New to ObCalc? Every calculator updates live as you type — there is no Calculate button. Number fields open a built-in number pad that does arithmetic and converts units: tap the tinted kg ⇄ badge to enter a value in pounds, feet & inches, or any other supported unit. Details are in Controls; every tool is covered in the Calculator Reference.
Getting started
The first time you open ObCalc, the Welcome screen lists every calculator.
Browse the list, type in Search tools, or tap a collection chip (All, Obstetrics, Perinatology / MFM, General, Thrombophilia, Genetics) to narrow it down.
Tap the calculators you use most — a check circle marks each selection.
Tap Get Started. It shows how many tools you picked and needs at least one.
Your picks become your library on the My Tools home screen. You can change it any time.
Managing your library
To…
Do this
Open a calculator
Tap its row on My Tools.
Filter by collection
Tap a chip at the top of the list. All shows everything; your last choice is remembered.
Add calculators
Tap + to open the Formulary, tap as many tools as you like, then tap Add (n). Tools already in your library are dimmed with a grey check.
Remove a calculator
Swipe its row left and tap Remove, or long-press it and choose Remove from Library.
Result card — the headline answer, with a colored interpretation pill and often a band (see Results).
Inputs — everything you enter.
Details — secondary results, derived values, and interpretation notes.
Disclaimer — how the tool should be used.
Toolbar icon
What it does
📖 Book
Opens References. Linked citations open in your browser.
↺ Reset
Clears every input back to its default.
⬆︎ Share
Maternal Sepsis Assistant only — exports a text summary.
Results recompute the moment an input changes. Until every required input is entered, results show “—”.
Controls
Number fields and the number pad
A number field is a row with a label on the left and its value on the right (a small shaded box when empty). Tap the value to open the ObCalc number pad; tap ⓧ beside a value to clear it.
CancelWeight
kg ⇄
ⓧ72.5
()⌫÷789×456−123+.0Done
Arithmetic built in. Use + − × ÷ and parentheses to calculate right in the field — the expression is evaluated when you tap Done. For example:
(128 + 131 + 126) ÷ 3 — average three measurements
2800 − 150 — adjust a weight
⌫ deletes one character; ⓧ in the display clears the entry.
Done evaluates, rounds to your Max Input Decimals, fills the field, and closes the pad. Cancel closes without changes.
Drag the pad upward to expand it to full height.
Built-in unit converter
The feature most people miss. When a field supports more than one unit, the unit badge at the top of the pad is tinted and shows ⇄. Tap it to split the pad into two cards:
lbs ⇄
ⓧ160
Source — type here
kg ⇄
72.57
Field’s unit — live result
Left card (source): type in the unit you have. Tap its badge to cycle through the other units — for weight: lbs → lbs + oz → g → oz…
Right card (target): shows the converted value in the field’s unit as you type.
Tap Done to insert the converted value. Tap the right card’s badge to go back to typing in the field’s own unit.
Arithmetic works here too — e.g. 150 + 12 in lbs.
Field type
Units you can enter
Adult / maternal weight
kg, lbs, lbs + oz, g, oz
Infant / fetal weight
g, lbs + oz, lbs, oz, kg
Height / length
cm, in, ft + in, ft, mm, m
Hemoglobin
g/dL, mg/dL
Urine protein / creatinine
mg/dL, g/dL
Doppler velocity
cm/s, m/s
Fields with a fixed unit — mEq/L, mmHg, %, and mm-based fetal biometry — show a plain grey badge with no ⇄. Every calculator computes in its own clinical base unit, and your entry is converted before the formula runs, so switching units never changes the result.
Feet & inches, pounds & ounces
When the unit is ft + in or lbs + oz, the pad shows two boxes, e.g. 5 ft7 in. Tap a box to choose which part you are typing. Feet and pounds take whole numbers; inches and ounces accept decimals. The field then reads 5′ 7″ or 7 lb 6 oz.
Steppers
Rows with − / + buttons step the value by a fixed amount (e.g. 0.1 cm, 1 year) within the calculator’s limits. Tap the value itself to open the number pad — with unit conversion where supported — and type an exact value.
