hCG doubling time calculator: two values, one piece of arithmetic

Doubling time between 100 and 200 mIU/mL, 48 hours apartworked example

48.0 hours

Change between the two values: +100%
Interval used: 48 hours
Slowest rise recorded over 48 hours: +53%Barnhart 2004, in women with a viable intrauterine pregnancy

That is the arithmetic between the two numbers you entered, and it is all this page will tell you. A doubling time is not a prognosis. Two hCG values are read alongside your history, your dates and any ultrasound findings, by somebody who can see all three - which is why the next section is a list of questions rather than a conclusion.

First value (mIU/mL)The earlier blood draw.Second value (mIU/mL)Ideally from the same laboratory as the first.Hours apartThe actual gap, not the scheduled one, if they differ.

The identity
$t_d = \frac{\Delta t \cdot \ln 2}{\ln (h_2 / h_1)}$
doubling time = (hours x ln 2) / ln(second / first)
The ratio
200 / 100 = 2
With your numbers
(48 x 0.6931) / ln(2) = 48.0 hours
Percentage change
(200 - 100) / 100 = 100%

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What to ask your clinician

These are the questions the arithmetic above raises but cannot answer. Take the actual lab reports with you if you can.

Were both values run by the same laboratory, and on the same assay?
Does the interval between the two draws support comparing them?
What would you like to see next, and when?
What should make me call before that?
Does an ultrasound change what these numbers can tell us at this stage?

What the research actually says about the rate of rise
Interval	Slowest rise recorded	Source
1 day	+24%	Barnhart 2004
2 days	+53%	Barnhart 2004
2 days (older teaching)	+66%	superseded by the above

Barnhart and colleagues followed 287 women with pain or bleeding and a non-diagnostic ultrasound who went on to have a viable intrauterine pregnancy. The slowest rise in that group was much slower than the older 66-percent rule, and the authors concluded that intervention should be more conservative, not less. These are floors observed in one population, not a test anyone passes or fails.

The sources this defers to

ACOG: Early Pregnancy Loss (patient FAQ) — how hCG blood tests and ultrasound are used together, and what happens next
NHS: miscarriage — what tests are offered, and how to reach an early pregnancy unit

Related, and also ad-free: the implantation window, which is where the hormone this measures starts rising. Every formula on this page is listed with its full citation on the methodology page.

This works out the doubling time between two hCG results and the percentage change between them, using the interval you enter. It shows the arithmetic and the slowest rise Barnhart and colleagues recorded in 2004 for a viable intrauterine pregnancy. It does not interpret your result. This page carries no ads.

hCG doubling timeconvention — doubling time = (hours between draws x ln 2) / ln(second value / first value). Verified 2026-07-30 against Barnhart et al., Obstet Gynecol 2004 (The identity assumes a constant rate of change between the two draws. Real hCG rises more slowly as the level gets higher, so a doubling time measured at 1,200 mIU/mL is not comparable to one measured at 120. Assay-to-assay variation also means two values should ideally come from the same laboratory.)
Slowest hCG rise seen with a viable intrauterine pregnancy — minimum rise: 24% at 1 day, 53% at 2 days. Verified 2026-07-30 against Barnhart et al., Obstet Gynecol 2004 (Derived from 287 symptomatic women (pain or bleeding, non-diagnostic ultrasound) who went on to have a viable intrauterine pregnancy. It is the slowest rise observed in that group, not a threshold anyone passes or fails, and it does not apply in reverse.)

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

No ads on this page, by design.

Frequently asked questions

How do you calculate hCG doubling time?

Doubling time equals the hours between the two draws multiplied by the natural log of 2, divided by the natural log of the second value divided by the first. Two values 48 hours apart that went from 100 to 200 give a doubling time of exactly 48 hours.

What rise did the research actually find?

Barnhart and colleagues studied 287 women with pain or bleeding and a non-diagnostic ultrasound who went on to have a viable intrauterine pregnancy. The slowest rise in that group was 24 percent at one day and 53 percent at two days - much slower than the older 66-percent-in-48-hours teaching.

Can this calculator tell me whether my pregnancy is viable?

No, and it will not try. It computes a doubling time and compares it with a published minimum. Interpreting two hCG values needs your history, your ultrasound findings and the same laboratory that ran them, all of which your clinician has and this page does not.

Why does the doubling time change as the numbers get bigger?

Because hCG does not rise at a constant rate. The rise slows as the level climbs, so a doubling time measured at 1,200 is not comparable with one measured at 120. The arithmetic here assumes a constant rate between your two draws, which is why it is only ever a description of that gap.

What should I ask my clinician?

Useful questions: were both values run by the same laboratory, does the interval between them support a comparison, what would you like to see next and when, and what should make me call before then. The tool prints these under the result so you can take them with you.

What if my second value went down?

The tool will show you the arithmetic of that - the percentage change and, if the level fell, the halving time - and then stop. What a falling level means depends on things a calculator cannot see. Please talk to your clinician, and if you are bleeding heavily or in pain, do so urgently.

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The next number

No body metric means much on its own. Here is what this one feeds into.

Implantation calculatorWhere the hormone this measures starts rising - also ad-free.Due date calculatorThe dating convention, once there is a date to work from.

Related calculators

Implantation calculatorEarly pregnancy Due date calculatorPregnancy dating Conception date calculatorEarly pregnancy

Every formula on this page is listed with its source on the methodology page, and changes to any reference value are logged in the data changelog. Canonical URL: https://myhealthmywellness.com/hcg-doubling-calculator