Progesterone Unit Converter
Progesterone Unit Converter
Convert progesterone between ng/mL, nmol/L and µg/L, with the mid-luteal thresholds for ovulation and why pulsatile release blunts a single value.
Progesterone converter
Mass ⇄ molarProgesterone 12 ng/mL, mid-luteal sample
Formula and conversion factors
ng/mL = nmol/L ÷ 3.18005
µg/L = ng/mL (numerically identical)
- 3.18005
- derived from the molecular mass of progesterone, 314.46 Da
- µg/L
- numerically identical to ng/mL; no conversion is needed between them
- 30 nmol/L
- the conventional mid-luteal threshold for ovulation, equal to about 9.4 ng/mL
- timing
- the sample belongs seven days before the next expected period, not on day 21 regardless of cycle length
Worked example
Progesterone 12 ng/mL, mid-luteal sample
12 × 3.18005 = 38.2 nmol/L
= 12.00 µg/L
Above 30 nmol/L, consistent with ovulation, and within the mid-luteal interval of 5–20 ng/mL
Mid-luteal thresholds in both units
| ng/mL | nmol/L | |
|---|---|---|
| Follicular phase | < 1 | < 3.2 |
| Mid-luteal, suggests anovulation | < 5 | < 16 |
| Mid-luteal, equivocal — repeat in a further cycle | 5 – 9.4 | 16 – 30 |
| Mid-luteal, consistent with ovulation | > 9.4 | > 30 |
Getting the timing right
| Cycle length | Sample day | Why |
|---|---|---|
| 28 days | Day 21 | Seven days before the next expected period, which is where day 21 comes from |
| 35 days | Day 28 | The luteal phase is roughly fixed at 14 days; the follicular phase is what varies |
| Irregular | Timing unreliable | A mid-luteal sample cannot be scheduled; consider repeated sampling or a different approach |
Timing, pulses, and the limits of one value
Progesterone is reported in ng/mL in the United States and in nmol/L elsewhere, with a factor of 3.18005 between them derived from its molecular mass of 314.46 daltons. A progesterone of 12 ng/mL is 38.2 nmol/L. The third unit shown, µg/L, is numerically identical to ng/mL, so no arithmetic is needed between those two.
The classic use of the test is the mid-luteal progesterone, taken to confirm that ovulation has occurred. It is often described as the day 21 progesterone, which is correct only for a 28-day cycle. The luteal phase is roughly fixed at about fourteen days while the follicular phase varies, so the sample belongs seven days before the next expected period: day 21 in a 28-day cycle, day 28 in a 35-day cycle. Mistimed sampling is the commonest reason for a low result in a woman who is ovulating normally.
A value above roughly 30 nmol/L, or about 9.4 ng/mL, is taken as evidence that ovulation occurred. That threshold is a convention rather than a physiological boundary, and it should not be read as a measure of luteal adequacy. Progesterone is secreted by the corpus luteum in pulses that follow luteinising hormone, and concentrations can vary several-fold within a few hours, so a single low value does not exclude ovulation. Repeating the measurement, often in a second cycle, is standard before drawing any conclusion.
In early pregnancy the test carries a different and more limited meaning. A low serum progesterone is associated with non-viable pregnancy, but the association is not strong enough to diagnose one, and guidance is explicit that progesterone should not be used on its own to decide management of a pregnancy of unknown location. Serial hCG measurement and transvaginal ultrasound remain the basis of that assessment. Progesterone supplementation, whether oral, vaginal or intramuscular, also raises measured levels and makes the result uninterpretable for that purpose.
Frequently asked questions
How do I convert progesterone from ng/mL to nmol/L?
Multiply by 3.18005, a factor derived from its molecular mass of 314.46 Da. A progesterone of 12 ng/mL is 38.2 nmol/L. To go the other way, divide by 3.18005.
When should a mid-luteal progesterone be taken?
Seven days before the next expected period. That is day 21 in a 28-day cycle and day 28 in a 35-day cycle, because the luteal phase is roughly fixed while the follicular phase varies. Mistiming is the commonest cause of a misleadingly low result.
What level confirms ovulation?
A mid-luteal progesterone above about 30 nmol/L, roughly 9.4 ng/mL, is the conventional evidence that ovulation occurred. Values below about 16 nmol/L suggest anovulation, and the range between the two is equivocal and usually repeated.
Can a single low progesterone exclude ovulation?
No. The corpus luteum releases progesterone in pulses following luteinising hormone, and levels can vary several-fold within hours. A low value is usually repeated, often in a further cycle, before it is acted on.
Is progesterone useful in early pregnancy?
Only as supporting information. A low level is associated with non-viable pregnancy but is not diagnostic, and it should not be used alone to decide management. Serial hCG and transvaginal ultrasound remain the basis of assessment, and supplementation raises the measured level.
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References
- National Institute for Health and Care Excellence. Fertility problems: assessment and treatment. NICE clinical guideline CG156. 2013, updated 2017.
- National Institute for Health and Care Excellence. Ectopic pregnancy and miscarriage: diagnosis and initial management. NICE guideline NG126. 2019, updated 2023.
- Practice Committee of the American Society for Reproductive Medicine. Diagnosis and treatment of luteal phase deficiency: a committee opinion. Fertil Steril. 2021;115(6):1416–1423.