hCG Doubling Time Calculator
Estimate the doubling time and daily rise between two hCG blood test values.
What it does
Human chorionic gonadotropin, or hCG, is a hormone produced by the placenta in early pregnancy. A quantitative blood test reports beta hCG in mIU/mL. Clinicians sometimes compare two values drawn about 48 hours apart to see how quickly the level is changing.
The doubling time is a compact way to describe that pace. It answers a simple question: at the observed rate, how many hours would the level need to double? The answer assumes exponential growth between the two samples, which is a useful approximation over a short interval.
This tool also reports the percentage change between the draws and an average daily change. Those figures can be easier to compare when the gap between tests is not exactly 48 hours. Enter the actual hours between draws, not a rounded target.
A single pair of hCG values cannot confirm a healthy pregnancy or diagnose a loss. Levels vary with gestational age, lab method, assay, and individual factors. Ultrasound and clinical findings are part of the picture. Use the result to prepare questions for a clinician, not as a stand-alone answer.
How to use
- Enter the first hCG value in mIU/mL.
- Enter the second hCG value in the same unit.
- Enter the exact number of hours between the two blood draws.
- Read the doubling time first, then the percent change and daily change.
- Confirm that both samples came from the same lab and assay when possible.
- Discuss the trend with your clinician rather than acting on the number alone.
Formula
- hcg1
- First quantitative hCG value in mIU/mL.
- hcg2
- Second quantitative hCG value in mIU/mL.
- hours
- Exact time between the two draws.
- ln(2)
- Natural logarithm of 2, about 0.6931.
- doubling time
- Hours needed to double at the observed pace.
If the second value is equal to or lower than the first, a doubling time does not apply, so the tool reports no change or a fall. The daily change is an average and can look large when the time gap is short. Values from different labs or assays may not be directly comparable.
Worked example
Example: 120 to 260 mIU/mL in 48 hours
First hCG 120, second hCG 260, time between tests 48 hours.
- Ratio: 260 / 120 = 2.1667.
- Natural logs: ln(2.1667) = 0.7732 and ln(2) = 0.6931.
- Doubling time: 48 x 0.6931 / 0.7732 = 43.0 hours.
- Percent change: (2.1667 - 1) x 100 = +116.7 %.
- Daily change: (2.1667^(24/48) - 1) x 100 = +47.2 % per day.
- The tool shows 43.0 hours, +116.7 %, and +47.2 % per day.
Reference table
| First to second | Change | Doubling time | Daily change |
|---|---|---|---|
| 100 to 110 | +10.0 % | 349.1 hours | +4.9 % per day |
| 100 to 125 | +25.0 % | 149.1 hours | +11.8 % per day |
| 100 to 150 | +50.0 % | 82.1 hours | +22.5 % per day |
| 100 to 200 | +100.0 % | 48.0 hours | +41.4 % per day |
| 100 to 300 | +200.0 % | 30.3 hours | +73.2 % per day |
Limits
- Two values cannot show a full trend beyond the assumed exponential curve.
- Results from different labs, assays, or units may not be comparable.
- Normal hCG ranges vary with gestational age and individual factors.
- The tool does not diagnose ectopic pregnancy, miscarriage, or a viable pregnancy.
Common mistakes
- Entering days between draws where the field asks for hours.
- Comparing values from different labs or different assay platforms.
- Assuming a slower rise always means a nonviable pregnancy.
- Using the result without ultrasound or clinical context.
- Treating another person's hCG curve as a target for your own.
Try it yourself
Quick checks. Answers sit tucked under each question.
100 to 200 mIU/mL in 48 hours: what is the doubling time?
48.0 hours, a +100.0 % change, and +41.4 % per day.
500 to 750 mIU/mL in 48 hours: what is the doubling time?
82.1 hours, a +50.0 % change, and +22.5 % per day.
200 to 220 mIU/mL in 72 hours: what is the doubling time?
523.6 hours, a +10.0 % change, and +3.2 % per day.
Questions
What is a normal hCG doubling time?
There is no single normal value. Early rises are often faster and slow as levels climb, and the expected pattern varies by starting value and clinical setting.
Does a slow rise mean miscarriage?
Not by itself. A slow rise can have several causes, and clinicians use repeat tests, ultrasound, and symptoms together.
Why does the tool use hours instead of days?
Doubling time is easier to compare in hours, especially when the gap between draws is not exactly 48 hours.
Can I use urine test results?
No. This calculator needs quantitative blood hCG values. Home urine tests are qualitative and do not provide a number.
What if the second value is lower?
The tool reports that the level is not rising and does not calculate a doubling time. Contact your clinician for interpretation.
Should I repeat the test at 48 hours?
Follow your clinician's plan. The timing and number of repeats depend on symptoms, ultrasound findings, and the first value.