Gestational-age wheels
Shown as a row like Gestational Age · 28 wk 3 days ›. Tap the row to reveal weeks and days (0–6) wheels; tap again to collapse. Some tools only need whole weeks and show just the weeks wheel. The weeks range depends on the calculator.
Segmented selectors
A row of two to four buttons — tap one to choose it. Scoring tools such as APGAR and Bishop start with nothing selected on purpose, so the score stays “—” until every row has a real answer. Longer lists appear as a drop-down menu instead.
Menu and sheet pickers
Menu: tap the value on the right to open a drop-down (e.g. cfDNA’s Chromosome Condition).
Sheet picker: tap the row to open a list; the current choice is checked, and tapping an option selects it and closes the sheet (e.g. Dating Method, Obstetric History). Pickers that start at Select… require an explicit choice.
Toggles
On/off switches for yes/no inputs. The grey hint under a label explains exactly what it covers.
Date pickers
Tap a date to open the calendar. An ⓘ beside a label explains what that date is for.
Results, pills, and bands
Result value — shown in your preferred units where relevant (e.g. weight gain in lbs).
Interpretation pill — a colored label such as Normal Weight or High Risk. Green is generally reassuring, orange borderline, red abnormal. These colors never follow your accent color.
Interpretation band — a colored bar of every range with a marker at your result and the cutoffs labelled beneath. Out-of-range values pin to the end of the bar.
Notes — tinted boxes in Details that interpret secondary results.
Settings
Tap the gear icon on My Tools.
Setting
Options
Effect
Appearance
System · Light · Dark
App theme; System follows your device.
Interface Accent Color
11 presets plus a custom color picker
Colors buttons, chips, and the number pad. Never changes result or interpretation colors.
Max Input Decimals
0 – 4 (default 2)
Most decimal places kept when you enter a value and shown in input fields.
Default Units
See below
Units fields display by default.
Acknowledgements
—
The clinicians and authors who helped build ObCalc. The list downloads when you're online and is then kept for offline use.
Default units
Category
Choices
Default
Applies to
Length (Height & Dimensions)
Centimeters, Inches, Feet & Inches, Meters
cm
Height and general length fields — BMI, weight gain, VBAC, CPAM dimensions, crown-heel length
Fetal Length (Biometry)
Millimeters, Centimeters, Inches
mm
Fetal ultrasound length fields that support unit conversion
Adult Weight
Kilograms, Pounds, Pounds & Ounces
kg
Maternal / adult body weight
Infant / Fetal Weight
Grams, Pounds & Ounces, Pounds, Ounces, Kilograms
g
Birth weight, twin weights
Your default only sets how a field displays. You can always enter one value in a different unit with the number pad’s converter.
Calculator reference
Each tool lists its inputs — with a chip showing which control is used (tap a chip to jump to how that control works) — and what it returns. For sources and per-tool disclaimers, see the Medical Disclosure.
Obstetrics
Bishop Score
Predicts likelihood of successful labor induction from the cervical exam.
Inputs
Cervical DilationSegmentedClosed · 1–2 cm · 3–4 cm · ≥ 5 cm
Total Bishop Score — ≤ 4 45–50% failure · 5–9 10% failure · ≥ 10 0% failure. Nothing is pre-selected; the score stays “—” until all five rows are chosen.
Estimated Due Date
EDD, key dates, screening windows, and milestones from any dating method.
Inputs
Dating MethodSheet pickerLast Menstrual Period · Ultrasound · Known Due Date · Conception Date · Ovulation Date · 3-Day Embryo Transfer · 5-Day Embryo Transfer
DateDateLabel follows the method — LMP Date, Ultrasound Date, Transfer Date, …
Measured Gestational AgeGA wheelsUltrasound method only (4–42 wk + days)
Results
Estimated Due Date and current gestational age. Pregnancy Dates — LMP, conception, ovulation. Prenatal Screening windows — first-trimester bloodwork (10w0d–13w6d), nuchal translucency (11w2d–14w2d), quad screen (15w0d–20w0d), anatomy scan (18w0d–20w0d), GDM OGTT (24w0d–28w0d), antepartum testing (32w0d–34w0d), GBS (35w0d–37w0d). Key Milestones — anti-D (28w0d), cervical length (after 16w0d), elective delivery (39w0d). Based on a standard 280-day gestation.
