hCG Levels Calculator (48-hour rise and doubling time)

Understanding the hCG levels calculator — concept and principle

This calculator compares two blood hCG (human chorionic gonadotropin) results taken some hours apart in early pregnancy. It gives the percentage rise over 48 hours and the doubling time, and compares the rise with the minimum seen in 99% of viable pregnancies in a large study.

Two blood tests, 48 h apart: with the default inputs the result is +92% (≥ 49% minimum · doubling 51.1 h).

How it is calculated

Ratio = second hCG ÷ first hCG

Rise over 48 h = ratio^(48 ÷ hours between tests) − 1

Doubling time (h) = hours × ln 2 ÷ ln(ratio); if falling, halving time = hours × ln 2 ÷ ln(1 ÷ ratio)

Minimum 48-hour rise (Barnhart 2016): 49% if the first value is under 1,500 mIU/mL, 40% for 1,500–3,000 and 33% above 3,000.

The principle

In early pregnancy hCG is made by the developing placenta and rises roughly exponentially, so the same percentage increase happens in each equal time span. That is why two values can be converted into a 48-hour rise or a doubling time regardless of the exact interval. As hCG climbs higher, its rate of rise slows, so the expected minimum rise is smaller at higher starting values. The thresholds were set so that 99% of women who turned out to have a viable pregnancy in the womb would be at or above them.

Worked example

Default inputs: first 1,200 mIU/mL, second 2,300 mIU/mL, 48 hours apart.

Ratio = 2,300 ÷ 1,200 = 1.917.
Rise over 48 h = 1.917 − 1 = 92% (same as the change between tests, since the gap is 48 h).
Doubling time = 48 × 0.693 ÷ ln(1.917) = 33.27 ÷ 0.651 = 51.1 h.
First value under 1,500 → minimum 49%; 92% is At or above the minimum.

Each point is the calculator re-run with only “Hours between the two blood tests” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Hours between the two blood tests	Rise over 48 hours
12	1,250%
24	267%
36	138%
48	92%
60	68%
72	54%
96	38%

How to read the result

A rise at or above the minimum is consistent with a viable pregnancy, but does not prove it. A slower rise does not prove a problem either: it may be an early pregnancy that is still viable, a miscarriage or an ectopic pregnancy. Doctors combine hCG trends with ultrasound and symptoms. A falling level usually needs prompt follow-up.

Common mistakes

Comparing a urine test with a blood test, or results from different labs.
Expecting every pregnancy to double in 48 hours; many viable pregnancies rise more slowly.
Interpreting a single value on its own.

Keep in mind: This is not a diagnosis. Pain, bleeding, dizziness or shoulder-tip pain in early pregnancy need urgent medical care.

Frequently asked questions

What is a normal hCG doubling time?

It varies; in early pregnancy it is often about two to three days, and it lengthens as levels rise.

Does a slow rise mean an ectopic pregnancy?

Not necessarily. It raises the need for follow-up, usually with ultrasound.

When can a pregnancy be seen on ultrasound?

That depends on the scan type and the hCG level; your doctor will advise on timing.

Sources: Barnhart KT, Guo W, Cary MS, et al., "Differences in Serum Human Chorionic Gonadotropin Rise in Early Pregnancy by Race and Value at Presentation", Obstet Gynecol 2016;128(3):504–511

Checked 9 Nov 2026. Barnhart KT, Guo W, Cary MS, et al., “Differences in Serum Human Chorionic Gonadotropin Rise in Early Pregnancy by Race and Value at Presentation”, Obstet Gynecol 2016;128(3):504–511 (minimum 2-day rise 49% / 40% / 33% by initial level); earlier Barnhart KT et al., Obstet Gynecol 2004;104(1):50–55. Thresholds as summarised in WikEM “Repeat B-hCG levels”.

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