Apple Inc. on Friday said reception problem with its latest iPhone was in part due to a wrong method to calculate signal strength, promising to fix the glitch with software updates in coming weeks.

Since Apple's iPhone 4 went on sale on June 24 in the United States and four other countries, complaints have been circulating that holding the handset in a certain way can lead to dramatic drop of signal bars.

Apple on Friday announced that it has found the cause of the dramatic drop, which it described as "both simple and surprising."

"Upon investigation, we were stunned to find that the formula we use to calculate how many bars of signal strength to display is totally wrong," Apple said in an open letter to users posted on its website.

The faulty formula can mistakenly display two more bars than it should for a given signal strength, the company said.

It explained that users observing a drop of several bars when they grip their iPhone in a certain way are most likely in an area with very weak signal strength, but they don't know it because the phone is erroneously displaying four or five bars.

"Their big drop in bars is because their high bars were never real in the first place," Apple said.

The company said it will release a free software update within a few weeks using a formula that can display signal bars more accurately, providing users a better indication of the reception they will get in a given area.

The mistake has been existing since the original iPhone that was launched in 2007, Apple said, adding that the software update will be available for the iPhone 3GS and iPhone 3G as well as for the latest model.