Plan B (Emergency Contraception) Calculator

What Is Plan B?

Plan B is a brand of levonorgestrel emergency contraception — a single high dose of progestin taken after unprotected sex to reduce the chance of pregnancy. It works mainly by delaying ovulation, so no egg is released to meet sperm. It is available without a prescription, is most effective the sooner it is taken, and is not an abortion pill: it does not work after implantation.

This calculator estimates two numbers: your baseline pregnancy risk from the timing of unprotected sex in your cycle, and the lower residual risk after taking Plan B. Both are educational estimates — emergency contraception is time-sensitive, so the most important step is always to act quickly and confirm your choice with a pharmacist or clinician.

How to Use This Calculator

1. Enter the date of unprotected sex and, if known, your last menstrual period and usual cycle length.
2. Enter when you took (or plan to take) Plan B. The time gap between sex and the dose is the biggest driver of effectiveness.
3. Read both numbers: baseline risk without emergency contraception, and estimated residual risk with Plan B taken at that timing.
4. Act on the result, not just the number. If timing or body weight makes Plan B look weak, ask a pharmacist about ulipristal or a copper IUD right away — speed matters more than any estimate.

How the Risk Estimate Works

The model first estimates baseline conception probability from cycle timing: using your LMP and cycle length, it infers likely ovulation timing and maps the intercourse date onto a fertility curve that peaks around ovulation. It then applies a time-to-dose effectiveness factor for levonorgestrel — strongest in the first 24 hours, still meaningful to 72 hours, and declining through 120 hours. A modest adjustment is applied for higher body weight, where research suggests reduced effectiveness.

Worked Example

Suppose unprotected sex happened on a Sunday evening, three days before your estimated ovulation — a high-fertility day where baseline risk might be around 20–25%. Take Plan B within 12 hours, and typical effectiveness near 85–90% would reduce that to roughly 2–4% residual risk. Wait until Wednesday evening (about 72 hours), and effectiveness has fallen meaningfully — residual risk climbs toward 6–10%.

The takeaway is the same one pharmacists give: every hour counts. The worked numbers are illustrative — your real risk depends on actual ovulation timing, which no calendar estimate can pin down exactly.

How Accurate Is It?

Honestly: it is a rough estimate, not a personal risk score. Cycle-based ovulation prediction can be off by days, sperm survival varies, and published Plan B effectiveness figures come from population studies with wide confidence intervals. The calculator is most useful for showing direction — how much timing matters — rather than a precise percentage.

The biggest real-world caveats: if ovulation already occurred, Plan B’s main mechanism is blunted; at higher body weight, effectiveness may be lower; and vomiting within a couple of hours of the dose may require a repeat dose — ask your pharmacist. None of these are reasons to skip emergency contraception; they are reasons to choose the right method quickly.

Other Emergency Contraception Options

Ulipristal (ella): prescription-only, effective up to 120 hours after unprotected sex, and appears to work better than levonorgestrel close to ovulation. Less affected by body weight. Copper IUD: placed by a clinician within 5 days, it is the most effective emergency contraception available — over 99% — and then provides up to 10 years of ongoing contraception. Both deserve a direct conversation with a pharmacist or doctor rather than an online decision.

As a rule of thumb: if it has been under 72 hours and you can act now, levonorgestrel is the fastest option. If more time has passed, if you may be near ovulation, or if body weight is a concern, ask specifically about ulipristal or the copper IUD — they outperform Plan B in exactly those situations.

What to Expect After Taking Plan B

Plan B is generally well tolerated. The most common side effects are nausea, fatigue, headache, breast tenderness, dizziness, and light spotting — all usually mild and short-lived. Taking the dose with food can reduce nausea. If you vomit within 2–3 hours of taking it, contact your pharmacist, since you may need another dose.

Your next period may come earlier or later than usual — a delay of up to a week is common and not by itself a sign of pregnancy. If your period is more than 7 days late, or it has been 3 weeks since the unprotected sex, take a home pregnancy test. Plan B does not protect against sexually transmitted infections, and it is not designed for regular use: if you find yourself needing it often, a doctor or pharmacist can help you switch to a reliable ongoing method.

Getting Plan B Quickly: Practical Tips

Speed is the entire game with emergency contraception, so remove friction in advance. The single best tip: keep a dose at home before you need it — levonorgestrel has a long shelf life, and having it in the drawer turns a stressful pharmacy run into a non-event. Generic levonorgestrel works the same as the brand name and usually costs less.

If you need it now, call ahead: not every pharmacy stocks every option, and confirming availability saves a wasted trip. Pharmacists can also tell you on the spot whether ulipristal or a copper IUD referral makes more sense for your timing. And if cost is a barrier, family planning clinics and many campus health centers provide emergency contraception at low or no cost.

Common Myths vs Facts

Myth: Plan B causes an abortion. Fact: it delays ovulation and does not work after implantation — it cannot end an existing pregnancy.

Myth: you need a prescription. Fact: levonorgestrel emergency contraception is available over the counter in most places, with no age restriction in many countries.

Myth: it harms future fertility. Fact: research shows no effect on long-term fertility; your cycle typically normalizes within a month or two.

Myth: it protects against STIs. Fact: it offers zero STI protection — use condoms for that, and consider STI testing after unprotected sex.

Myth: taking it twice is dangerous. Fact: repeat use is safe but unreliable as a routine method, and it disrupts cycle predictability. Needing it more than occasionally is a practical signal — not a medical emergency — to discuss ongoing contraception with your provider.

When to Talk to a Pharmacist or Doctor

• Right now, if it has been more than 72 hours — a pharmacist can advise on ulipristal or copper IUD availability near you.
• If you take regular medications (some anti-seizure drugs, St. John’s wort, and others can reduce effectiveness).
• If you vomit within 2–3 hours of taking the dose.
• If your period is more than 7 days late afterward, or it has been 3 weeks since the unprotected sex — take a pregnancy test.
• If you need emergency contraception more than occasionally — a doctor can help you find a reliable ongoing method.

Pharmacists are an excellent first stop: in many places they can dispense emergency contraception directly and answer timing questions on the spot.