A Buffer Holds Its pH by a Ratio, So Dilution Changes Nothing
Equal amounts of a weak acid and its base give pH = pKa exactly, at any concentration. Blood runs the same equation at a 20:1 ratio and lands on 7.40.
Sofia Nunes · 3 min read
Health
89 guides on health. Showing the 48 most recent.
Weight, nutrition, sleep and the body's own numbers. These guides follow published health-authority guidance and name the equation behind every estimate, because the formulas disagree with one another. None of it replaces a professional who knows your case.
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- 1How Much Milk Does a Baby Need? The Rule of Thumb and Its LimitsA formula-fed baby takes roughly 2.3 fl oz per pound of body weight per day in the first months, split across the day's feeds. Here is the table by age and weight, why breastfed babies are not measured this way, and the signs that outrank any number.
- 28.25 a Day Is 3,011 a Year, and Only One of Those Numbers Changes AnythingFifteen cigarettes a day from packs of twenty at 11 costs 8.25 daily and 3,011.25 a year. The same amount saved instead, for twenty years at 5 %, comes to about 99,570.
- 3Four Hours a Day of Screen Is Sixty-One Days a YearFour hours a day is 28 a week and 1,460 a year — 60.8 full days. Three hours is 45.6 days and seven is 106.5. The conversion is arithmetic; what it means is not.
Weight & body23 guides
Medical & clinical17 guides
Nutrition16 guides
Pregnancy & baby16 guides
- How Much Milk Does a Baby Need? The Rule of Thumb and Its Limits
- IVF Dating Is the One Case Where the Date Is Actually Known
- What Is a Normal Menstrual Cycle Length? The Numbers, by Life Stage
- Pregnancy Week by Week: The Calendar and What Each Stage Means
- hCG Doubling Time, and What a Single Number Cannot Tell You
Sleep & alertness5 guides
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October 19, 2026
October 16, 2026
Four Hours a Day of Screen Is Sixty-One Days a Year
Four hours a day is 28 a week and 1,460 a year — 60.8 full days. Three hours is 45.6 days and seven is 106.5. The conversion is arithmetic; what it means is not.
Sofia Nunes · 3 min read
October 6, 2026
One Milligram per Litre Is One ppm — in Water, and Only There
50 mg in a litre is 50 ppm, because a litre of water weighs a million milligrams. Change the liquid, or move to air, and the two units stop agreeing.
Sofia Nunes · 3 min read
October 2, 2026
8.25 a Day Is 3,011 a Year, and Only One of Those Numbers Changes Anything
Fifteen cigarettes a day from packs of twenty at 11 costs 8.25 daily and 3,011.25 a year. The same amount saved instead, for twenty years at 5 %, comes to about 99,570.
Sofia Nunes · 3 min read
July 17, 2026
How Heart Disease Risk Scores Work — and Why They Disagree About You
Framingham, SCORE2 and the pooled cohort equations use overlapping but different inputs and were calibrated on different populations, so the same person gets three different percentages. Here is what each one asks for and what the number means.
Sofia Nunes · 6 min read
July 16, 2026
Biological Age vs Chronological Age: What Each One Actually Measures
Chronological age is exact. Biological age is not one thing but three — epigenetic clocks, blood-marker scores and lifestyle quizzes — and only the first two have published validation. Here is how to tell which one you are looking at.
Sofia Nunes · 6 min read
July 15, 2026
Waist-to-Hip Ratio Explained: The Thresholds, and Where to Put the Tape
WHO puts abdominal obesity at a ratio of 0.90 or more for men and 0.85 or more for women. But measuring in the wrong place changes the answer more than the threshold does — here is exactly where the tape goes.
Sofia Nunes · 6 min read
July 14, 2026
What Is a Normal Menstrual Cycle Length? The Numbers, by Life Stage
FIGO puts a normal cycle at 24 to 38 days; ACOG's older range is 21 to 35. Here is why two respected bodies give different numbers, how much variation is normal, and how cycles change after menarche and before menopause.
