Ovulation test calculator: when to start testing

How the ovulation test calculator works

Ovulation tests (also called LH tests or ovulation predictor kits) look for a surge of luteinizing hormone in your urine. According to the FDA, that surge happens about 1 to 1½ days before ovulation. The surge is short, so the trick is to start testing a few days before you expect it and test every day until it shows.

Ovulation usually comes about 14 days before your next period. Starting 17 days before your period is due gives you a buffer of about three days before the likely surge. The calculator takes the first day of your last period, adds your cycle length to find your next period, and counts back 17 days. It never suggests a day earlier than cycle day 7.

When to start ovulation tests by cycle length

Day 1 is the first day of real bleeding. If your cycles vary, use your shortest recent cycle to pick the start day, so you don't miss an early surge, and keep testing until you see a positive. Not sure of your cycle length? Our cycle length calculator works it out from a few period dates.

Clearblue, First Response and other brands

Many people search for a Clearblue ovulation calculator or a First Response ovulation calculator. The good news: the timing logic is the same for every LH test, so this calculator works whichever brand you buy. Lulu isn't affiliated with any test brand. A few practical differences are worth knowing:

• Strip tests show two lines; a positive usually means the test line is as dark as or darker than the control line. A faint second line is not a positive.
• Digital tests read the result for you and show a symbol instead of lines.
• Dual-hormone tests also detect estrogen (E3G), so they can flag a few "high fertility" days before the LH peak.
• Brand leaflets have their own start-day tables, and some start a day earlier than this one. If they differ, follow the instructions that came with your test.

Best time of day to do an ovulation test

Unlike pregnancy tests, many LH test brands don't recommend first-morning urine. A good routine for most people:

1. Test once a day at about the same time, often between late morning and evening.
2. Try not to pee for about 2 hours before testing, so the urine isn't too dilute.
3. Don't drink a lot of fluid beforehand: MedlinePlus warns that large amounts of fluid can affect the result.
4. Some people with short surges test twice a day once lines start getting darker.

Worked example

Your last period started on 4 June and your cycles are usually 30 days. Your next period is due around 4 July. Counting back 17 days gives 17 June, which is cycle day 14. You test daily from then; on 19 June the test line is as dark as the control line. Ovulation most likely follows within about a day or two, so the best days for sex are that day and the next two. Our fertile window calculator shows the whole window.

Accuracy limits

• The FDA says these tests detect LH reliably about 9 times out of 10, but only when the instructions are followed carefully.
• A positive LH test shows the surge, not proof that an egg was released.
• PCOS can cause high LH levels or several surges, so tests may read positive often. Perimenopause can do the same.
• Fertility medicines containing hCG or LH can cause false positives. Ask your clinic how to test.
• Ovulation tests are not birth control. The FDA says plainly they are not reliable for preventing pregnancy.

Around the surge, many people also notice a lift in mood, energy and libido, then a dip a week later. Logging LH results next to how you feel shows your own pattern over a few cycles.

When to talk to a doctor