About this tool
Standard pregnancy dating counts 280 days (40 weeks) from the first day of the last menstrual period. Conception itself happens about 14 days later, around ovulation, so we subtract 266 days from the due date to estimate it.
How it works
Conception date = due date - 266 days
Conception window = conception date - 5 days to + 1 day (sperm survival)
This calculator uses standard obstetric conventions. Early ultrasound dating may revise the estimate by a few days.
BMI, BMR, TDEE - what they actually measure
Three numbers often confused; they answer different questions:
| Metric | Formula | What it tells you |
|---|---|---|
| BMI (Body Mass Index) | weight kg / height m^2 | Rough screen for under/normal/over weight. Bad for athletes. |
| BMR (Basal Metabolic Rate) | Mifflin-St Jeor equation | Calories your body burns AT REST in 24 hours |
| TDEE (Total Daily Energy Expenditure) | BMR x activity multiplier | Total calories burned per day, including activity |
| Body fat % | DEXA / calipers / bioimpedance | More accurate than BMI for athletes and very lean people |
WHO BMI categories
| Category | BMI range | Notes |
|---|---|---|
| Underweight | < 18.5 | Increased risk of malnutrition, osteoporosis |
| Normal | 18.5 - 24.9 | Lowest health risk for most adults |
| Overweight | 25.0 - 29.9 | Increased risk of cardiovascular disease, diabetes |
| Obese class I | 30.0 - 34.9 | High risk; lifestyle change recommended |
| Obese class II | 35.0 - 39.9 | Very high risk; clinical intervention often warranted |
| Obese class III | >= 40.0 | Extreme risk; bariatric options often discussed |
Asian-specific BMI cutoffs
South Asians have higher cardiovascular risk at lower BMI. India, Singapore, and many Asian countries use:
- Underweight: < 18.5
- Normal: 18.5 - 22.9
- Overweight: 23.0 - 27.4
- Obese: >= 27.5
Calorie math for weight change
The simplified energy balance:
- 1 kg of body fat = approximately 7,700 kcal
- 1 lb of body fat = approximately 3,500 kcal
To lose 0.5 kg / 1 lb per week, eat ~500 kcal/day below TDEE. To gain muscle, eat 200-500 kcal/day above TDEE with adequate protein (~1.6g per kg body weight per day) and resistance training.
Caveats:
- "Calories in vs out" is the right framework, but the body adapts - metabolism slows ~10-15% during sustained weight loss, so progress slows over months.
- Hormones (insulin, leptin, ghrelin, thyroid) modulate hunger and storage. Not all calories are equally easy to eat.
- Sleep and stress affect both intake and expenditure measurably.
Protein, fat, carbs - what's needed
| Macronutrient | Daily target | Energy density |
|---|---|---|
| Protein | 0.8 g/kg sedentary; 1.6-2.2 g/kg if exercising | 4 kcal/g |
| Fat | 20-35% of total calories; minimum ~0.5 g/kg | 9 kcal/g |
| Carbohydrate | Fills the rest; minimum 130 g/day for brain function | 4 kcal/g |
| Fiber | 25-38 g/day | ~2 kcal/g (variable) |
| Water | 30-40 ml/kg body weight | 0 kcal/g |
Frequently asked questions
How is conception date calculated from due date?
Subtract 266 days from the due date. Pregnancy is dated from the first day of the last menstrual period (280 days, Naegele rule), but actual conception happens about 14 days later, at ovulation.
How accurate is this estimate?
For a typical 28-day cycle the estimate is within 2 to 3 days. If your cycle is shorter or longer, conception likely happened a few days earlier or later. Early ultrasound dating is the most accurate for true conception window.
What is the LMP date?
LMP is the first day of your last menstrual period before pregnancy. Doctors use this date to calculate gestational age and due date because it is more reliably remembered than ovulation or conception.
Why is the due date 280 days from LMP, not conception?
It is a convention from the 19th century (Naegele rule) that simplifies calculation when the LMP is known but ovulation is not. Real pregnancy from conception is closer to 266 days or 38 weeks.
Is BMI accurate for me?
BMI is a population screening tool, not an individual diagnosis. It overestimates body fat in muscular athletes and underestimates it in people with low muscle mass. Body fat percentage (DEXA, calipers) is more accurate. Waist-to-height ratio under 0.5 is another useful single number.
How much weight can I safely lose per week?
0.5-1 kg (1-2 lb) per week is sustainable for most adults. Faster loss is possible but typically comes from water, glycogen, and muscle - not just fat. Crash diets also slow metabolism and rebound when normal eating resumes.
Are calorie calculators accurate?
BMR estimates are within ±10% of measured rates for most people. TDEE depends on self-reported activity, which is usually overestimated by 20-50%. Track actual weight change over 2-4 weeks and adjust intake to match.
Why does my weight fluctuate 2-3 kg in a single day?
Almost all daily fluctuation is water (sodium, carbohydrate storage as glycogen which holds 3-4x its weight in water) and digestive contents. Fat loss is on the order of 100-150g per day at most. Weigh weekly at the same time for trend, not daily.
Is fasting better than calorie restriction?
For weight loss, the two are equivalent if total calories are the same. Some people find fasting easier to comply with (skipping breakfast = automatic deficit). Others find it harder. The best diet is the one you can sustain.
How accurate is the Pregnancy Conception Date Calculator?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the Pregnancy Conception Date Calculator free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Can I use the Pregnancy Conception Date Calculator on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Does the Pregnancy Conception Date Calculator work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
How do I report a bug or suggest improvement to the Pregnancy Conception Date Calculator?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Can I share results from the Pregnancy Conception Date Calculator?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another pregnancy conception date tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.