hCG Doubling Time Calculator (with Chart)

Updated Oct 2025 · Written by

Enter two hCG values and their dates to see your doubling time and rate. Compare your rise to typical ranges and view a chart you can save.

hCG draws
Draw 1
Draw 2
Enter at least two draws with date/time and value. Sample data is prefilled so you can test immediately.
Projection (optional)
If left empty, we will project to 48 hours after your latest draw.
Methodology and sources
Educational only, not medical advice: this explains how the tool estimates doubling time, how “typical” bands are chosen by gestational age, and how projection and charting work. Follow your clinician’s plan.

How the doubling time is calculated

  • Sort and clean: entered draws are sorted by date and time. Only rows with a valid date/time and a positive hCG value are used.
  • Log-rise model: early hCG change is treated as exponential. The calculator fits a line to ln(hCG) versus hours since your first draw.
  • Two draws: slope k equals log change divided by the time gap. Doubling time = ln(2) ÷ k hours.
  • Three or more draws: a least-squares line is fit to all points, then doubling time is ln(2) ÷ k.
  • Declining or flat: if k ≤ 0 the tool shows a halving time when k < 0, or “no rise” when the slope is zero within rounding.

“Typical” rise bands by stage

  • No GA given: a general early-pregnancy band of about 48–72 h is used for comparison.
  • With GA context: when gestational age is available the comparison band widens with advancing GA, reflecting slower rises at higher hCG:
    • ≤ ~6 weeks (≤ 42 days): 48–72 h
    • ~6–7 weeks (≤ 49 days): 72–96 h
    • > ~7 weeks: 96–144 h
  • Badge: the headline badge reads Declining when k ≤ 0, otherwise classifies as Faster than typical, Within typical, or Slower than typical against the stage-matched band.

Projection and chart

  • Projection: the chosen “Project to” date/time is predicted from the fitted slope using the last measured value as the start. If empty, projection defaults to +48 h after the last draw.
  • Chart: the Y-axis is logarithmic (IU/L). The X-axis is hours since the first draw. A shaded envelope shows the stage-appropriate “typical” rise from the first point.
  • Progress bar: the bar maps doubling times from about 24–144 h to a 0–100% fill to give quick visual context.

Data entry tips

  • Use the exact report date/time for each draw and the same laboratory when possible.
  • Intervals of 24–48 hours are most informative. Very short or very long gaps are harder to interpret.
  • Later in the first trimester, rises slow naturally; a longer doubling time can still be normal.

Limitations

  • Serial hCG trends should be interpreted with symptoms and, when appropriate, ultrasound.
  • Assay variability and biological variation can shift values between labs and over time.
  • Atypical patterns can occur with ectopic pregnancy, miscarriage, or trophoblastic disease; this calculator does not diagnose these conditions.
Key sources

How to use this calculator

  1. Enter your draws: type the exact date/time and hCG value for at least two draws. Use the same lab if possible.
  2. Add more points (optional): click “Add another draw” to include extra results. Wider gaps of 24–48 hours are most useful.
  3. Set a projection (optional): choose a future date/time or leave blank to project +48 h from the last draw.
  4. Calculate: press Calculate to see the doubling time, a quick context badge, the projection value, and a chart.

How to read the chart

  • X-axis: hours since the first draw.
  • Y-axis: hCG (IU/L) on a log scale, so equal vertical steps mean equal percent change.
  • Shaded band: a stage-appropriate “typical” rise envelope used for comparison.
  • Dark points and pink line: your measured draws and the fitted trend.
  • Projection point: the predicted hCG at the chosen time. The result notes show the projected value and gestational age context when available.
  • Badge and bar: classifies the rise as Faster than typical, Within typical, Slower than typical, or Declining. The bar fills more with shorter doubling times.

Educational tool only. It does not diagnose any condition and does not replace a clinician review.

Last updated: September 2025

hCG calculator: FAQs

How is hCG doubling time calculated?

The tool fits an exponential trend to your results. With two points it uses the exact slope. With three or more it uses a least squares line on ln(hCG) vs time. If levels fall, it shows a halving time.

What is a typical doubling time in early pregnancy?

About 48–72 h before ~6 weeks. About 72–96 h around 6–7 weeks. It can be 96–144 h after ~7 weeks. Ultrasound provides the definitive assessment.

How many draws do I need?

At least two. A third draw can smooth noise and improve the fit. Gaps of 24–48 h are the most informative.

Does a slow rise always mean something is wrong?

No. hCG is one piece of information. Some viable pregnancies rise slower. Some non-viable cases can rise faster. Trends are interpreted with timing and ultrasound.

Can I compare results from different labs or times of day?

Use the same lab when you can. Assays differ. Time-of-day effects are small compared with multi-day change, but consistency helps.

How accurate is the projection to a future date?

It assumes your recent rate continues. Biology varies. Treat the projection as a guide and follow your clinician’s plan.

Why does the chart use a log scale?

hCG changes by percentages, not constant amounts. A log Y-axis turns equal percent changes into equal steps, so slopes reflect rate cleanly over wide ranges.