Short answer
It equals a Hajj in reward, not in obligation. The Prophet ﷺ said an umrah in Ramadan is equal to a Hajj, but it does not remove the duty of the fard Hajj. No umrah, at any time, replaces Hajj. The four schools agree.
Umrah in Ramadan is the most virtuous time for umrah. It does not discharge the fard Hajj, which remains due on whoever is able. Umrah is makruh tahrimi on 9 to 13 Dhul Hijjah for those not performing it as part of their Hajj.
The four schools agree on this.
Explanation
When a woman of the Ansar could not join the Prophet's Hajj, he told her: when Ramadan comes, do umrah, for an umrah in it equals a Hajj. In another wording, "equals a Hajj with me". Scholars agree the equality is in reward, since the Prophet ﷺ did not tell her that her Hajj duty was finished.
- Fard Hajj still due: if you can afford Hajj, you must still go, even after many Ramadan umrahs.
- Best timing: any time in Ramadan; the last ten nights are the most crowded.
- Fasting during umrah: a traveller may break the fast, which can make tawaf and sa'i easier in the heat. Once you are a resident in Makkah under the travel rules, you fast.
- Some scholars held the virtue was special to that woman, but the majority apply it to everyone.
- Disliked days: in the Hanafi school, do not start an umrah on the day of Arafah or the four days after it.
For why Hajj and umrah differ, see is umrah fard or sunnah. For the reward of umrah in general, see does umrah erase sins.
The evidence
Quran, Surah Al-Baqara 2:196
وَأَتِمُّوا۟ ٱلْحَجَّ وَٱلْعُمْرَةَ لِلَّهِ ۚ فَإِنْ أُحْصِرْتُمْ فَمَا ٱسْتَيْسَرَ مِنَ ٱلْهَدْىِ ۖ وَلَا تَحْلِقُوا۟ رُءُوسَكُمْ حَتَّىٰ يَبْلُغَ ٱلْهَدْىُ مَحِلَّهُۥ ۚ فَمَن كَانَ مِنكُم مَّرِيضًا أَوْ بِهِۦٓ أَذًۭى مِّن رَّأْسِهِۦ فَفِدْيَةٌۭ مِّن صِيَامٍ أَوْ صَدَقَةٍ أَوْ نُسُكٍۢ ۚ فَإِذَآ أَمِنتُمْ فَمَن تَمَتَّعَ بِٱلْعُمْرَةِ إِلَى ٱلْحَجِّ فَمَا ٱسْتَيْسَرَ مِنَ ٱلْهَدْىِ ۚ فَمَن لَّمْ يَجِدْ فَصِيَامُ ثَلَٰثَةِ أَيَّامٍۢ فِى ٱلْحَجِّ وَسَبْعَةٍ إِذَا رَجَعْتُمْ ۗ تِلْكَ عَشَرَةٌۭ كَامِلَةٌۭ ۗ ذَٰلِكَ لِمَن لَّمْ يَكُنْ أَهْلُهُۥ حَاضِرِى ٱلْمَسْجِدِ ٱلْحَرَامِ ۚ وَٱتَّقُوا۟ ٱللَّهَ وَٱعْلَمُوٓا۟ أَنَّ ٱللَّهَ شَدِيدُ ٱلْعِقَابِ ﴿١٩٦﴾
And complete the Hajj and 'umrah for Allah. But if you are prevented, then [offer] what can be obtained with ease of sacrificial animals. And do not shave your heads until the sacrificial animal has reached its place of slaughter. And whoever among you is ill or has an ailment of the head [making shaving necessary must offer] a ransom of fasting [three days] or charity or sacrifice. And when you are secure, then whoever performs 'umrah [during the Hajj months] followed by Hajj [offers] what can be obtained with ease of sacrificial animals. And whoever cannot find [or afford such an animal] - then a fast of three days during Hajj and of seven when you have returned [home]. Those are ten complete [days]. This is for those whose family is not in the area of al-Masjid al-Haram. And fear Allah and know that Allah is severe in penalty.
Hajj and umrah are two separate acts of worship.
Sahih Muslim 1255
Sahih... فإذا جاء رمضان فاعتمري فإن عمرة فيه تعدل حجة ...
"When Ramadan comes, perform umrah, for an umrah in it equals a Hajj."
The reward of umrah in Ramadan.
Sahih al-Bukhari 1863
Sahih... فإن عمرة في رمضان تقضي حجة معي ...
"An umrah in Ramadan is like a Hajj with me."
Another wording of the same promise.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Hajj, chapter on umrah. Umrah is best in Ramadan and disliked on the five days of Hajj.
- al-Majmu' (Nawawi), Kitab al-Hajj, on the time of umrah. The equality is in reward, not in discharging the obligation.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.