Free timing tool
Plan B Calculator
Enter when unprotected sex happened and when you’re planning to take Plan B. We’ll show you the elapsed time and where that falls in the usual 72-hour window.
This calculator is for educational purposes and gives an estimate based on what you enter. It cannot tell you whether you’re pregnant, confirm when you ovulated, or guarantee that emergency contraception will prevent pregnancy. It doesn’t replace advice from a pharmacist, nurse, or doctor.
How Long Do You Have to Take Plan B?
You generally have up to 72 hours — three days — after unprotected sex to take Plan B (levonorgestrel). But that number is a ceiling, not a target. Plan B works best the sooner it’s taken, and its ability to prevent pregnancy appears to drop the longer you wait.
There’s no single moment where it suddenly “stops working” before 72 hours. It’s more of a sliding scale: earlier is better, and every extra hour of delay can matter. If you’re not sure whether you’re still in the window, the calculator above will do the math for you.
Plan B Time Frame
Here’s a simple way to think about the time frame, based on how long it’s been since unprotected sex:
| Time since unprotected sex | What it generally means |
|---|---|
| 0–24 hours | The window when Plan B is generally considered most effective. Taking it now is usually better than waiting. |
| 24–48 hours | Still within the commonly recommended time frame. Take it as soon as you reasonably can. |
| 48–72 hours | You’re near the edge of the usual window. Plan B may still help, but time is short — don’t delay further. |
| More than 72 hours | Outside the typical Plan B window. Other emergency contraception options may still be available — talk with a pharmacist or clinician promptly. |
We haven’t listed exact effectiveness percentages here on purpose. Studies on emergency contraception timing vary in their methods and populations, and precise numbers can be misleading out of context. The honest takeaway is simpler: sooner is better, and 72 hours is the commonly cited outer edge for this particular medication.
When Should You Take Plan B?
As soon as possible after unprotected sex. That’s the direct answer. You don’t need to wait for a particular time of day, wait to “see if” you might be pregnant, or wait until your next dose of another medication wears off in most cases.
Plan B is available over the counter without a prescription or age restriction in the United States, which is part of why speed matters — there’s usually nothing standing between you and taking it right away, other than getting to a pharmacy.
How Long Does Plan B Work?
This is a different question from “how long do you have to take it,” and it trips people up. Plan B is emergency contraception for one specific episode of unprotected sex. It is not an ongoing form of birth control, and it doesn’t protect you against pregnancy from sex that happens afterward — even later that same day.
Plan B mainly works by delaying or preventing ovulation, the release of an egg. Once you’ve ovulated, it’s much less effective, and it doesn’t work at all if you’re already pregnant. If you have unprotected sex again after taking Plan B, that new instance isn’t covered — you’d need to consider emergency contraception again, or start using ongoing birth control.
Does Plan B Work Around Ovulation?
Timing relative to ovulation matters a lot for how well Plan B works. Because it mainly delays or prevents ovulation, it’s less effective if you take it after you’ve already ovulated that cycle.
The problem is that most people can’t know exactly when they ovulated. Cycle length varies month to month, tracking apps and calendar math give estimates rather than certainties, and stress, illness, and travel can all shift ovulation earlier or later than expected. That’s why the optional cycle fields in the calculator above are labeled as a rough estimate, not a diagnosis — they can’t tell you with confidence whether ovulation has already happened.
The practical takeaway is the same regardless of where you might be in your cycle: taking Plan B sooner gives it a better chance of working, because it doesn’t depend on guessing ovulation correctly.
When Should You Take a Pregnancy Test After Plan B?
Wait until your period is at least a few days late, or about three weeks after the unprotected sex, before testing. Testing too early — even a day or two after taking Plan B — won’t give you a reliable result, because it takes time for pregnancy hormone (hCG) to build up to a detectable level.
Home pregnancy tests check urine for hCG, and taking Plan B itself does not affect that test or cause a false positive. If your period arrives close to when you expected it, that’s a reassuring sign, though it isn’t a substitute for testing if you have ongoing concerns.
