free hCG calculator
hCG Doubling-Time Calculator
Calculate the percentage change and estimated doubling time between two quantitative beta-hCG blood results, with a plain explanation of what the numbers can and cannot tell you.
What hCG is
Human chorionic gonadotropin (hCG) is a hormone made by the developing placenta. It is what pregnancy tests detect. A home urine test only tells you whether hCG is present above a threshold; a quantitative blood test measures the actual amount, which is why only blood results can be used in this calculator.
Clinicians sometimes order two blood tests about 48 hours apart in early pregnancy, for example after fertility treatment, after bleeding or pain, or when an ultrasound is too early to show a pregnancy. Comparing the two results shows whether the level is rising, staying the same or falling.
How the calculation works
hCG rises roughly exponentially in early pregnancy, so the calculator uses the standard exponential formula:
Doubling time = hours between tests × ln(2) ÷ ln(second result ÷ first result)
It also shows the simple percentage change: (second − first) ÷ first × 100. For example, a rise from 120 to 250 mIU/mL over 48 hours is a 108% increase, with an estimated doubling time of about 45 hours. If the second result is lower than the first, the calculator reports the percentage decrease instead of a doubling time.
Why the number is only part of the picture
- The rate of rise is not the same for everyone and slows down as levels get higher later in the first trimester.
- Different laboratories and machines can give slightly different values, so results from the same lab are easier to compare.
- The exact time between blood draws matters: 40 hours and 56 hours give different doubling times from the same two numbers.
- Twins, IVF timing and how many days after conception the first test was taken all change what is expected.
- From around the time a pregnancy can be seen on ultrasound, the scan usually tells your care team more than further hCG tests.
That is why a doctor never judges a pregnancy on the doubling time alone. They combine it with how many weeks you are, your symptoms and ultrasound findings.
When not to wait for a second test
Common questions
What is an hCG doubling time?
It is the estimated time it would take for your hCG level to double, worked out from two quantitative blood results and the number of hours between them.
Does hCG always double every 48 hours?
No. The rise is usually fastest in the first weeks and slows as levels get higher, and healthy pregnancies vary. A slower or faster change is not a diagnosis on its own; your clinician interprets it together with your symptoms and, when possible, an ultrasound.
Can I use home pregnancy test lines to calculate doubling time?
No. Home urine tests are designed to say yes or no. Line darkness depends on how concentrated your urine is and on the test brand, so it cannot be converted into an hCG number.
Can this calculator confirm a healthy pregnancy?
No. It cannot tell whether a pregnancy is developing normally or where it is located. Only your care team can assess that.
Why is my hCG going down?
A falling level can have several explanations, including a pregnancy that is not continuing. Contact the clinic or doctor who ordered the tests so they can explain your results and next steps.
Sources & editorial process
- MedlinePlus — Pregnancy test (hCG)
- NHS — Finding out you are pregnant
- NHS — Ectopic pregnancy
- ACOG — Ectopic pregnancy
- ACOG — Early pregnancy loss
Written by the Elara editorial team from the sources above. This page has not been reviewed by an independent clinician. Read how we write and update health pages. Last updated .
Medical disclaimer: this page is general education, not medical advice, diagnosis or treatment. If you are worried about yourself or your baby, contact your midwife, doctor or maternity unit straight away. In an emergency call your local emergency number.