Free Plan B effectiveness calculator — estimate emergency contraception effectiveness based on time since unprotected sex, body weight, and cycle day. Includes ella and copper IUD comparison with FDA-sourced data.
✓ Last verified: March 2026 · FDA prescribing information & ACOG guidelines
Plan B Effectiveness Calculator
Estimate effectiveness based on timing, weight, and cycle
⚠️ Medical notice: This calculator is for educational purposes only and is not a substitute for medical advice. If you have taken or are considering emergency contraception, please consult a pharmacist, healthcare provider, or call the Plan B helpline at 1-800-330-1271. Emergency contraception does not protect against STIs.
American College of Obstetricians and Gynecologists clinical guidance on EC options, weight thresholds, and prescribing recommendations
Methodology: Base effectiveness by time window per FDA prescribing data: within 24hrs = 95%, 24-48hrs = 85%, 48-72hrs = 58%, over 72hrs = not recommended. Weight adjustments per ACOG: under 165 lbs = standard, 165-175 lbs = approximately 20% reduction, over 175 lbs = approximately 35% reduction. Cycle day pregnancy risk estimates: Days 1-7 = 5%, Days 8-14 = 30%, Days 15-21 = 12%, Days 22-28 = 3%, unknown = 15% conservative estimate. Risk with EC = baseline risk x (1 - adjusted effectiveness). Medication interactions reduce effectiveness by approximately 30-45%. Educational estimates only.
Last reviewed: March 2026 — effectiveness data verified against current FDA prescribing information and ACOG Practice Bulletin on Emergency Contraception.
Plan B Effectiveness Calculator — Complete Guide to Emergency Contraception Timing
Understanding how effective Plan B is in your specific situation requires knowing three key variables: time elapsed since unprotected sex, your body weight, and where you are in your menstrual cycle. This Plan B effectiveness calculator combines all three factors using FDA prescribing data and WHO guidance to estimate your pregnancy risk with and without emergency contraception.
Plan B Effectiveness by Time -- The Most Important Factor
Time is the single most important variable. Plan B works primarily by delaying or preventing ovulation, and this mechanism is time-sensitive. Every hour that passes after unprotected sex reduces the window for Plan B to act before a potential fertilization event.
Estimated Pregnancy Risk With EC = Baseline Cycle Risk x (1 - Plan B Effectiveness)
Example: Day 10 (near ovulation, 30% baseline risk). Plan B taken within 24 hours (95% effective).
Risk with Plan B = 30% x (1 - 0.95) = 30% x 0.05 = 1.5%
Same scenario at 60 hours (58% effective): 30% x 0.42 = 12.6%
Every hour matters -- taking Plan B earlier significantly reduces remaining risk.
Time Since Sex
Plan B Effectiveness
ella Effectiveness
Copper IUD
Within 24 hours
~95%
~98%
>99%
24 to 48 hours
~85%
~95%
>99%
48 to 72 hours
~58%
~85%
>99%
72 to 120 hours
Not recommended
~65%
>99%
Over 120 hours
Not effective
Not effective
>99% up to 7 days
How Body Weight Affects Plan B Effectiveness
Multiple studies indicate that levonorgestrel (the active ingredient in Plan B) may be less effective for people weighing over 165 pounds (75 kg) and substantially less effective over 175 pounds (79 kg). ACOG and WHO acknowledge this weight-related reduction.
For individuals over 165 lbs, ella (ulipristal acetate) is the clinically preferred alternative. ella maintains more consistent effectiveness across a wider weight range and is effective up to 120 hours. It requires a prescription but same-day telehealth prescriptions are widely available. The copper IUD is over 99% effective regardless of weight.
Cycle Day and Ovulation -- Why Timing Within Your Cycle Matters
Plan B works by preventing or delaying ovulation. If ovulation has already occurred it cannot prevent fertilization of a released egg. Days 8 to 14 carry the highest pregnancy risk (near ovulation) and are the most critical window for taking Plan B as soon as possible.
