Subtract 17 from your shortest recent cycle length. That is the cycle day to start testing. With a 28-day cycle, that means starting on day 11, counting day 1 as the first day of your period. Keep testing once a day until you get a positive, which usually takes 3 to 7 days. If your cycles vary, use your shortest cycle from the past six months, because starting a few days early costs a strip and starting late can cost the whole window.
Start day by cycle length
Find your usual cycle length in the left column. Cycle length is counted from the first day of one period to the day before the next one starts.
| Your cycle length | Start testing on cycle day | Ovulation is likely around |
|---|---|---|
| 21 days | Day 4 | Day 7 |
| 24 days | Day 7 | Day 10 |
| 26 days | Day 9 | Day 12 |
| 28 days | Day 11 | Day 14 |
| 30 days | Day 13 | Day 16 |
| 32 days | Day 15 | Day 18 |
| 35 days | Day 18 | Day 21 |
| 40 days | Day 23 | Day 26 |
| Irregular or unknown | Use your shortest cycle, or start on day 7 | Varies, test for longer |
Why the day 14 rule is not enough
Ovulation does not land on the same day for everyone, or even for the same person every month. In a prospective study of 696 cycles, the fertile window fell entirely between cycle days 10 and 17 in only about 30% of women, and on any day from day 6 to day 21 there was at least a 10% chance of being in the fertile window.
Large app datasets show the same spread: in an analysis of more than 600,000 cycles, cycle length and ovulation timing varied widely between people and from cycle to cycle. That is the argument for starting a few days earlier than the textbook says, and for testing rather than counting.
How many days should you keep testing?
Test once a day until you get a positive, then stop for that cycle. Most people need 3 to 7 days of testing, and a wider margin is normal in the first month or two while you learn your own pattern.
If a cycle passes with no positive, start two or three days earlier next cycle and keep testing a few days longer. A missed surge is far more often a timing problem than a sign that you did not ovulate.
What time of day to test
For strip tests, late morning to early evening works best. LH rises in the blood first and takes a few hours to show up in urine, so clinics generally suggest testing between about 10 a.m. and 8 p.m., at roughly the same time each day. Avoid drinking large amounts of fluid for about two hours beforehand.
Some digital systems ask for first morning urine instead, so follow the instructions for your product. If your surges tend to be short, testing twice a day around your expected window makes them easier to catch.
When to start testing after your period
It depends on your cycle length rather than on when bleeding stops. With a 28-day cycle you will usually start about 5 or 6 days after your period ends. With a short cycle, such as 21 to 24 days, testing can begin within a day or two of your period finishing, because ovulation comes much earlier.
This is the part people most often get wrong. Waiting until a week after your period feels natural, and on a short cycle it can mean testing after the window has already closed.
If your cycles are irregular or long
Use the shortest cycle you have had in the past six months as your starting point, and plan on testing for longer. With very irregular cycles, or with PMOS, formerly called PCOS, LH can behave unpredictably, and tests are easier to read as a pattern across the cycle than as a single result.
If several cycles pass with no positive at all, or positives that last for many days, that is worth a conversation with your doctor rather than another pack of strips.
Testing on clomid or letrozole
Follow your clinic's schedule. Clomiphene changes the LH pattern during a treatment cycle, so a test taken soon after your last pill can be hard to interpret, and there is no single start day that fits every protocol or brand.
The most common mistake I see is starting too late. Counting to day 14 works for a minority of cycles, and the cost of testing a few days early is one strip, while the cost of starting late is the whole window.
— Inna Galitsky, MD, PhD, medical reviewer at PregmateWhat to do once the test is positive
A positive means your LH surge has been detected and ovulation usually follows about a day later, so have sex that day and the next. ASRM guidance is sex every one to two days across the fertile window, which means you do not need to wait for a positive to start.
Frequently asked questions
Subtract 17 from your shortest recent cycle length. For a 28-day cycle that is day 11, for a 30-day cycle day 13, and for a 24-day cycle day 7. Count day 1 as the first day of your period.
Only in the sense that you use more strips. An early negative costs nothing but the test, while starting late can mean missing the surge entirely, so err on the early side.
With a 28-day cycle, about 5 to 6 days after bleeding stops. With cycles of 21 to 24 days, within a day or two of your period ending, because ovulation arrives earlier.
Usually one to two days. The test detects the LH surge, and ovulation typically follows within about 24 to 36 hours, which is why the positive day and the next day matter most.
- Wilcox AJ, Dunson D, Baird DD. The Timing of the Fertile Window in the Menstrual Cycle: Day Specific Estimates From a Prospective Study. BMJ. 2000;321(7271):1259-1262. pubmed.ncbi.nlm.nih.gov
- Bull JR, Rowland SP, Scherwitzl EB, et al. Real-World Menstrual Cycle Characteristics of More Than 600,000 Menstrual Cycles. npj Digital Medicine. 2019;2:83. nature.com
- Marriott L, Zinaman M, Abrams KR, Crowther MJ, Johnson S. Analysis of Urinary Human Chorionic Gonadotrophin Concentrations in Normal and Failing Pregnancies. Annals of Clinical Biochemistry. 2017. journals.sagepub.com
- CCRM Fertility. When to Take an Ovulation Test. ccrmivf.com
- American Society for Reproductive Medicine. Optimizing Natural Fertility: A Committee Opinion. Fertility and Sterility. 2022. asrm.org