Can Plan B Make Your Period Early or Late?
Yes, this is common and usually not a cause for alarm. Plan B can shift your next period a few days earlier or later than expected, and some people notice spotting or a heavier or lighter flow than usual.
Most periods return to a normal pattern within a cycle or two. If your period is more than a week late, it’s a reasonable time to take a pregnancy test. If your period doesn’t return at all, or if you have ongoing irregular bleeding, it’s worth checking in with a healthcare provider.
What If More Than 72 Hours Have Passed?
You still have options. Levonorgestrel Plan B is the most widely known emergency contraceptive, but it isn’t the only one. Depending on availability and your individual situation, other options — including some that work later than 72 hours after unprotected sex — may be appropriate.
The most useful next step is to contact a pharmacist, clinic, or doctor as soon as you can. They can talk through what’s realistic for your timing and circumstances, rather than guessing on your own.
What to Do After Taking Plan B
- Take it as directed on the package — most versions are a single dose.
- If you vomit within a couple of hours of taking it, contact a pharmacist about whether you need another dose.
- Keep using or start using ongoing birth control if you don’t want to become pregnant later in this cycle — Plan B doesn’t cover future unprotected sex.
- Note the date of your next expected period so you know when it would be reasonable to test if it doesn’t arrive.
- Mild side effects like nausea, fatigue, headache, or breast tenderness are common and usually pass within a day or two.
When Should You Seek Medical Care?
Most people take Plan B without any serious problems. Contact a doctor, clinic, or emergency care promptly if you experience:
- Severe or worsening abdominal or pelvic pain, especially on one side
- Fainting, dizziness that doesn’t pass, or feeling like you might pass out
- Unusually heavy vaginal bleeding, or bleeding that soaks through pads quickly
- Pain in your shoulder along with abdominal pain (this combination, along with the symptoms above, can be associated with ectopic pregnancy and needs prompt evaluation)
- Signs of an allergic reaction, such as swelling, hives, or trouble breathing
These symptoms don’t automatically mean something is wrong, but they’re not ones to wait out on your own. When in doubt, it’s reasonable to call a healthcare provider or seek urgent care.
Frequently Asked Questions
How long do you have to take Plan B?
Generally up to 72 hours after unprotected sex, though taking it as early as possible within that window is better.
Can you take Plan B after 72 hours?
Levonorgestrel Plan B is intended for use within 72 hours. After that, other emergency contraception options may still be available — a pharmacist or clinician can advise on what fits your timing.
Does Plan B work after ovulation?
It’s less effective after ovulation has already happened, since it mainly works by delaying or preventing the release of an egg. Because ovulation timing is hard to know for certain, taking it as soon as possible gives it the best chance of working.
How long does Plan B work?
It addresses one episode of unprotected sex. It isn’t ongoing birth control and doesn’t protect against pregnancy from unprotected sex that happens afterward.
When should I take a pregnancy test after Plan B?
Wait until your period is a few days late, or about three weeks after the unprotected sex, for a reliable result.
Can Plan B delay your period?
Yes. An earlier or later period, and lighter or heavier bleeding than usual, are common after taking emergency contraception.
A note on this calculator: it’s an educational timing tool, not a medical test. It can’t tell you whether you’re pregnant, confirm exactly when you ovulated, or guarantee that any form of emergency contraception will work. If you have concerns about your specific situation, a pharmacist, nurse, or doctor can give you guidance based on your health history.
Sources / Medical References
- U.S. Food & Drug Administration — emergency contraceptive labeling and guidance
- Centers for Disease Control and Prevention — contraception guidance
- Planned Parenthood — emergency contraception information
- American College of Obstetricians and Gynecologists (ACOG) — emergency contraception practice guidance
- NHS — emergency contraception information
- Mayo Clinic / Cleveland Clinic — emergency contraception patient education
This page is for general education and doesn’t replace individualized medical advice. If you have symptoms that concern you, contact a healthcare professional.