What is the hCG Doubling Time Calculator?
In early pregnancy, the hormone human chorionic gonadotropin (hCG) typically rises rapidly. This calculator takes two beta hCG blood test results, along with the time of each draw, and works out the doubling time in hours, the total percent change, and the rate of rise scaled to a full day. It is an educational tool and is not a substitute for medical advice from your doctor.
How to use it
Enter your first hCG value and the hour it was measured (you can leave this as 0), then enter your second hCG value and the hour of that draw. The calculator returns how many hours it takes the level to double and how fast it is rising.
The formula explained
Doubling time uses exponential growth: $$t_d = (t_2 - t_1) \cdot \frac{\ln 2}{\ln(\text{hCG}_2 / \text{hCG}_1)}$$ The total change is simply \((\text{hCG}_2/\text{hCG}_1 - 1) \times 100\). The rate of rise per day takes that total change and scales it to 24 hours by multiplying by \(24 / (t_2 - t_1)\).
Worked example
Suppose hCG goes from 100 to 200 over 48 hours. The ratio is 2, so doubling time $$= 48 \times \frac{\ln 2}{\ln 2} = 48 \text{ hours.}$$ The total change is \((2 - 1) \times 100 = 100\%\). The rate of rise per day $$= 100\% \times \left( \frac{24}{48} \right) = 50\% \text{ per day.}$$
FAQ
What is a normal doubling time? In early pregnancy hCG often doubles every 48 to 72 hours, though this varies and slows as levels rise.
Should hCG always double? A slower rise can still be normal, especially at higher levels. Always discuss results with your healthcare provider.
Why might I get no result? The two hCG values must be positive and different, and the two times must differ, or the math is undefined.
