I rebuilt the physical "OB wheel" doctors carry in their coat pocket — as a free web app

diamond sharma — Thu, 23 Jul 2026 06:52:13 +0000

If you've ever spent time on a labour ward, you've seen one: a small plastic disc with two dials that an obstetrician spins to work out a due date. It's called an OB wheel (or pregnancy wheel / obstetric wheel), and despite being decades old, it's still standard kit in most Indian clinics — mine included.

I'm a doctor who also writes code, and I got tired of two things:

Physical wheels drift out of calibration and get lost
Every "due date calculator" online does one thing (LMP → EDD) and stops, when the actual back of a real OB wheel has a lot more printed on it — fetal biometry by week, a prenatal test schedule, a β-hCG dating chart

So I built a digital one: OB Wheel Calculator — part of RxMedCalc, a free calculator suite I maintain for Indian clinicians.

What it does that a basic due-date calculator doesn't

Most "pregnancy calculators" online are a single form: enter LMP, get a due date. That's maybe a third of what a physical OB wheel actually does. This one has five calculation modes because that's what you actually need across a real antenatal visit:

GA by LMP — the classic Naegele's Rule calculation, adjusted for cycle length (not everyone has a 28-day cycle, and most calculators quietly assume they do)
GA by Scan — back-calculate from ultrasound dating, since first-trimester CRL is more reliable than LMP for irregular cycles
GA by IVF transfer date — day-3 vs day-5 blastocyst transfers shift the math, and this is a case generic calculators almost never handle
GA on a specific date — for backdating OT notes, discharge summaries, or referral letters
Reverse EDD → LMP — useful when you're reviewing a referral and only the due date was recorded

All four dating paths funnel through the same Naegele's Rule engine (EDD = LMP + 280 + (cycle length − 28) days), so results stay internally consistent no matter which mode you enter from.

Digitizing the "back of the wheel"

The part I actually enjoyed building was the reference data — the stuff printed on the reverse of a physical wheel that most digital calculators skip entirely:

Expected fetal biometry (CRL/BPD/HC/AC/FL) by gestational week, from the Hadlock nomograms
A trimester-wise prenatal test and precautions table (DIPSI GDM screening, Anti-D timing, GBS swab windows)
A quantitative β-hCG dating chart, since a slow-rising hCG is one of the first flags for a non-viable or ectopic pregnancy
A bleeding-in-pregnancy differential table, split by trimester

None of this changes the due-date math, but it's the reason a physical OB wheel is genuinely useful beyond one number — and it's why I didn't just clone the fifty other "due date calculator" pages that already exist.

Stack notes, for the dev crowd

Nothing exotic — the whole RxMedCalc site runs on Cloudflare Pages/Workers/D1/KV, static HTML/CSS/JS, no backend framework, no login, no tracking beyond privacy-respecting analytics. Keeping it framework-free has made it trivial to keep every calculator fast on low-end Android phones, which is what a lot of clinicians in India are actually using between patients.

Try it / break it

It's free, no signup: rxmedcalc.com/medical-calculators/obs-calc

If you're a developer who's also clinical staff (or married to one, or just curious about medical calculators as a genre of software), I'd genuinely like feedback on the UX — particularly whether the five-mode tab layout is intuitive on first use, or whether it needs to be simplified.

I Built a Rabies Vaccine Schedule Calculator (Because People Still Miscalculate It) using Claudee

diamond sharma — Thu, 26 Mar 2026 17:26:45 +0000

Rabies is almost always fatal once symptoms appear—but completely preventable with timely vaccination.

What surprised me is how often people (even in healthcare) still manually calculate vaccine schedules.

So I built a small tool to fix that:

👉 https://thezerowhisper.github.io/rabies-scheduler/

The Problem

Rabies post-exposure vaccination follows fixed schedules like:

Day 0
Day 3
Day 7
Day 14
Day 28

But in real life:

“Day 0” confuses people (it’s the first dose day, not exposure)
Manual counting leads to off-by-one errors
Different protocols exist
Missed doses happen

The Solution

This tool:

Takes a start date
Instantly calculates all future doses
Removes manual errors
Works on mobile (no login required)

Tech Stack

Vanilla JavaScript
GitHub Pages
No dependencies

Other Tools

Growth Chart

https://thezerowhisper.github.io/medical-calculators/growth-chart
Milestones Tracker

https://thezerowhisper.github.io/medical-calculators/milestones
Full Hub

https://thezerowhisper.github.io/medical-calculators

Final Thought

Sometimes the most useful tools are the simplest ones.

If this prevents even one mistake, it’s worth it.

DEV Community: diamond sharma