FSH LH Ratio Calculator
FSH and LH are the two pituitary signals that drive the ovaries — and their ratio says something neither value says alone. Enter both values from a day-3 (early follicular) blood draw in mIU/mL, and get your ratio in both directions with reference context and an interpretation table.
LH ÷ FSH
From a day 2–4 (early follicular) blood draw; mIU/mL and IU/L are the same unit
Same report, same units as LH
Enter your value above to see the result.
FSH ÷ LH
From a day 2–4 (early follicular) blood draw
Same report, same units as FSH
Enter your value above to see the result.
This result is a screening estimate, not a diagnosis. Discuss your labs with a qualified healthcare professional.
Want a deeper AI analysis of your symptoms?
Our free 3-minute assessment maps your symptoms to possible root cause patterns and emails you a personalized report.
Take the free health assessment →For practitioners: see the Hormone Health Assessment →
How it works
The formula
LH:FSH ratio = LH ÷ FSH, with both values from the same blood draw in the same units (mIU/mL, which is numerically identical to IU/L). The inverse (FSH:LH) is the same division flipped. No unit conversion is ever needed — the units cancel.
Why the day-3 draw matters
FSH and LH both swing across the cycle — LH surges 10-fold mid-cycle. The ratio is only interpretable from an early-follicular (day 2–4) baseline draw, which is how the research cohorts behind the reference bands were sampled.
What the ratio was studied for
An elevated LH:FSH ratio was historically studied as a marker in PCOS research cohorts, reflecting altered pituitary signaling. Modern diagnostic frameworks (Rotterdam criteria) do not include the ratio — it survives as a context clue, not a criterion. Low ratios appear in hypothalamic patterns; high FSH-dominant ratios appear with ovarian reserve changes.
Limitations
Hormonal birth control, recent cycle irregularity, pregnancy, perimenopause, and assay differences can all shift the ratio. It is one signal among many — a screening reference, not a diagnosis — and interpretation belongs with a qualified practitioner who has your full panel.
LH:FSH ratio quick reference
Reference bands for a day-3 (early follicular) draw in cycling reproductive-age women. Research-cohort conventions, not official guideline cut-offs.
| LH:FSH ratio | Reading | Common context |
|---|---|---|
| < 1 | LH below FSH | Common in early follicular phase; also seen in hypothalamic (low-signal) patterns |
| 1 – 2 | Common window | Typical for reproductive-age cycling women on a day-3 draw |
| 2 – 3 | Elevated | Historically studied in PCOS research cohorts — a reference signal, not a criterion |
| ≥ 3 | Markedly elevated | Review with a qualified practitioner; assay, cycle day, and medications all shift it |
A screening signal, not a diagnosis. The Rotterdam criteria for PCOS do not include the LH:FSH ratio.
Frequently asked questions
How do I calculate my FSH LH ratio?
Take both values from the same day 2–4 blood draw and divide: LH ÷ FSH for the LH:FSH ratio, or FSH ÷ LH for the inverse. This calculator runs both directions instantly — no unit conversion needed as long as both inputs come from the same report.
What is a normal LH:FSH ratio?
In reproductive-age cycling women, a day-3 LH:FSH ratio between roughly 1 and 2 is the most common window, and ratios under 1 are also frequently seen early in the follicular phase. These are research-cohort conventions, not official cut-offs — your lab's own reference interval comes first.
Does a high LH:FSH ratio mean I have PCOS?
No. An elevated ratio was studied historically in PCOS research cohorts, but it also appears in healthy cycles, and the current Rotterdam diagnostic criteria do not include the ratio at all. It is a reference signal to discuss with a qualified practitioner — never a diagnosis on its own.
When in my cycle should I draw the labs?
Early follicular phase — days 2–4 of your cycle, counting day 1 as the first day of full flow. Both hormones surge and dip across the month, so a mid-cycle or luteal draw can't be compared against the day-3 reference bands.
Does the ratio still apply in perimenopause or menopause?
It loses most of its meaning. As ovarian reserve declines, FSH climbs and dominates the ratio regardless of LH — at that stage the individual FSH value (with clinical context) is more informative than the ratio.
My report says IU/L instead of mIU/mL — do I need to convert?
No — for FSH and LH, IU/L and mIU/mL are numerically identical (1 IU/L = 1 mIU/mL). Enter the values exactly as printed, using the same units for both inputs.
Sources
- Saadia Z. Follicle Stimulating Hormone (LH:FSH) Ratio in Polycystic Ovary Syndrome (PCOS) — Obese vs. Non-Obese Women. Med Arch, 2020.
- Rotterdam ESHRE/ASRM-Sponsored PCOS Consensus Workshop Group. Revised 2003 consensus on diagnostic criteria and long-term health risks related to PCOS. Fertil Steril, 2004.
This calculator is for informational purposes only and is not medical advice, diagnosis, or treatment. Reference ranges are general population guidelines — always discuss your lab results with a qualified healthcare professional.