Where do atheists get their morality from?

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How do atheists know what is morally right or morally wrong? Is morality cultural, gut feel, are there any basic principled you can use? Opinions?

The Golden Rule predates religious morality. It has been used by social animals (including man) for thousands of years.

Tsze-kung asked, saying, "Is there one word which may serve as a rule of
practice for all one's life?" The Master said, "Is not Reciprocity such a
word? What you do not want done to yourself, do not do to others."

— Confucius, Analects XV.24

I did a study on Golden Rules a few months ago. Here's part of it

ARE ALL RELIGIONS THE SAME? A COMPARISON OF “GOLDEN RULES”
Christianity: Matthew 7:12 New International Version (NIV) 12 So in everything, do to others what you would have them do to you, for this sums up the Law and the Prophets.
The Christian Golden Rule is far more comprehensive and inclusive than the comparable “rule” in other religions. Most other religions only say that you shouldn’t do bad things to others, an omission that leaves out the love and kindness that is the character of Christ. Some religions such as Islam and Taoism encourage only positive regard or include only some people. Christianity excels!
Judaism "What is hateful to you, do not to your fellow man. That is the entire Law; all the rest is commentary" (Talmud, Shabbat 31a)
Does not tell followers to do good to others; tells them not to do bad. Applies only to actions that are substantial enough to qualify as “hateful”, rather than the Christian version which says “in everything.”
Brahmanism (a form of Hinduism) "This is the sum of duty: Do naught unto others which would cause you pain if done to you" (Mahabharata 5:5157).
Does not tell followers to do good to others; tells them not to do bad. Applies only to actions that are seen as “pain”, rather than the Christian version which says “in everything”
Buddism "Hurt not others in ways that you yourself would find hurtful" (Udana-Varga 5:18).
Does not tell the follower to do good to others; tells them only not to do bad. Applies only to actions “you yourself would find hurtful”, rather than the Christian version which says “in everything”.
Islam “No one of you is a believer until he desires for his brother that which he desires for himself" (Sunnah)
Applies only to thoughts, not to action. Includes only to those regarded as “brothers”. Excludes women. Includes the judgment that one isn’t “a believer” if he doesn’t wish his brother well.
Confucianism "Surely it is the maxim of loving-kindness: Do not unto others what you would not have them do unto you" (Analects 15:23).
Does not tell followers to do good to others; tells them only not to do bad.
Taosim "Regard your neighbor's gain as your own gain and your neighbor's loss as your own loss" (T'ai Shang Kan Ying P'ien).
Applies only to thoughts, not to action. Lacks the important qualifier in Christianity, "in everything." Excludes those that are not perceived as neighbors.
Zoroastrianism "That nature alone is good which refrains from doing unto another whatsoever is not good for itself" (Dadistan-i-dinik 94:5).
Does not tell followers to do good to others; tells them not to do bad.