Do I have to make up missed Ramadan fasts?

Yes, missed Ramadan fasts generally have to be made up, one day for each day missed. This applies to days missed for illness, travel, a period or childbirth. You can make them up any time before the next Ramadan, on separate days or in a row.

The rule in the Quran

Allah allows people who are sick or traveling to skip fasting, and then tells them to make up the days later:

وَمَن كَانَ مَرِيضًا أَوْ عَلَىٰ سَفَرٍ فَعِدَّةٌ مِّنْ أَيَّامٍ أُخَرَ "And whoever is ill or on a journey, then an equal number of other days." Quran 2:185

Women who miss days because of their period also make them up. Aisha said they were told to make up the fasts but not the prayers (Sahih Muslim 335). For travel rules, see Can I skip fasting while traveling?

When and how to make them up

Any day except Eid days: you can't fast on Eid al-Fitr or Eid al-Adha.
Before next Ramadan: Aisha used to make up her missed days in Sha'ban, the month before Ramadan (Sahih al-Bukhari 1950).
In a row or spread out: both are fine. Many people do Mondays and Thursdays, or short winter days.
Intention the night before: say in your heart that tomorrow is a make-up (qada) Ramadan fast.

Winter days in Europe and North America are short, which makes catching up much easier. Ramadan 2027 is expected to start around 8 February 2027; see When does Ramadan start in 2027?

What if I can't ever make them up?

If a long-term condition means you will never be able to fast (for example old age or a chronic illness your doctor says won't improve), you don't make up the days. Instead you pay fidya: feeding a poor person for each missed day. The same applies to some mental health conditions; see Can I skip fasting because of a mental illness?

What if I delayed past the next Ramadan?

If you had no excuse and let a whole year pass, you still make up the days. The Maliki, Shafi'i and Hanbali schools add fidya for each day as well, while the Hanafi school says making up the days is enough. If you're catching up on many years, for example from years when you weren't practising, make a plan, start slowly and ask a local scholar about your case.

What about fasts from before I became Muslim?

None. Islam wipes out what came before it, so converts don't make up any Ramadans from before their shahada.

Key points

Make up one day for each Ramadan day missed.
Do it before the next Ramadan if you can.
People who can never fast pay fidya instead.
Converts don't make up fasts from before Islam.

Tip: Write the number of days you owe in your phone notes right after Eid, and pick a few short winter days to clear them. Fasting with a friend who is also catching up makes it far easier.