Sofia Nunes · 6 min read
July 3, 2026
How Blood Type Is Inherited: The Full Parent-to-Child Table
Each parent passes on one ABO allele — A, B or O — and Rh follows a separate gene. Here is every parent combination, which child types it can and cannot produce, and why this is not a paternity test.
Sofia Nunes · 7 min read
How Much Milk Does a Baby Need? The Rule of Thumb and Its Limits
A formula-fed baby takes roughly 2.3 fl oz per pound of body weight per day in the first months, split across the day's feeds. Here is the table by age and weight, why breastfed babies are not measured this way, and the signs that outrank any number.
Sofia Nunes · 6 min read
June 29, 2026
HbA1c to Average Blood Sugar: The Full Conversion Table
An HbA1c of 7 percent corresponds to an average glucose of about 154 mg/dL, or 8.5 mmol/L, or 53 mmol/mol. Here is the whole table, the formulas behind it, and when the number cannot be trusted.
Sofia Nunes · 4 min read
June 25, 2026
16/8, 5:2 or Alternate Day: The Fasting Protocols Compared
Five protocols side by side, what each demands, and the uncomfortable finding from the controlled trials: matched for calories, fasting loses about the same weight as simply eating less.
Sofia Nunes · 4 min read
How to Calculate Keto Macros (Without Starting From the Percentages)
Set carbs as a fixed gram figure, protein by body weight, and let fat fill what is left. The famous 75 percent fat is an output of that calculation, not an input — and it drops on its own in a deficit.
Sofia Nunes · 5 min read
How Much Weight to Gain in Pregnancy: The Guidelines by Starting BMI
The recommended range depends entirely on your BMI before pregnancy: 28 to 40 lb if you started underweight, 11 to 20 lb if you started with obesity. Here is the full table and the weekly rate behind it.
Sofia Nunes · 4 min read
June 24, 2026
Pregnancy Week by Week: The Calendar and What Each Stage Means
Pregnancy is counted in 40 weeks from your last period, not from conception — so you are already two weeks pregnant at conception. Here is the full calendar, the trimester boundaries and what term actually means.
Sofia Nunes · 4 min read
How to Predict Your Next Period (And How Far Off the Estimate Can Be)
Add your average cycle length to the first day of your last period. Here is how to measure that length properly, why the estimate has a range rather than a date, and when irregularity is worth a consultation.
Sofia Nunes · 5 min read
June 9, 2026
How Long Does Alcohol Stay in Your System?
The body clears roughly one standard drink per hour. See how long alcohol stays in blood, breath and urine — and why you can't rush it.
Sofia Nunes · 2 min read
Is BMI Reliable? The Limitations of Body Mass Index
BMI is a quick screen, not a diagnosis. Learn what it misses — muscle, fat distribution, age — and what to check alongside it.
Sofia Nunes · 2 min read
How Much Water Should You Drink During Exercise?
Match your drinking to how much you sweat. Learn to estimate sweat rate and how much to sip per hour during a workout.
Sofia Nunes · 2 min read
June 3, 2026
How Many Calories Are in a Pound of Fat?
A pound of body fat holds about 3,500 calories. See where the number comes from, why it's a rough guide, and how to use it for weight loss.
Sofia Nunes · 2 min read
May 29, 2026
What Is a Child Growth Percentile?
Growth percentiles compare a child's height and weight to others the same age and sex. Here's how to read them and what really matters.
Sofia Nunes · 2 min read
May 27, 2026
How to Estimate Your Conception Date
Estimate conception from your last period or your due date, understand ovulation timing, and know why the result is a range rather than a single day.
Sofia Nunes · 4 min read
May 22, 2026
How Much Caffeine Is Safe Per Day?
The 400 mg daily guideline for adults, how much caffeine is in coffee, tea, cola and energy drinks, and why sensitivity varies from person to person.
Sofia Nunes · 3 min read
May 18, 2026
How Much Fiber Per Day Do You Need?
Daily fiber targets of 25 to 38 grams by sex and age, why fiber matters for digestion and heart health, and the best food sources to hit your goal.
Sofia Nunes · 3 min read
May 12, 2026
How Much Sodium Per Day Is Safe?
