How this hCG calculator works
The calculator uses only the two values and two times you enter. Here is the exact arithmetic behind every number it shows.
Interval (hours) = second sample time − first sample time
Change (%) = (hCG2 − hCG1) ÷ hCG1 × 100
Doubling time (hours) = interval × ln(2) ÷ ln(hCG2 ÷ hCG1)
Rise over 48 hours (%) = ((hCG2 ÷ hCG1)(48 ÷ interval) − 1) × 100
- Doubling time is shown only when the second value is higher than the first. If it is the same or lower, the page reports that there is no rise instead of returning a meaningless number.
- The 48-hour rise is projected from your real interval. A 30-hour gap and a 60-hour gap can then be compared on the same 48-hour scale. It is not calculated when the samples are less than an hour apart.
- Times are read as the clock times you enter. The two timestamps are subtracted directly, so a daylight-saving change cannot shift the interval by an hour.
- Units are not converted. Enter mIU/mL as printed on your report, and use results from the same laboratory wherever possible.
- No number is invented. Missing values, values that are not greater than zero, and a second sample dated before the first all produce an error message instead of a result.
What hCG does in early pregnancy
In early pregnancy hCG roughly doubles every 48 to 72 hours (ACOG Practice Bulletin 200, 2018). As the pregnancy advances, the rate of increase slows, so the same percentage rise is not expected at every stage.
Normal hCG values cover a very wide range at any given week. Two people at the same stage of pregnancy can have values that differ by a large factor, which is why a single number — however high or low — cannot tell you how a pregnancy is progressing or where it is located.
Different laboratories may use different assays, units or reference ranges. Results from two different laboratories should not be compared with each other as if they came from the same test; serial values are usually interpreted within one laboratory.
An ultrasound is the key test for confirming where a pregnancy is developing and how it is progressing. hCG values are read alongside imaging and your history, not instead of them.
What the 35% figure means — and what it does not
ACOG Practice Bulletin 200 (2018) describes a rise of at least 35% over 48 hours as the lower limit of what is expected in a normal early pregnancy. This page uses that single published figure and labels which side of it your projected rise falls on.
- At or above 35%. The page says your rise is at or above that lower limit. It does not say the pregnancy is healthy, correctly located, or certain to continue. A pair of values cannot show any of those things.
- Below 35%. The page says your rise is slower than that lower limit and that a doctor should assess you, usually with an ultrasound. It does not mean a miscarriage is happening.
- No rise. If the second value is the same or lower, the page says there is no rise and points you to a clinician. It does not interpret what that pattern means.
We do not use hCG to date a pregnancy, to diagnose a miscarriage, or to rule out an ectopic pregnancy. Those decisions need a clinician, your history and imaging.
Not medical advice. Early Pregnancy Tools is an educational calculator. It cannot diagnose a pregnancy, confirm that a pregnancy is progressing normally, or detect a problem. Always rely on your doctor, midwife or emergency service for medical decisions.
When to contact a clinician
Do not wait for a calculator result if you have symptoms that worry you. Contact your maternity service, doctor or emergency service straight away if you have:
- Pain in your lower abdomen, especially if it is one-sided or severe, or pain at the tip of your shoulder.
- Vaginal bleeding, particularly with pain or dizziness.
- Feeling faint, dizzy or unwell.
- Any symptom that frightens you, even if your hCG result looks reassuring on this page.
If your two results came from different laboratories, ask your doctor whether the same assay was used, because values from different methods may not be directly comparable.
Frequently asked questions
What is hCG doubling time?
Doubling time is the number of hours hCG would need to double if it kept rising at the same rate as it did between your two samples. This page calculates it as interval in hours multiplied by ln(2), divided by ln(second value divided by first value). It is only shown when the second value is higher than the first.
What rise is expected over 48 hours?
In early pregnancy hCG usually doubles roughly every 48 to 72 hours, and the rise slows as the pregnancy advances. ACOG Practice Bulletin 200 (2018) describes a rise of at least 35% over 48 hours as the lower limit expected in a normal early pregnancy. Normal values vary widely, so a result above or below that line is not a diagnosis.
Can a single hCG level tell me if my pregnancy is OK?
No. Normal hCG values cover a very wide range at any given week, so one number — however high or low — cannot show how a pregnancy is progressing or where it is located. An ultrasound is the key test for confirming the location and development of a pregnancy, and hCG values are interpreted alongside it.
Why is there no doubling time in my result?
The calculator only shows a doubling time when the second hCG value is higher than the first. If the second value is the same or lower, it reports that there is no rise rather than producing a misleading number. That pattern needs to be assessed by a doctor, usually with an ultrasound.
Sources and disclaimer
Primary sources used for the method and the threshold on this page:
This page is informational only and is not a medical device. It does not create a clinician–patient relationship and must not be used to make medical decisions. hCG results are interpreted by a clinician together with your history and an ultrasound.
Your values and dates are processed only in this browser tab. Nothing is uploaded, stored or shared.
Related tools
If you already have a due date, the due date calculator shows your current gestational age and the milestone dates ahead. The implantation calculator estimates when implantation may have happened after ovulation.