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Cortisol to ACTH Ratio Calculator

Enter cortisol and ACTH lab values.

How it is calculated

The calculation is a single division: ratio = cortisol ÷ ACTH. Cortisol must be entered in μg/dL and ACTH in pg/mL, the units printed on many lab reports. The ratio carries the combined unit (μg/dL per pg/mL), which is not a standard clinical unit, so it only makes sense when both inputs follow these units. It is shown to three decimals, but a very small ratio gets extra decimals so it does not print as 0.000 (0.01 over 500 shows 0.000020). The tool also echoes your two values to two decimals so you can confirm what was read. ACTH must be greater than zero and cortisol cannot be negative; otherwise there is no result. A cortisol of exactly 0 is accepted and gives a ratio of 0. The tool does not label the ratio low, normal or high, because that depends on the lab's reference ranges and the clinical picture. For reference only, not medical advice.

Worked example

A lab report lists cortisol 15 μg/dL and ACTH 30 pg/mL. Divide cortisol by ACTH: 15 ÷ 30 = 0.500. The echo lines read 15.00 μg/dL and 30.00 pg/mL. If both hormone values had doubled (30 and 60), the ratio would still be 0.500, which shows that the ratio reflects the relationship between the two values, not their size.

Try this example →

Common mistakes

  • Entering cortisol in nmol/L from a laboratory that reports in SI units. The tool expects μg/dL, so convert first, otherwise the ratio is off by a large factor.
  • Entering ACTH in pmol/L instead of pg/mL. The two are different units, and the ratio is only comparable when both inputs match the stated units.
  • Comparing the ratio with a range from a different lab or different units. Each laboratory sets its own reference ranges for the hormones themselves.
  • Using values from blood drawn at different times of day. Both hormones vary through the day, so the pair should come from the same sample.
  • Reading a very small ratio as proof of a condition. It can come from a tiny cortisol or a huge ACTH, and a single number cannot tell those apart.

About this calculator

Cortisol and ACTH (adrenocorticotropic hormone) work as a feedback loop: the pituitary releases ACTH to stimulate the adrenal glands to produce cortisol, and rising cortisol normally suppresses further ACTH release. Looking at their ratio rather than either value alone helps distinguish between different causes of adrenal dysfunction — a low ratio can point toward primary adrenal insufficiency (Addison's disease, where ACTH is high but the adrenals can't respond), while patterns of both being low or both elevated suggest different pituitary or ectopic causes.

This calculator simply divides cortisol (μg/dL) by ACTH (pg/mL) to produce that ratio. It's meant as a quick reference for students and clinicians reviewing lab panels — the ratio's clinical interpretation depends on the full clinical picture and reference ranges from the testing lab, so this is for reference only and not medical advice.

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