The 2,300 mg FDA limit, the 1,500 mg ideal from the AHA, and why most sodium is hidden in packaged food — plus how to read labels and cut back.
Sofia Nunes · 3 min read
May 6, 2026
How Much Sugar Per Day Is Healthy?
The WHO limit on free sugars, how to turn a percentage of calories into grams, and where hidden sugar comes from — with practical daily targets.
Sofia Nunes · 3 min read
May 1, 2026
How to Convert Steps to Distance
To turn a step count into distance, multiply your steps by your stride length. Learn how to estimate your stride and convert steps into miles.
Sofia Nunes · 3 min read
April 27, 2026
Best Time to Wake Up: Using Sleep Cycles
Sleep runs in cycles of about 90 minutes. Waking at the end of a cycle can help you feel more refreshed. Here's how to count back from your wake time.
Sofia Nunes · 3 min read
April 23, 2026
IVF Dating Is the One Case Where the Date Is Actually Known
Every other pregnancy is dated from an estimate. After IVF, fertilisation is timed to the hour, so gestational age is arithmetic: 266 days from fertilisation, minus the embryo's age at transfer. Here is the derivation for day-3, day-5 and day-6 embryos, why frozen and fresh transfers use the same rule, why an early scan rarely moves an IVF date, and what the two-week offset on a scan report means.
Sofia Nunes · 12 min read
April 22, 2026
Am I Overweight? Why One Number Cannot Say
BMI reads muscular bodies as heavy and cannot see where fat sits at all — which is exactly why the waist measures exist. Here are the published waist and waist-to-height thresholds, a worked case where BMI and waist give opposite answers, an honest account of the contested mortality curves, and the 2025 shift toward defining obesity by adiposity and organ function rather than by a ratio.
Sofia Nunes · 13 min read
April 21, 2026
Am I Underweight? What the Question Is Really Asking
The 18.5 cut-off is a population screening line, not a diagnosis. Here is where it comes from, what the mortality data below it actually show, and the three things it cannot see: body composition, recent change — which is far more informative than the level — and the fact that the same line does not fit every age or every population.
Sofia Nunes · 11 min read
Blood Pressure Ranges Explained
Blood pressure has two numbers: systolic and diastolic, measured in mmHg. Learn the ACC/AHA categories from normal to hypertensive crisis and what they mean.
Sofia Nunes · 3 min read
April 20, 2026
What Breastfeeding Actually Costs in Energy — and Why the Published Number Is Smaller Than the Milk
Milk production has a computable cost: volume times energy density divided by the efficiency of conversion. Do the sum and you get about 675 kcal a day, not the 500 that gets quoted. Here is the derivation, why the published increment is deliberately lower, how much the real spread is between mothers, what the water requirement actually rests on, and why cutting calories while feeding is a clinical question.
Sofia Nunes · 12 min read
April 17, 2026
Eating for Two Is About Three Hundred Calories — and Almost None of Them in the First Trimester
The extra energy a pregnancy costs is far smaller than the phrase suggests, and it arrives on a schedule: near zero in the first trimester, modest in the second, real in the third. Here are the published increments from EFSA, FAO/WHO/UNU and the 2023 US reference intakes, what they look like as food, and why protein and folate rise proportionally faster than calories do.
Sofia Nunes · 13 min read
April 9, 2026
Resting Heart Rate by Age: What's Normal?
A normal resting heart rate for adults is usually 60 to 100 beats per minute. Learn what affects it, why athletes run lower, and when a reading may be concerning.
Sofia Nunes · 3 min read
April 6, 2026
How Many Calories Does a Run Actually Burn?
Running costs roughly 0.73 kcal per pound per mile — one kilocalorie per kilogram per kilometre in the units the physiology is published in — and, this is the counterintuitive part, the figure barely moves with pace. Here is the derivation from the ACSM equation, the corrections that matter, and why grade changes it far more than speed.
Sofia Nunes · 13 min read
What Is Blood Alcohol Content (BAC)?
Blood alcohol content measures the concentration of alcohol in your blood. Learn how BAC is estimated with the Widmark formula and why legal limits differ by country.
