hCG Doubling Time Calculator
In early pregnancy, beta hCG typically rises in an exponential pattern. This calculator takes two blood test values and the hours between them, then computes the doubling time and the percentage rise using the standard exponential growth formula. It is an educational arithmetic tool only. A doubling time on its own does not confirm or rule out any outcome; serial hCG results must be interpreted by your healthcare provider alongside ultrasound and your clinical picture.
Doubling time formula
Doubling time (hours) = (hours * ln 2) / ln(second / first)
Percentage rise = (second / first - 1) * 100
Daily growth rate = (second / first)^(24 / hours) - 1
This assumes pure exponential growth between the two measurements. The doubling time is only meaningful when the second value is higher than the first; a flat or falling level has no positive doubling time.
Worked example
First value 500, second value 1,000, 48 hours apart:
- Ratio = 1,000 / 500 = 2; ln(2) = 0.693147.
- Doubling time = (48 * 0.693147) / 0.693147 = 48.00 hours (2.00 days).
- Percentage rise = (2 - 1) * 100 = 100.00 percent.
hCG doubling time: frequently asked questions
How is hCG doubling time calculated?
Beta hCG is assumed to grow exponentially in early pregnancy. The doubling time is the number of hours for the level to double, computed as elapsed hours times the natural log of 2, divided by the natural log of the ratio of the second to the first value. The formula is doubling time = (hours * ln 2) / ln(second / first).
What doubling time is considered reassuring in early pregnancy?
In early intrauterine pregnancy, beta hCG commonly rises so that it roughly doubles every two to three days, though the normal rate slows as levels get higher. A single doubling time outside a textbook range does not by itself confirm or rule out a healthy pregnancy. Only your clinician can interpret your results in context.
Is this calculator a diagnosis?
No. It is an educational arithmetic tool that applies the exponential growth formula to two numbers you enter. It does not diagnose ectopic pregnancy, miscarriage, or a viable pregnancy. Always discuss serial hCG results with your healthcare provider, who considers ultrasound and your full history.
What does the percentage rise mean?
The percentage rise is how much the second value exceeds the first, as a percentage: (second / first - 1) times 100. Clinicians sometimes look at the rise over 48 hours rather than the doubling time, so the calculator reports both the doubling time and the raw percentage rise.
Official sources
- U.S. National Library of Medicine, MedlinePlus: hCG Test.
- U.S. National Library of Medicine, MedlinePlus: Pregnancy.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.