Pregnancy Re-dating
Compares GA by LMP vs. ultrasound using ACOG Committee Opinion 700 thresholds.
A Recommendation — keep the LMP-based EDD or re-date by ultrasound — with rationale, plus the discrepancy in days, the cutoff for the window, and the GA window by LMP. Re-date when the discrepancy exceeds 5 days (< 9 0/7 wk), 7 (9 0/7–15 6/7), 10 (16 0/7–21 6/7), 14 (22 0/7–27 6/7), or 21 (≥ 28 0/7).
Pregnancy Weight Gain
Tracks gestational weight gain against IOM (2009) targets by pre-pregnancy BMI.
Status — Below Recommended / On Track / Above Recommended; pre-pregnancy BMI and category; weight gained so far; expected gain to date, recommended total gain, and recommended weekly gain (2nd/3rd trimester), each as a low–high range. Weights display in your preferred adult-weight unit.
Urine Protein Creatinine Ratio (UPCR)
Screens for significant proteinuria from a spot urine sample.
Obstetric HistorySheet pickerNo previous vaginal delivery · Previous VD only before prior cesarean · Previous VBAC
Arrest Disorder IndicationTogglePrior cesarean for arrest of dilation or descent
Treated Chronic HypertensionToggleDiagnosed and pharmacologically treated before pregnancy
Results
Predicted VBAC success (%) on a likelihood gauge — Low < 60%, Moderate 60–89%, High ≥ 90%. BMI is shown for reference only (it is not a model input). Shows “—” until every required input is entered.
VBAC at Admission
The same prediction made at delivery admission — Grobman 2024 MFMU model.
Survival for actively treated infants with the hospital range (10th–90th percentile); survival and death for Actively Treated and All Infants; and neurodevelopmental outcomes among survivors — profound and moderate impairment, blindness, deafness, moderate cerebral palsy, cognitive impairment.
Fetal Biometry
Multi-panel biometry with z-scores, percentiles, Hadlock EFW, and long-bone soft-marker ratios.
Inputs
PanelSegmentedChip bar: EFW · Head · Thorax · Doppler · GI / Liver · Renal · Skeletal (scroll sideways for all)
EFW panelNumber padBPD, OFD, HC, AC, FL (mm). EFW formula: HC + AC + FL · BPD + AC + FL · BPD + HC + AC + FL — or turn on Enter EFW directly and type grams
Head panelNumber padBPD, OFD, HC, TCD, binocular, mandible
Percentile and z-score for every measurement (red below the 10th or above the 90th). On the EFW panel: an Estimated Fetal Weight card with percentile, and Proportionality — cephalic index (normal 70–86%), HC/AC, FL/expected FL (short femur < 0.92), HL/expected HL (short humerus < 0.90). Measurements carry across panels — AC entered on EFW also appears on Thorax.
IUT Transfusion
Packed-RBC volume and weight-based procedure medications for fetal intravascular transfusion.
Inputs
Estimated Fetal WeightNumber padg — warns outside 100–6000 g
Donor HctNumber padDefault 78% — verify against the unit label
Results
RBC transfusion volume (Mandelbrot: fetoplacental volume × ΔHct ÷ donor Hct), with the Giannina cross-check (EFW × ΔHct × 0.002) and a warning if they differ by more than 25%. Procedure medications dosed by EFW: paralysis/sedation/analgesia (vecuronium, rocuronium, midazolam, fentanyl), emergency/resuscitation (atropine, epinephrine, calcium chloride, sodium bicarbonate, amiodarone, adenosine), and local anesthetic (lidocaine, flagged for protocol verification). Rescan if the EFW is more than 7 days old.
MCA Doppler
Middle cerebral artery peak systolic velocity MoM to screen for fetal anemia.
Estimated rates (%) of respiratory distress syndrome, necrotizing enterocolitis, intraventricular hemorrhage, and sepsis, each with its own risk note — or Not Applicable beyond the model’s GA range.
Ponderal Index
Neonatal proportionality — symmetric vs. asymmetric growth restriction.
FL/AC (flag < 0.16), TC/AC (flag < 0.60), cardiac/thoracic ratio (flag > 0.50), and a screen result — No Features Flagged · Concerning (1 feature) · Concerning (multiple features).
