Before you pick up the tape
Four conditions do more for accuracy than any technique. Measure over well-fitting underwear and nothing else, or the thinnest layer you can manage; a t-shirt adds a quarter inch to every circumference and a jumper adds an inch and a half. Wear the bra you intend to wear with the finished garment, because it changes both the height and the volume of the bust, and a pattern fitted over one bra will not fit over a different one. Stand as you normally stand rather than as you would in a photograph — sucking in your stomach or squaring your shoulders produces a garment that only fits when you are performing. And breathe out normally before you read the tape, not all the way in and not all the way out.
Use a fabric or fibreglass tape measure, not a metal builder's tape. An old fabric tape that has been stretched over years reads short, which will send you a size down at every point on your body; if yours is soft and floppy at the ends, check it against a ruler and replace it if it has drifted. A full-length mirror is worth more than a helper here — it is the only way to see the back of the tape, which is where nearly every error lives.
The rule that fixes most bad measurements
The tape must be level all the way around, and the place it is usually not level is the back. Reaching around yourself pulls the tape upward behind you, so you read a circumference taken across your shoulder blades rather than across your bust. That single error routinely costs an inch and a half, which is a whole size in most charts and sometimes two.
Stand square to a mirror, take the reading at the front, then turn and check the back in the mirror before you trust it. If the tape rides up behind you, drop it and start again. It is worth measuring each point twice on separate occasions — if the two readings differ by more than a quarter of an inch, something in your technique is moving, and it is almost always the back of the tape.
Snug, not tight. The tape should sit against your skin without compressing it: you should be able to slide one finger under it comfortably, and it should not leave a mark. Pulling tight is the second most common error and it reads a half inch small at every point.
The six measurements
High bust — the one nobody knows about
Run the tape directly under your armpits, high across the chest and above the fullest part of the bust, straight across the top of the bust tissue at the front and level across the back. It should feel high — if it is sitting on the swell of the bust, it is too low.
This measures your frame: ribcage, shoulders and chest, with the bust volume mostly excluded. That is why it matters. Commercial patterns are drafted for a B cup, so on anyone larger the full bust measurement describes bust tissue rather than body width, and choosing a size from it produces a garment that is too big in the shoulders, neck and armhole while still being tight where it counts. The high bust is the measurement that tells you which body the pattern should fit. It is also the single most common input error, because most people have never heard of it and measure it far too low.
Full bust
Around the fullest part of the bust, level all the way round, tape snug but not compressing. Check the back in the mirror — this is the measurement that suffers most from the tape riding up. If your bust is very full or low, it can help to lean forward slightly, settle everything into the bra, then stand straight before reading.
The gap between this and your high bust is the number that drives the whole full bust adjustment question, and it is worth writing both down every time.
Waist
Not where your jeans sit. The pattern waist is your natural waist — the narrowest part of your torso, usually about an inch above the navel and roughly level with the bottom of your ribcage. If you cannot see it in the mirror, bend sideways: the crease that forms is your natural waist.
The reliable trick is to tie a length of narrow elastic or a piece of string around your middle and wriggle. It settles at the narrowest point on its own and stays there while you measure. Leave it on afterwards, because you need it for the next two measurements.
Hip
Around the fullest part of the hip and seat, feet together, level all the way round. For most people that is seven to nine inches below the natural waist, but the only way to know is to measure at several heights and take the largest reading. Include everything — a full high hip or a shelf at the top of the thigh has to go through the garment.
Feet together matters. Standing with your feet apart narrows the reading by up to an inch.
Height
Barefoot, back against a wall, heels touching it, looking straight ahead. Rest a book flat on your head against the wall, mark the underside, then measure from the floor to the mark.
Height is not used to pick your size — it decides whether the pattern's length needs to move. Most commercial patterns are drafted for someone around five foot five to five foot six, so the further you are from that, the more the waistline, the armhole depth and the hem all sit in the wrong place on you.
Back waist length
From the prominent bone at the base of your neck — the one that sticks out when you drop your chin — straight down your spine to the string at your natural waist.
This is the measurement that tells you whether you are long or short in the body rather than in the legs, which is what actually decides where a bodice waist seam lands. Two people of identical height can differ by an inch and a half here. It genuinely needs a second person or a mirror at your back; if you must do it alone, hold the tape end at your neck with one hand, let it hang, and read it in a second mirror rather than twisting to look.
Worked example. High bust 36, full bust 39, waist 32, hip 42, height 5'4", back waist length 16½.
Against the Big 4 misses chart that is a bust of 39 falling between size 16 (bust 38) and size 18 (bust 40), so it rounds up to 18; a waist of 32 landing exactly on size 18; and a hip of 42 landing exactly on size 18. But the high bust of 36 matches the size 14 bust line exactly — and on a pattern drafted for a B cup, the high bust is the honest guide to frame size. The three-inch difference between high and full bust is the borderline case: a small adjustment is likely, and cutting the shoulders and armhole from the 14 with a one-inch full bust adjustment will fit far better than cutting a straight 18.
Run the same numbers through the pattern size calculator and it will draw the blend for you and switch brands on demand.
Metric or imperial — pick one and stay there
Sewing is unusually split between the two, and the split runs through the brands as well as the sewists: the Big 4 publish in inches, most British and European independents publish in centimetres. Converting each measurement by hand introduces rounding at every step, and the errors do not cancel — they accumulate into the size boundaries.
Record your measurements in whichever unit your tape is marked in, and let the tool convert once. A quarter inch is about 6 mm; charts step in half inches or in one-to-two centimetre increments, so a sloppy conversion really can move you a size.
How often to measure again
Bodies change, and a set of measurements from two years ago is not data, it is a memory. Measure again at the start of any project you care about, after any noticeable change in weight or fitness, and after pregnancy — the ribcage in particular often does not return to where it was. Keeping the numbers saved means you notice the drift instead of discovering it halfway through cutting.
Where measurements stop and fitting begins
These six numbers get you to the right size and the right blend. They cannot tell you about shoulder slope, a forward-rounded upper back, a swayback, a high or low bust apex, or a ribcage that is deep front to back rather than wide side to side. All of those show up in a muslin and none of them show up in a tape measure.
That is not an argument against measuring carefully — it is the reason to. The better your starting size, the fewer variables are left in the muslin, and the easier it is to see what your body is actually asking the pattern for.
General guidance following standard commercial pattern measuring practice. Where a pattern company's own instructions differ, follow theirs — they drafted the block. Not professional fitting advice.