Four calculations most people do occasionally, each with a specific and predictable way of going wrong.
What BMI is, and what it was for
The formula is weight in kilograms divided by the square of height in metres. It was devised by Adolphe Quetelet in the 1830s as a way of describing the distribution of a population, and it carried his name until Ancel Keys renamed it the Body Mass Index in 1972.
That origin is the key to reading it sensibly. It was built to characterise groups, and it does that job well and cheaply. Applied to one person it is a crude proxy, because it measures nothing about the body except a ratio of mass to height.
| Category | BMI | Note |
|---|---|---|
| Underweight | Below 18.5 | |
| Healthy range | 18.5 – 24.9 | Lower ceiling for some populations |
| Overweight | 25 – 29.9 | From 23 in several guidelines |
| Obese | 30 and above | From 27.5 in several guidelines |
The right-hand column matters. The WHO and bodies including NICE in the UK recognise that cardiometabolic risk rises at lower BMI values in people of South Asian, Chinese and some other ancestries, and recommend lower thresholds accordingly. The standard cut-offs were derived largely from European populations, so applying them universally understates risk for a very large number of people.
Where it misleads
- It cannot distinguish muscle from fat. Muscular people are routinely classified as overweight or obese on a measure that has no way to tell.
- It ignores where fat is carried, and abdominal fat carries the risk that matters most.
- It is invalid for children, who need age-and-sex percentiles instead.
- It is not meaningful in pregnancy.
- It behaves poorly at the extremes of height, because the squared term is an approximation.
- It says nothing about fitness, diet, blood pressure or blood glucose.
Waist-to-height ratio is a better single predictor of the risks people usually have in mind, and it needs only a tape measure — keeping your waist under half your height is the guidance NICE has adopted. Use BMI as one rough number among several, and take anything that matters to a clinician. Nothing here is medical advice.
Counting working days
The error is almost always the same one: whether both endpoints count.
"Within five working days" from a Monday can mean the following Monday or the Friday before it, depending on whether the starting day is included. Contracts and service agreements are frequently silent on this, and the two readings differ by a day.
- Decide inclusive or exclusive first, and say which in writing.
- Public holidays are regional. Scotland, England and Northern Ireland differ; US states differ; Germany varies by Land.
- Some sectors have their own calendars. Banking and court days are not always the same as working days.
- Half-days and shutdowns between Christmas and New Year are handled by custom rather than rule.
When a deadline actually matters, state the date rather than the interval. "By 17:00 on Friday 12 September" cannot be miscounted, and it removes the whole question. Date Calculator works out the interval so you can quote the date.
Your exact age in days
The tempting shortcut — years multiplied by 365 — drifts by about a day every four years, so someone in their forties is out by ten days or more.
The correct method is to subtract the dates directly, which handles leap years by construction. Any date library does this; the arithmetic by hand needs care.
Leap-day birthdays are the interesting case. Someone born on 29 February has a birthday in the calendar only every fourth year, and jurisdictions differ on which day counts in non-leap years for legal purposes — some treat 28 February as the anniversary and others 1 March. It rarely matters, and when it does it usually concerns a legal age threshold. Age Calculator counts the actual elapsed days regardless.
Splitting a bill and tipping
Two questions get conflated: what to tip, and how to divide it. The first is convention and varies enormously by country; the second is arithmetic and has a fairness dimension people feel strongly about.
On the first, check the bill before adding anything. A service charge already applied is common in the UK and much of Europe, and adding a tip on top is a choice rather than an obligation. In much of East Asia tipping is not customary at all and can be awkward.
| Where | Typical | Watch for |
|---|---|---|
| United States | 18–20%, on the pre-tax subtotal | Auto-gratuity for large groups |
| United Kingdom | 10–12.5%, often already added | Discretionary service charge |
| Much of Europe | Rounding up, or 5–10% | Service often included |
| Japan, South Korea | Not customary | Can cause confusion |
| Australia, NZ | Not expected | Staff are paid a full wage |
On dividing it, an even split is simplest and is unfair when consumption differed sharply — one person on tap water beside a shared bottle of wine. Splitting by item and sharing tax and service proportionally is fairer and slower. The workable compromise is to split evenly by default and let anyone who ate or drank markedly less say so, which is easier when it is offered rather than requested. Tip Calculator handles the per-person arithmetic including the rounding.
Frequently asked questions
How is BMI calculated?
Weight in kilograms divided by height in metres squared. In imperial units, weight in pounds divided by height in inches squared, multiplied by 703.
Is BMI accurate for individuals?
It is a crude proxy. It was designed in the 1830s to describe populations, and it cannot distinguish muscle from fat or account for where fat is carried. It is invalid for children, not meaningful in pregnancy, and behaves poorly at extremes of height.
Do BMI thresholds differ between populations?
Yes. The WHO and bodies including NICE recognise that cardiometabolic risk rises at lower BMI values in people of South Asian, Chinese and some other ancestries, with overweight from 23 and obesity from 27.5. The standard cut-offs came largely from European populations.
Is there a better measure than BMI?
Waist-to-height ratio predicts the risks people usually have in mind better and needs only a tape measure — the guidance NICE has adopted is to keep your waist under half your height. Use several measures, and discuss anything that matters with a clinician.
How do I count working days correctly?
Decide first whether both endpoints count, since "within five working days" differs by a day between readings. Then account for public holidays, which vary by country and often by region. Where a deadline matters, quote the actual date rather than an interval.
How do I work out my exact age in days?
Subtract the dates directly rather than multiplying years by 365, which drifts about a day every four years. Direct subtraction handles leap years automatically.
Should I tip on the total or the subtotal?
The convention in the US is to tip on the pre-tax subtotal. Elsewhere, check the bill first — a service charge is often already applied in the UK and much of Europe, and tipping is not customary in much of East Asia or in Australia and New Zealand.