don't have a problem with accumulated earnings because earnings are taxed to S corporation shareholders even if they're not distributed to them.