Twin Discordance
Estimated fetal or birth-weight discordance between twins.
Male SexToggleUsed for the Devine ideal-body-weight formula
Results
BMI with WHO category (Underweight < 18.5 · Normal 18.5–24.9 · Overweight 25–29.9 · Class I / II / III Obesity from 30 / 35 / 40) and Ideal Body Weight (Devine) in your preferred weight unit.
ContextToggleSuspected or confirmed infection · ≥ 20 wk and ≤ 72 h postpartum (pregnancy-adjusted criteria) · In labor (raises the lactate threshold to ≥ 4 mmol/L)
Lactate & ShockNumber padLactate, plus a toggle for vasopressor to keep MAP > 65 despite adequate fluids
Results
Appears once anything is entered: Screen Negative · Serious Infection · Elevated Lactate (in labor) · Sepsis · Septic Shock, with screen and EOI counts, the recommended action, and the list of criteria met. Each field’s threshold is shown as a hint under its label.
Gestational Age
Projects gestational age to any date from a measured GA and scan date.
Project GA toDateDefaults to today — pick a future date to project forward or a past date for a retrospective GA (tap ⓘ for help)
Results
GA in weeks + days as of the chosen date, and an interval card (days, weeks, months). Switch Measure from between Today and Scan Date to change the interval’s starting point.
Management tab — M4 BundleToggleTap-to-check first-hour bundle with progress bar; vasopressor escalation questions when vasopressors are in use
Results
Classification card — Screen Negative · Sepsis Suspected · Septic Shock — with a time-critical banner for shock; qSOFA score out of 3; recommended antibiotic regimen(s) and coverage; active vasopressor tiers with dose and target; and an M5 Delivery Decision once a source is chosen. The Share button exports a text summary.
Antepartum and postpartum thromboprophylaxis for inherited thrombophilia — ACOG Practice Bulletin 196.
Inputs
ThrombophiliaSheet pickerFactor V Leiden or prothrombin G20210A (heterozygous / homozygous), compound heterozygous, antithrombin, protein C, or protein S deficiency
Prior VTESegmentedNone · Single episode · ≥ 2 episodes
The matching ACOG Table 3 row and summary; Antepartum and Postpartum (×6 weeks) cards with the dosing level — Surveillance · Prophylactic · Intermediate · Adjusted-dose. Where ACOG allows more than one level, a Clinician selects one control appears. Reference doses for LMWH and UFH follow. For protein C/S deficiency, a literature-conflict banner lets you choose high-risk (default) or low-risk. Acquired thrombophilia is out of scope.
Genetics
cfDNA PPV/NPV
Positive and negative predictive value of cfDNA (NIPT) screening by maternal age.
Estimated GA by CRL, expected (median) NT, and NT percentile for CRL. NT interpretation — < 3.0 mm below counseling threshold · 3.0–3.4 borderline · 3.5–4.4 increased · 4.5–5.4 moderately · 5.5–6.4 markedly increased · ≥ 6.5 very large.
Tips & FAQ
Where is the Calculate button?
There isn’t one — results update as you type. If you see “—”, a required input is still empty or a scoring row hasn’t been chosen.
How do I enter a height in feet and inches when the field shows cm?
Tap the field, tap the cm ⇄ badge, then tap the left card’s badge until it reads ft + in. Type feet, tap the inches box, type inches, and tap Done. To always use feet and inches, set Settings → Default Units → Length to Feet & Inches.
Can I do quick math in a field?
Yes — the number pad accepts + − × ÷ and parentheses, e.g. (2.1 + 2.3) ÷ 2.
Why was my value rounded?
Entries are rounded to Settings → Max Input Decimals (default 2). Increase it for more precision.
Why can’t I convert units on some fields?
Conversion is offered for weights, lengths, hemoglobin, urine concentrations, and Doppler velocity. Other fields use a single clinical unit and show a grey badge without ⇄.
Does changing units change the result?
No. Each calculator computes in its own base unit; your entry is converted before the formula runs.
Where do the numbers come from?
Tap the book icon on any calculator for its references, or see the Medical Disclosure page.
Still have a question?
Reach us through Support — reports of a formula that doesn’t match your protocol or source get priority.