Day 21 Progesterone Test Calculator (Any Cycle Length)
Find the right day for your “day 21” progesterone blood test when your cycle is not 28 days long, and check whether your result confirms ovulation (nmol/L or ng/mL).
How it works
Test day = cycle length − 7 · date = first day of period + (test day − 1) days · ng/mL × 3.18 = nmol/L · ≥ 30 nmol/L (about 10 ng/mL) is consistent with ovulation
The “day 21” progesterone test checks for ovulation by measuring progesterone in the mid-luteal phase, about 7 days after ovulation, when the corpus luteum makes the most. Day 21 is only right for a 28-day cycle; NICE guidance says to test 7 days before the expected next period, so a 32-day cycle is tested on day 25 and a 35-day cycle on day 28. A level of about 30 nmol/L (10 ng/mL) or more is generally taken as evidence of ovulation; a lower value may mean no ovulation or simply that the test was taken on the wrong day, so it is often repeated.
Worked example
Your period started on October 1, 2026 and your cycles last 32 days: test day = 32 − 7 = cycle day 25, so book the blood test for October 25, 2026. Ovulation is estimated around October 18, 2026 and the next period around November 2, 2026.
Sources: NICE CG156: Fertility problems — mid-luteal serum progesterone · ASRM Practice Committee (2021) Diagnostic evaluation of the infertile female. Fertil Steril 116(5)
Frequently asked questions
When should I take a day 21 progesterone test if my cycle is longer?
Test 7 days before your next expected period: cycle length minus 7. For a 30-day cycle that is day 23, for 35 days day 28. If your cycles are irregular, your doctor may repeat the test weekly until your period starts.
What progesterone level confirms ovulation?
Most labs and fertility clinics take about 30 nmol/L (roughly 10 ng/mL) or more in the mid-luteal phase as a sign that ovulation happened. Some use a lower threshold of around 16 nmol/L (5 ng/mL).
What if my progesterone result is low?
A low result can mean you did not ovulate that cycle, or that the test was taken too early or too late. Check the timing against the day your next period actually started and ask your doctor about repeating it.
How do I convert ng/mL to nmol/L?
Multiply ng/mL by 3.18 to get nmol/L, or divide nmol/L by 3.18 to get ng/mL. For example, 10 ng/mL is about 31.8 nmol/L.
Related calculators
This calculator gives population averages for education only. It is not a method of contraception or a diagnosis — talk to your doctor, gynecologist or fertility specialist about your own situation.