Ultrasound Calculators
Volumes, obstetric biometry and dating, Doppler indices, haemodynamics and safety indices
Educational reference, not a dating, growth or diagnostic report. Formulae differ between vendors and software packages: check which regression your machine implements before comparing numbers. Reference ranges are population-typical starting points, never a threshold for an individual patient. Obstetric outputs in particular must not drive management.
Obstetric dating
One input set · two datingsGestational age & EDD
Naegele: EDD = LMP + 280 d · Robinson–Fleming: GA(d) = 8.052·√CRL + 23.73
CRL dating is the standard for 10–84 mm (≈6–14 wk, ±5 days). Beyond 84 mm, date by head circumference. Naegele assumes a 28-day cycle; EDD is shifted by (cycle − 28).
By LMP Dating policy: ISUOG practice guidelines; ACOG Committee Opinion 700.
By CRL Robinson HP, Fleming JEE. A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol 1975;82:702–710. Dating policy: ISUOG practice guidelines; ACOG Committee Opinion 700.
Standalone calculators
One plate eachEllipsoid volume & PSA density
V = 0.52 × L × W × H · PSAD = PSA ÷ V
0.52 ≈ π/6, the prolate-ellipsoid coefficient. Same method for bladder, ovary, prostate and thyroid lobe. PSA density above 0.15 ng/mL/cc is a common biopsy-decision threshold, not a diagnosis.
PSA density PI-RADS v2.1.
Estimated fetal weight (Hadlock IV)
log₁₀EFW = 1.3596 + 0.0064·HC + 0.0424·AC + 0.174·FL + 0.00061·BPD·AC − 0.00386·AC·FL
All four inputs are in centimetres. If your report gives BPD/FL in millimetres, divide by 10. Hadlock 1985 four-parameter regression. Random error ≈ ±15% (1 SD ≈ 8–10%); accuracy falls at weight extremes.
EFW Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol 1985;151:333–337.
Doppler indices
RI = (PSV−EDV)/PSV · PI = (PSV−EDV)/TAMV · S/D = PSV/EDV
RI (Pourcelot) and PI (Gosling) are angle-independent. PI uses time-averaged maximum velocity. Reversed end-diastolic flow makes RI > 1 and S/D negative; interpret against gestation-specific ranges.
Doppler indices Gosling RG, King DH (PI); Pourcelot L (RI). ISUOG practice guidelines: use of Doppler in obstetrics.
Volume flow
Q = TAMV × π(d/2)² × 60
Uses the time-averaged mean velocity, not TAMX. Diameter is squared, so a 10% diameter error becomes a 21% flow error. Angle correction must be ≤ 60°.
Formula Volume flow = time-averaged mean velocity × cross-sectional area.
Doppler velocity & angle correction
V = (Δf · c) ÷ (2 · f₀ · cosθ)
Speed of sound taken as 1540 m/s. Above 60° the cosθ term makes velocity error rise steeply: at 80° a 5° angle error changes velocity by roughly 30%.
Formula The Doppler equation rearranged for velocity; speed of sound 1540 m/s.
Simplified Bernoulli
ΔP = 4V²
Bernoulli ignores the proximal velocity term, so it overestimates when V₁ exceeds ~1.5 m/s.
Formula Simplified Bernoulli relation; the proximal velocity term is omitted.
Mechanical index
MI = P₋ ÷ √f
The FDA output limit is MI 1.9; ALARA still applies below it, and obstetric scanning should keep MI and TI as low as practicable.
Output indices AIUM/NEMA Output Display Standard; BMUS safety guidelines.
Ankle–brachial index
ABI = higher ankle ÷ higher brachial
ABI 1.0–1.4 is typical; above 1.4 suggests incompressible calcified vessels rather than health.
Formula Higher ankle systolic pressure ÷ higher brachial systolic pressure.
Mean sac diameter
MSD = (L+W+H) ÷ 3
MSD ≥ 25 mm with no embryo is diagnostic of failed pregnancy (SRU 2013); a single measurement, made by an experienced operator.
Failed early pregnancy Doubilet PM et al. Diagnostic criteria for nonviable pregnancy early in the first trimester. N Engl J Med 2013;369:1443–1451 (SRU multispecialty panel).
Sources
EFW: Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol 1985;151:333–337.
CRL dating: Robinson HP, Fleming JEE. A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol 1975;82:702–710. Dating policy: ISUOG practice guidelines; ACOG Committee Opinion 700.
Doppler indices: Gosling RG, King DH (PI); Pourcelot L (RI). ISUOG practice guidelines: use of Doppler in obstetrics.
Failed early pregnancy: Doubilet PM et al. Diagnostic criteria for nonviable pregnancy early in the first trimester. N Engl J Med 2013;369:1443–1451 (SRU multispecialty panel).
Output indices: AIUM/NEMA Output Display Standard; BMUS safety guidelines. PSA density: PI-RADS v2.1.
Educational content only. Not medical advice, and not a substitute for a reported study.