Sofia Nunes · 3 min read
April 3, 2026
The Calories in a Drink Are Mostly the Alcohol
Ethanol carries 7 kcal per gram and the arithmetic is exact: volume × ABV × 0.789 × 7. A dry spirit is almost pure alcohol calories; a liqueur is nearly half sugar. Here is the derivation, the table, and the metabolic caveat stated without overclaiming.
Sofia Nunes · 12 min read
April 2, 2026
What a Meal Plan Generator Can and Cannot Know
A generator solves a constrained arithmetic problem — hit an energy target, hit the macro splits, stay inside a food list — and it does that well. Here is the arithmetic that makes a plan work, and the honest list of what no generator can see.
Sofia Nunes · 13 min read
April 1, 2026
How Much Carbohydrate Do You Actually Need?
There is a small hard answer — the brain's glucose demand, published as 130 g a day — and a very large soft one. Above the minimum, carbohydrate need scales with training volume in grams per kilogram, not with a percentage.
Sofia Nunes · 14 min read
March 31, 2026
How Long Until You Are Sober? Zero-Order Elimination Explained
Alcohol leaves the body at a roughly constant mass per hour, whatever the concentration. That one property — zero-order elimination, unlike caffeine's exponential decay — explains why doubling the dose doubles the time, why nothing speeds it up, and why every estimate carries a wide error bar. With the Widmark relation worked through, and each market's legal limit stated separately.
Sofia Nunes · 16 min read
March 30, 2026
Caffeine Against Sleep Pressure: What It Blocks, and What It Cannot
Sleep is regulated by two processes: a pressure that builds with every waking hour, and a clock. Caffeine acts on neither directly — it blocks the receptors that report the pressure. The pressure keeps building behind the block, which is why the crash arrives and why an afternoon dose costs that night's sleep even when you do not feel it.
Sofia Nunes · 13 min read
March 27, 2026
Baby Growth, and What Crossing Centiles Actually Means
The WHO infant standard is prescriptive: it was built from healthy breastfed babies and describes how infants should grow. National references describe how they did grow. In the first year the two disagree by enough to move the same baby forty percentile points, with no change in the baby. Here is the size of the gap, and what the published thresholds for concern actually say.
Sofia Nunes · 14 min read
Where a Percentile Comes From: Which Population, Measured When
A percentile is a rank against a reference population, so the first question is never the arithmetic — it is which population, measured when. The same 5'11" man is the 38th percentile in one national dataset and the 86th in another, and the 91st against a reference of people born a century ago. Here is why, and what a z-score adds.
Sofia Nunes · 12 min read
March 26, 2026
How Tall Will a Child Be? The Arithmetic Is Easy; the Error Bar Is the Story
Average the parents' heights, shift by a sex constant, and you have a target height. The honest part is what comes next: the published prediction interval is about ±4 inches, wide enough to cover most of the adult population. Here is the formula, the interval, why bone age does better, and why children keep out-growing the target.
Sofia Nunes · 14 min read
March 25, 2026
Body Frame Size, and What Ideal-Weight Formulas Assume
Frame size exists because height alone does not fix skeletal mass, and the published methods are crude proxies with real cut-offs. The bigger story is the ideal-weight formulas: four of them, all built for drug dosing rather than health, disagreeing by up to 27 lb at the same height.
Sofia Nunes · 12 min read
The Insulin Sensitivity Factor and the 1800 Rule
The correction factor says how far one unit of insulin moves glucose, and the classical estimate is 1800 divided by the total daily dose in mg/dL — or 100 divided by it in mmol/L. Those are the same rule, and here is the arithmetic that proves it, plus why stacking is the error that actually hurts.
Sofia Nunes · 10 min read
March 24, 2026
The Carbohydrate-to-Insulin Ratio, and the Rule That Estimates It
The ratio says how many grams of carbohydrate one unit of insulin covers, and the usual starting estimate is 500 divided by the total daily dose. Here is where that number comes from, how sharply it moves across daily doses, why a 450 version exists, and the three things it quietly assumes.
Sofia Nunes · 10 min read