The Four Calculators, Honestly: What Each One Measures and Where It Stops

A soft tape measure costs about as much as a coffee and spends most of its life coiled in a drawer under the scissors. It comes out when something has gone wrong: a bra riding up at the back by lunchtime, a dress ordered online that fits nothing like the photograph, a haircut you will live with for four months.

There are four calculators on this site and every one is arithmetic: useful, and narrower than a confident headline result makes them look. Here is what each does with your numbers, how to give it numbers worth having, and where it stops.

Before any of them: the tape

Most wrong results are wrong inputs. The maths is not subtle, so almost all the error is in how the tape was held.

• Bare skin, or a thin unpadded bra. A jumper adds an inch, and the arithmetic cannot tell wool from you.
• Level all the way round. A tape that slopes down at the back reads long, so check it in a mirror.
• Snug, not pulled - flat against you, no gap, no dent in the skin. Under the bust is the one place to draw it firm against the ribcage, because that is what a band does.
• Breathe out normally, then measure, and stand how you actually stand, not braced for the tape.
• Late in the day is the honest reading, and if you are comparing against older numbers, measure in the same week of your cycle.

Write them down once. All four calculators reuse the same handful.

Bra size: two numbers, five countries

The bra size calculator wants two measurements, underbust and bust, in inches or centimetres. Both are rounded to the nearest whole inch first. The band uses the modern method, with no four inches added to anything: an even underbust is your band, an odd one rounds up to the next even number, so 31 and 32 both come out as a 32 band. The cup is the difference between the two rounded numbers, one inch per cup step, stopping at twelve.

It then reads that result out in five systems at once, and they are not translations of one another. A five inch difference is a DD in the US, UK and Australia, an E in Europe and a D in Japan. US and UK letters agree up to F and then part company: an eight inch difference is a G in the US and an FF in the UK.

Where it stops. It is arithmetic on two numbers, and a bra is a manufactured object with a shape. Two people with identical measurements can be built differently, one shallow and wide across the ribcage, one narrow and projected, and the same labelled size fits one and not the other. Brands differ, cuts differ within a brand, elastic ages. What you get is a size to try first, plus its sister sizes: a better starting point than the one you buy out of habit, not a fact about you.

It also takes one bust number, and plenty of people do not have one. If your two sides want different cups it cannot say so, and One Breast Is Bigger Than the Other covers what to do instead. The bra fitting guide has the full routine.

Body shape: four numbers, ten names, three checkboxes

The body shape calculator takes shoulder, bust, waist and hip, then works down a list of rules and returns the first that matches. The thresholds are fixed gaps in inches, not percentages. Hourglass needs bust and hips within an inch of each other and a waist at least nine inches smaller than the bust. Rectangle is the exception: bust, waist and hips within five per cent of each other. The shoulder number only appears in some of the rules, so it matters less than its position suggests. And because the cutoffs are whole inches, an inch matters: a tape sitting slightly high on the hips can move you into a different name. Take each measurement twice.

The three optional checkboxes are not equal. A visible shelf above the hips turns Pear into Spoon, and only if you already met the Pear rule. Visibly muscular returns Athletic, but only when shoulders and hips are within an inch and the waist is not much smaller than the bust. Small and slender overall returns Skinny immediately, before any measurement is looked at: deliberate, because no tape can read frame size, but it does mean ticking it makes the other four boxes irrelevant.

What the result is for. It is shopping vocabulary: which proportions a garment has to accommodate, so you can stop trying the cut that has never once worked. Ten names, none of them a grade. Hourglass is not a compliment and Apple is not bad news. The body shapes guide has what tends to fit each.

Face shape: four numbers, and Oval is the default

The face shape calculator takes forehead width, cheekbone width, jaw width and face length, and checks them in order: Oblong, Heart, Triangle, Diamond, Square, Round. Worth knowing before you read your result: Oval is what you get when none of the others matched. It is the fallback, not a finding.

It is also the least reproducible of the four, for physical rather than mathematical reasons. The tolerances run from half an inch to an inch and a half, and these are hard measurements to take on yourself: hair in the way, a flat tape on a curved skull, "the widest part of your jaw" a judgement call you make differently each time. Measure twice; if the attempts disagree by half an inch, so can the answer. Face Shapes Explained describes all seven if you would rather recognise yours than measure.

Hair type: three questions, one of which sometimes does not count

The hair type quiz is not measurement at all. Three questions, run through the classification created by the hairstylist Andre Walker, which is his system rather than settled science and is worth weighing as such.

Pattern picks the number: straight 1, wavy 2, curly 3, coily 4. Then the paths differ. For straight and wavy hair the letter comes from strand thickness, fine giving A, medium B, coarse C. For curly and coily hair it comes from curl size instead, and the thickness answer is collected but does not change the result. If you are a 3 or a 4, the last question decides your letter.

Answer the pattern question about freshly washed hair, air dried, no product. That is the only state in which the pattern is yours rather than your routine's. And pattern is not what most routines hinge on: porosity and density do more work, so the hair porosity test is the more useful half of the answer. Hair Types Decoded covers all twelve types and porosity together.

The numbers move, and that is not a failure to keep up

Every result here has a shelf life. Bust, waist and ribcage change across a cycle. Pregnancy changes them quickly, feeding changes them again, and things settle afterwards without landing exactly where they started. Weight settles differently in different decades. Menopause alters what breast tissue is made of, and so what size fits. Hair pattern shifts with hormones, medication, colour and age. A result you saved six months ago being wrong today is not something you let slip. It is a body doing what bodies do, and running it again takes two minutes.

Which one to open

• Never been measured, or your band rides up and the wire sits on tissue instead of behind it: bra size, today. It is the one with a daily physical payoff.
• Buying for an event, or ordering online where returns are a nuisance: body shape, for the vocabulary to filter with.
• Changing your hair, or choosing frames: face shape, held lightly.
• Products not doing what the bottle says: hair type, then porosity.

All four are free and need no account, and every result page has a button that says this is not right. Press it when it is not.