Emergency Contraceptive Pill Effectiveness Calculator

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See how levonorgestrel emergency contraceptive pills compare with ulipristal acetate emergency contraceptive pills by hours since intercourse, weight/height, and number of encounters. Get your estimated pregnancy chance and your first accurate test day.

Enter a number from 0 to 168.
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Methodology & sources

How this estimate works

  1. Baseline per-act pregnancy chance by age. We anchor the unprotected, per-act chance to age-banded baselines (below), derived from day-specific fecundability studies and averaged across cycles where timing is uncertain.
  2. Adjust for any method used at the time. If a contraceptive was in use (e.g., condom, pill on time), we apply a residual-risk multiplier to the baseline to reflect protection before emergency contraceptives (EC).
  3. Apply EC effectiveness by hours since intercourse and BMI. We model a time-sensitive effectiveness curve for levonorgestrel (LNG) and ulipristal acetate (UPA), then adjust for BMI (reduction is stronger for LNG). The calculator reports an effectiveness range and the resulting pregnancy-probability range.
  4. Combine if multiple acts. For n similar-timed acts, overall probability is 1 − (1 − p)n. (If acts occurred at very different times, re-run with the most relevant hours.)
  5. Testing timing. We show a first realistic test day (~14 days after the act). If your period date is unknown/irregular, many services advise testing ≥21 days after intercourse.
1) Baseline per-act chance (unprotected)
AgeBaseline per-act
<203.5%
20–244.0%
25–294.5%
30–343.6%
35–392.6%
40–441.5%
45+0.6%

These values are calibrated from day-specific conception probabilities around ovulation (10–33% near the fertile days) averaged over typical cycles when exact timing is unknown. They keep estimates conservative for a general-audience self-assessment.

2) Residual risk if a method was used at the time
Method at time of intercourseResidual multiplier (× baseline)
None1.00
Condom (used correctly)0.20
Withdrawal0.50
Combined pill (on time)0.05
POP (on time)0.07

Multipliers approximate per-act protection so results align with well-known annual “typical/perfect-use” patterns. They are heuristics, not direct conversions of annual failure rates.

3) EC effectiveness by hours and BMI (what the slider drives)

Labeled windows: UPA is licensed up to 120 hours after intercourse; LNG up to 72 hours (some services consider use to 120 hours with reduced effectiveness). Copper IUD is the most effective EC (not modeled here).

Levonorgestrel (1.5 mg)
Hours since intercourseMidpoint effectiveness
0–24 h~85%
24–48 h~70%
48–72 h~55%

After 72–120 h we taper to ~35% (and further beyond), reflecting off-label decline.

Ulipristal acetate
Hours since intercourseMidpoint effectiveness
0–24 h~90%
24–48 h~85%
48–72 h~75%
72–96 h~65%
96–120 h~60%

BMI adjustment (applied to the % above):

BMI bandLNG adj.UPA adj.
<25×1.00×1.00
25–29.9×0.80×0.90
30–34.9×0.60×0.80
≥35×0.40×0.70

Evidence suggests LNG performance falls with higher BMI/weight; UPA is less affected. Some guidelines recommend UPA first-line or double-dose LNG when BMI >26 kg/m² or weight >70 kg; the calculator reflects this by reducing modeled LNG effectiveness as BMI rises.

4) Putting it together
p_base = baseline(age)
p_pre  = p_base × residual(method_at_time)
EC_effect ∈ [min,max] from hours & BMI
per-act risk range = p_pre × (1 − EC_effect)
overall risk range (n acts) = 1 − (1 − per-act)n
5) Important limits
  • Educational estimate only; not medical advice. If you are outside labeled windows (LNG >72 h or UPA >120 h) or have uncertainty about ovulation timing/medication interactions, seek local clinical guidance.
  • Results assume ovulation has not already occurred (pills primarily delay ovulation).
  • Multiple-act combination assumes similar timing and independence; widely separated acts should be assessed separately.
  • Copper IUD EC is not modeled here but is the most effective EC option if available and acceptable.
Sources

This tool cannot replace advice from your doctor or midwife.

FAQs

How effective is levonorgestrel emergency contraception (LNG) compared with ulipristal acetate emergency contraception (UPA)?
Both reduce pregnancy risk after unprotected intercourse. Effectiveness declines with time. UPA generally maintains higher effectiveness up to 120 hours, while LNG is labeled for up to 72 hours. The calculator personalizes your estimate by hours and body size.
How late can I take it?
LNG is labeled up to 72 hours after intercourse; UPA up to 120 hours. If you are beyond those windows or think ovulation already happened, seek local clinical advice. A copper IUD is the most effective emergency contraception if available and acceptable.
Does weight or BMI affect effectiveness?
Higher body size may reduce effectiveness, especially for LNG. Enter weight and height to compute BMI; the calculator adjusts estimates and may suggest UPA when appropriate.
Can I use emergency contraception pills more than once in a cycle?
Emergency contraception pills can be used more than once if needed, but they are not intended as a primary contraceptive method. Consider starting or resuming a regular method for ongoing protection; talk to a clinician for personalised guidance.
When should I take a pregnancy test?
Many services recommend testing from your first missed period, or at least 21 days after intercourse if your period date is uncertain. The calculator also shows a first accurate test day based on your inputs.
Will it protect me for intercourse later in the cycle?
No. Emergency contraception pills mainly delay ovulation for the encounter that already happened. They do not protect against later encounters, so use ongoing contraception for future intercourse.

This content is educational and not a substitute for professional medical advice.