hCG Doubling Time Calculator
Work out how fast hCG is rising or falling from two or more blood test results: the doubling time, the rise over 48 hours and per day, with the working shown.
How to Use
- Choose Two results for a pair of blood tests, or Several results for three or more.
- Enter the hCG values in mIU/mL (the same as IU/L) and the time between the tests in hours or days. For several results, type one line per test: the time, then the value, such as
48 170. - Read the doubling time, the rise over 48 hours and the rise per day. If the level fell, you get the halving time and the fall instead.
- The chart shows your results and the steady rise through them; the dashed line is a published reference rise, for context only.
- Show Work lists each step, each interval and the published figures. Discuss your results with your clinician.
Worked Example
Two tests 48 hours apart. The first result was 120 mIU/mL and the second, 48 hours later, 250 mIU/mL. The ratio is 250 ÷ 120 = 2.0833, so the doubling time is 48 × ln 2 ÷ ln 2.0833 = 48 × 0.6931 ÷ 0.7340 = 45.3 hours (1.89 days). The rise over the 48 hours is 2.0833 − 1 = 108.3%, which is 2.0833½ − 1 = 44.3% per day.
The common mistake: dividing the rise by the days. Tests 3 days (72 hours) apart going from 500 to 1,100 mIU/mL rose 120%. Taking two-thirds of that, 80%, as the 48-hour rise overstates it, because the rise compounds: the true figure is 2.248/72 − 1 = 69.2%, with a doubling time of 72 × 0.6931 ÷ ln 2.2 = 63.3 hours.
Three results. 85, 170 and 330 mIU/mL at 0, 48 and 96 hours: the first interval doubles in 48.0 hours and the second in 50.2 hours. A straight line through the logarithms of all three gives a doubling time of 49.1 hours and a 97.0% rise per 48 hours.
Show Work
Formulas
How the “Minimum Rise” Has Changed
In 1981 Kadar, Caldwell and Romero (Obstetrics & Gynecology) proposed comparing two hCG samples drawn 48 hours apart and suggested a rise of 66% or more. They noted that about 15% of normal pregnancies would fall below that figure.
Later studies of women with pain or bleeding and an early pregnancy whose location was not yet clear found that viable pregnancies can rise more slowly. Barnhart and colleagues (2004) put the slowest rise at 53% over 2 days; Seeber and colleagues (2006) proposed 35% as a cautious minimum, to avoid ending a wanted pregnancy; and in 2016 Barnhart’s group showed that the minimum depends on the starting level: 49% for a first value under 1,500 mIU/mL, 40% for 1,500–3,000 and 33% above 3,000. Each step made the figure slower, which is why a single slow rise is read together with ultrasound and repeat tests rather than on its own.
About This Calculator
This calculator works out the doubling or halving time of hCG and the change over 48 and 24 hours from two results, or from a best-fit line through up to 12 results, and draws them on a chart. It quotes the published figures above for context; it does not interpret your results, and a slower rise doesn’t by itself mean anything is wrong.
Your results stay in your browser; nothing is sent anywhere.
For general information only — not medical advice. Talk to a doctor or other qualified professional about your own health.
Related tools: Period Calculator, Pregnancy Weight Gain Calculator, and Corrected Age Calculator.
Frequently Asked Questions
How is the hCG doubling time calculated?
Assuming a steady rise between the tests, doubling time = hours between tests × ln 2 ÷ ln(second value ÷ first value). For 120 then 250 mIU/mL 48 hours apart: 48 × 0.6931 ÷ ln(2.0833) = 48 × 0.6931 ÷ 0.7340 = 45.3 hours, about 1.89 days.
What is the rise over 48 hours?
The percentage increase the level would make in 48 hours at the same steady rate: (second ÷ first)48 ÷ hours − 1. Tests 72 hours apart going from 500 to 1,100 mIU/mL rose 120% in 3 days, which is a 69.2% rise per 48 hours and a doubling time of 63.3 hours.
Is there a minimum rise that is considered normal?
Studies of women with pain or bleeding and an early pregnancy whose location was not yet clear give figures clinicians use alongside ultrasound. Barnhart and colleagues (2004) found 99% of pregnancies that proved viable rose by more than 53% in 2 days; Seeber and colleagues (2006) proposed 35% as a cautious minimum; Barnhart and colleagues (2016) found the figure depends on the first value: 49% under 1,500, 40% at 1,500–3,000 and 33% over 3,000 mIU/mL. A slower rise doesn’t by itself mean anything is wrong. Discuss your results with your clinician.
Why does hCG rise more slowly at higher levels?
The rate of rise slows as the level climbs, which is why the 2016 study found a 49% minimum 2-day rise for first values under 1,500 mIU/mL but 33% for values over 3,000. A rise from 4,000 to 5,600 mIU/mL in 48 hours is 40%, with a doubling time of 98.9 hours.
What if my hCG went down?
The calculator then gives the halving time and the fall. 900 then 500 mIU/mL 48 hours apart is a 44.4% fall, halving every 56.6 hours (2.36 days). A falling level can happen for several reasons, so it needs your clinician, who will look at it alongside your history and any scan.
How do I use the hCG Doubling Time Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Two tests 48 hours apart
120 then 250 mIU/mL: a doubling time of 45.3 hours and a 108.3% rise over the 48 hours.
Tests on different days
500 then 1,100 mIU/mL 3 days apart: a 69.2% rise per 48 hours, doubling every 63.3 hours.
Three or more tests
85, 170 and 330 mIU/mL at 0, 48 and 96 hours: the best-fit doubling time is 49.1 hours, a 97.0% rise per 48 hours.
Higher levels
4,000 then 5,600 mIU/mL over 48 hours is a 40.0% rise, an 18.3% rise per day.
Before an appointment
Having the doubling time and the 48-hour rise worked out (and the working) makes it easier to follow what your clinician explains.
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