HCG Calculator
Use this hCG calculator to check whether the rise between two hCG test results meets the minimum seen in early viable pregnancies.
Updated October 22, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How HCG Doubling Time Is Calculated
hCG rises predictably in early pregnancy, which is why a single value means less than a trend between two values taken days apart. The expected pace of that rise has been mapped since the early 1980s and remains one of the most useful early signals available before ultrasound can confirm anything directly¹.
What This Calculator Measures
This turns your two hCG values and the days between them into a 48-hour rise and a doubling time. It then compares the rise with the slowest rise seen in early pregnancies that turned out to be viable. It tells you whether your rise meets that minimum, not whether a pregnancy is viable.
The Formula
The tool works out two numbers from your values and the time between tests:
48-hour rise (%) = ((second value ÷ first value) ^ (48 ÷ hours between tests) − 1) × 100
Doubling time (hours) = hours between tests × ln(2) ÷ ln(second value ÷ first value)
The tool counts the hours between your two test dates, so tests two days apart count as 48 hours, and it shows the rise as a whole percent and the doubling time to one decimal place. If the second value is not higher than the first, it shows "Not rising" instead of a doubling time.
In a 2016 study of 285 women with early pain or bleeding whose pregnancies turned out to be viable, the slowest normal 2-day rise (the 1st percentile) depended on the first hCG value⁷:
First hCG value (mIU/mL) | Minimum 2-day rise |
Under 1,500 | 49% |
1,500 to 3,000 | 40% |
Over 3,000 | 33% |
Older rules of thumb, such as doubling every 48 to 72 hours, describe a typical rise rather than the minimum, and the rise slows as hCG climbs¹ ².
How This Calculator Computes Your Result
Enter both hCG values and their test dates. The tool scales your rise to 48 hours, so tests one or three days apart can be compared with the same 2-day benchmark, then checks it against the minimum for your first value. If the second value is not higher than the first, it shows "Not rising". Above 3,000 mIU/mL it also notes that an ultrasound usually tells you more than another blood test³.
Why Doubling Time Is Only Half the Picture Once Levels Rise Further
Doubling time is most informative very early on, but it isn't the only tool used to track early pregnancy. Once hCG reaches a certain level, typically somewhere between about 1,500 and 3,500 mIU/mL depending on the specific ultrasound equipment and protocol used, a gestational sac should become visible on transvaginal ultrasound, this is known as the discriminatory zone³.
In a 2013 study of early viable pregnancies, a gestational sac was first seen at an hCG of 390 mIU/mL and was seen 99% of the time by 3,510 mIU/mL⁸. A 2023 study found a similar 99% level of 3,994 mIU/mL³.
In practice, this means doubling time and ultrasound aren't competing methods, they're used together at different stages, doubling time matters most before a sac would be expected to be visible, and ultrasound takes over as the more direct check once hCG has climbed into that range.
A Note on Accuracy
A slower-than-expected rise does not necessarily mean a nonviable pregnancy, and a normal doubling time does not guarantee a viable one. hCG trends are one data point interpreted alongside ultrasound and clinical evaluation, never used alone to conclude anything about pregnancy outcome. Still, a slow rise or a falling level can be a sign of miscarriage or of an ectopic pregnancy (a pregnancy growing outside the uterus), which can cause life-threatening internal bleeding¹ ⁴ ⁵. Get emergency care now if you have sudden, severe pain in your belly or pelvis, shoulder pain, or feel very dizzy, weak or faint⁴ ⁶. Do not wait for your next blood test. Call your doctor right away if you have heavy bleeding, or pain or cramping low down on one side⁴ ⁵ ⁶.
Frequently Asked Questions
What does this calculator actually tell me?
It works out how much your hCG rose over 48 hours and its doubling time, then shows whether the rise meets the minimum seen in early pregnancies that turned out to be viable⁷. It is not a viability verdict.
Why does the minimum rise depend on the hCG level?
hCG rises fastest when levels are low and more slowly as they climb. That is why the minimum normal 2-day rise is 49% for a first value under 1,500 mIU/mL but 33% above 3,000⁷.
What happens once hCG is already high?
Beyond roughly 6 weeks gestation, or once levels climb into the 1,500 to 3,500 mIU/mL range, ultrasound becomes the more direct way to check on the pregnancy, since a gestational sac is usually visible by then, and almost always by about 3,500 to 4,000 mIU/mL³ ⁸. The calculator still checks your rise against the 33% minimum for values above 3,000, but an ultrasound is usually more useful at that point.
Does a slow rise mean the pregnancy isn't viable?
No. A slower-than-expected rise does not necessarily mean a nonviable pregnancy, and hCG trends should always be interpreted alongside ultrasound and clinical evaluation, never used alone.
Sources
A Method of Screening for Ectopic Pregnancy and Its Indications, Kadar, Caldwell & Romero, Obstetrics & Gynecology, 1981
Symptomatic Patients with an Early Viable Intrauterine Pregnancy: HCG Curves Redefined, Barnhart et al., Obstetrics & Gynecology, 2004
Association of HCG Level with Ultrasound Visualization of the Gestational Sac in Early Viable Pregnancies, Park et al., Reproductive Sciences, 2023
Ectopic Pregnancy, American College of Obstetricians and Gynecologists
Early Pregnancy Loss, American College of Obstetricians and Gynecologists
Ectopic Pregnancy, NHS
Differences in Serum Human Chorionic Gonadotropin Rise in Early Pregnancy by Race and Value at Presentation, Barnhart et al., Obstetrics & Gynecology, 2016
Reevaluation of Discriminatory and Threshold Levels for Serum β-hCG in Early Pregnancy, Connolly et al., Obstetrics & Gynecology, 2013