Cycle Day
Pregnancy Risk Without EC
Ovulation Status
Plan B Likely Action
Days 1-7
~5% (low)
Distant
Likely effective -- ovulation far off
Days 8-14
~30% (highest)
Approaching / occurring
Most critical -- take immediately
Days 15-21
~12% (moderate)
Likely passed
Reduced benefit if ovulation occurred
Days 22-28
~3% (low)
Completed
Low risk period overall
When to Choose ella or Copper IUD Instead
Choose ella if: it has been 48 to 120 hours since unprotected sex, you weigh over 165 pounds, or you want the most effective non-IUD option. ella requires a prescription -- same-day telehealth prescriptions are available through Wisp, Nurx, and Hey Jane for $30 to $60.
Choose a copper IUD if: maximum effectiveness is the priority (>99%), it has been up to 7 days since unprotected sex, or you want long-term contraception (lasts up to 10 years). The copper IUD is hormone-free and works immediately as emergency contraception.
💡 Free and low-cost options: Many Title X family planning clinics and Planned Parenthood locations offer emergency contraception on a sliding fee scale, including free services for qualifying individuals. Generic levonorgestrel (Take Action, My Way, Next Choice) is chemically identical to Plan B and typically costs $10 to $20 less. Call 1-800-230-PLAN to find a Planned Parenthood near you.
Frequently Asked Questions
How effective is Plan B? +
Plan B is approximately 95% effective when taken within 24 hours of unprotected sex, 85% effective between 24 and 48 hours, and 58% effective between 48 and 72 hours. These percentages represent the reduction in pregnancy risk compared to not using emergency contraception. Actual pregnancy risk also depends on your menstrual cycle day and body weight.
Does weight affect Plan B effectiveness? +
Yes. Research indicates Plan B may be less effective for people weighing 165 pounds or more, and significantly less effective over 175 pounds. For individuals over 165 pounds, ella (ulipristal acetate) is clinically preferred as it is less affected by body weight. The copper IUD is over 99% effective regardless of weight.
How long does Plan B work after sex? +
Plan B is approved for use up to 72 hours (3 days) after unprotected sex. Within 24 hours effectiveness is approximately 95%. Between 24 and 48 hours it is about 85%. Between 48 and 72 hours it drops to approximately 58%. After 72 hours Plan B is not recommended. ella remains effective up to 120 hours, and a copper IUD can be inserted up to 7 days after unprotected sex.
What is the difference between Plan B and ella? +
Plan B contains levonorgestrel and is effective up to 72 hours after unprotected sex. ella contains ulipristal acetate and is effective up to 120 hours (5 days). ella is also less affected by body weight and maintains more consistent effectiveness across a wider weight range. ella requires a prescription but same-day telehealth prescriptions are widely available. Both work primarily by preventing or delaying ovulation.
Can you get pregnant if you take Plan B? +
Yes, pregnancy is still possible after taking Plan B. It reduces but does not eliminate the risk. Taken within 24 hours it reduces risk by approximately 95% but is not guaranteed. Factors that reduce effectiveness include waiting too long, body weight over 165 pounds, vomiting within 2 hours of taking it, and certain medications. If your period is more than a week late after taking Plan B, take a pregnancy test.
Does Plan B affect your period? +
Yes. Plan B commonly causes changes to your next menstrual period. It may come earlier or later than expected, often by 1 to 2 weeks, and may be heavier, lighter, or different in duration. Spotting after taking Plan B is common and normal. If your period is more than 3 to 4 weeks late after taking Plan B, take a pregnancy test.
How much does Plan B cost? +
Plan B typically costs $40 to $50 at pharmacies without insurance. Generic levonorgestrel versions (Take Action, Next Choice, My Way) are chemically identical and cost $30 to $45. ella costs approximately $50 to $60 and requires a prescription. Many Planned Parenthood and Title X clinics offer emergency contraception free or at reduced cost based on income. Call 1-800-230-PLAN to find resources near you.
Does Plan B work if you are already ovulating? +
Plan B is significantly less effective if ovulation has already occurred because it works primarily by delaying or preventing ovulation. Once an egg is released it cannot prevent fertilization. Days 8 through 14 of a 28-day cycle carry the highest pregnancy risk. If you are on or past day 14, a copper IUD is a more reliable option as it can prevent implantation regardless of whether fertilization has already occurred.