Work out beta-hCG doubling time from your blood tests
This hCG calculator takes two or three quantitative blood (serum) beta-hCG results and the time between them, and works out the doubling time — how long the level takes to double at the rate it changed — and the percentage rise per 24 and 48 hours. If the level fell, it shows the halving time instead.
People often have serial hCG tests in early pregnancy, after IVF or another fertility treatment, or when a scan is not yet clear. The numbers are easier to talk through with your clinic when you know how to read them. Everything stays in your browser.
This page does the arithmetic only. It does not tell you whether a pregnancy is healthy, and it is not a substitute for your doctor, midwife or fertility team.
How to use the hCG calculator
- Enter your first and second hCG results in mIU/mL (the same as IU/L).
- Enter how long after the first test the second was taken, in hours or days.
- If you have a third result, add it with its time after the first test; the calculator then fits all three.
- Read the doubling time, the rise per 24 and 48 hours, the chart and the interval table, and copy the result for your notes.
hCG Doubling Time Calculator features
Two or three tests
Each interval separately, and an overall rate from all three results.
Doubling or halving
Rising levels give a doubling time; falling levels a halving time.
Rise per 48 hours
The percentage change scaled to 24 and 48 hours, the way clinics usually describe it.
Log-scale chart
Steady exponential change shows as a straight line.
The formula and a worked example
hCG rises roughly exponentially in early pregnancy, so the calculator uses doubling time = t × ln 2 ÷ ln(hCG₂ ÷ hCG₁), where t is the time between the tests. The change over 48 hours is (hCG₂ ÷ hCG₁)48 ÷ t − 1.
Example: 120 mIU/mL, then 250 mIU/mL 48 hours later. ln(250 ÷ 120) = 0.734, so the doubling time is 48 × 0.693 ÷ 0.734 = 45.3 hours, and the level rose by 108% in 48 hours. With three results the calculator fits a straight line to the logarithms of all three, which smooths out a single unusual reading.
Published figures for context (not applied to your results)
These come from studies of people with symptoms in early pregnancy whose pregnancies turned out to be ongoing intrauterine pregnancies. They describe the slowest rises seen in those groups, not a target, and clinicians use them alongside ultrasound and symptoms — never alone.
| Starting hCG | Slowest 2-day rise seen | Study |
|---|---|---|
| Under 1,500 mIU/mL | 49% | Barnhart et al., Obstet Gynecol 2016;128:504–511 |
| 1,500–3,000 mIU/mL | 40% | Barnhart et al., 2016 |
| Over 3,000 mIU/mL | 33% | Barnhart et al., 2016 |
| All levels | 53% (24% in 1 day) | Barnhart et al., Obstet Gynecol 2004;104:50–55 |
Why the numbers need a clinician
- The rise slows as the level climbs and as pregnancy progresses, so one doubling time cannot be judged on its own.
- Twin pregnancies, IVF transfers and laboratory differences change the picture; use results from the same lab where possible.
- A single level says little; the trend together with an ultrasound scan is what clinicians use.
- This calculator does not work out hCG injection doses for fertility treatment — those come from your clinic’s prescription.
- To estimate dates, try the due date calculator or the conception calculator.
hCG Doubling Time Calculator FAQ
How do I calculate hCG doubling time?
Multiply the hours between tests by 0.693 (ln 2) and divide by the natural log of the second result divided by the first. The hCG calculator above does this for you.
Can I use the hCG calculator with 3 tests?
Yes. Add the third result and its time after the first test. You get each interval’s doubling time plus an overall rate fitted to all three.
Does it work for IVF beta-hCG results?
The arithmetic is the same for any two serum results. Your clinic will interpret IVF betas against their own experience and the transfer date.
What units does hCG use?
Labs report mIU/mL or IU/L, which are the same number. Use the same unit for every test.
My hCG is not doubling in 48 hours — is something wrong?
Not necessarily. Many healthy pregnancies rise more slowly than doubling every two days, especially at higher levels. Only your clinician, usually with a scan, can say what your results mean.
Is this hCG calculator a diagnosis?
No. It is a calculation for information only and cannot tell you whether a pregnancy is viable. Contact your doctor or clinic about any concern.