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How to take a measurement at home that is worth plotting

Why technique is the whole story

A growth chart is exact arithmetic run on an inexact number. The curves are published to four decimal places; the measurement you feed them was taken on a bathroom scale with a wriggling toddler on it. Almost every surprising percentile a parent meets at home is a measurement problem, not a growth problem.

This guide covers technique only: how each of the three measurements is defined and taken, and where the error comes from. It says nothing about what any resulting number means, because that is not this site’s job.

Weight

What is being measured: total body mass, in kilograms or pounds, with as little else on the scale as possible.

• Under about two years, use an infant scale that reads in small increments, with the baby lying or sitting on the tray. A standing adult bathroom scale is not designed for the range or the resolution, and its readings move around under a load that shifts.
• Weigh naked, or in a known amount of clothing. A wet diaper is easily 100 to 200 grams, and at 6 months that alone can move a percentile several points. If undressing is not practical, weigh the same way every time so at least the error is consistent.
• Zero the scale, on a hard floor. Carpet compresses and throws the reading off. Zero it with the tray or blanket already in place, not after.
• The tared-adult trick is a last resort. Weighing yourself, then yourself holding the child, and subtracting works, but the difference of two large numbers inherits both of their errors. On a scale that reads to 0.1 kg, that method has an error band wider than a month of infant weight gain.

Where error comes from: feeding state, diaper, clothes, and a scale that is not level. Weigh at a consistent time of day if you are comparing readings.

Length, under 24 months

What is being measured: recumbent length, crown to heel, with the child lying down.

This is genuinely difficult to do accurately at home, and it is worth saying so plainly. Clinics use a length board with a fixed headpiece and a sliding footpiece, and it normally takes two people: one holding the head in position against the headboard, one straightening the legs and bringing the footpiece to the heels.

• The head goes against a fixed surface, looking straight up, so the line from the ear canal to the lower edge of the eye socket is vertical.
• The legs must be straight and together, with the knees pressed gently flat, and the feet flexed so the soles are at a right angle to the shins. A bent knee shortens the reading.
• The tape or ruler runs parallel to the body, not diagonally across it, and not draped over a curve.

A tape measure on a soft mattress with a baby who does not want to be there can easily be a centimetre out either way. At 12 months, one centimetre is worth about sixteen percentile points on the WHO length standard, which is more than most parents expect and a good reason to treat a home length reading as approximate.

Standing height, from 24 months

What is being measured: stature, heels to crown, with the child standing.

From two years, the reference tables switch from length to standing height, so the measurement technique switches with them.

• Shoes and bulky hair accessories off. Heels, bottom, shoulder blades and the back of the head against a flat wall, feet together and flat.
• Look straight ahead, chin level, not tipped up to see what you are doing.
• Bring a flat rigid object down onto the crown, a book or a ruler held horizontally against the wall, and mark the wall at its underside. Measure from the floor to the mark afterwards, not while the child is standing there.
• A door frame notch is a memory, not a measurement. It is a lovely tradition and a poor data source, because the pencil angle and the child’s posture change each time.

Standing height reads shorter than recumbent length for the same child, typically by around a centimetre, because the spine compresses under load. That is one of the reasons a percentile can shift at a second birthday without anything happening to the child. There is more on that in why the WHO and CDC charts differ.

Head circumference

What is being measured: the largest circumference of the head, called the occipitofrontal circumference.

• Use a flexible non-stretch tape. A cloth tailoring tape stretches over time and will read short; a paper or fibreglass tape does not.
• Position matters more than tension. The tape sits just above the eyebrows and the ears at the front, and around the most prominent part at the back of the skull. Slide it up and down a little at the back and take the largest reading you can find, because that is the definition.
• Snug, not tight. Firm enough to compress the hair, not so tight that it indents.
• Hair defeats it. Braids, buns, and thick curls all add to the reading. Flatten what you can.

Head circumference is the measurement most sensitive to tape position: a tape that rides low at the back can read one to two centimetres short, which is a large move on a curve whose whole 3rd-to-97th range at 12 months is under five centimetres.

Recording it so the chart can use it

Two habits make home measurements far more useful when you plot them:

1. Write the date, not the age. Ages get rounded and misremembered; a date is a fact. The chart tool accepts a date of birth plus a date measured and works out the age itself, precisely.
2. Record what you actually read, before rounding, and note the conditions (naked or clothed, which scale). A series measured consistently is far more informative than a series measured accurately once.

The boundary of this page

Everything above is about how to take a number. What a number or a series of numbers means for a particular child is a clinical judgement that depends on things a measurement cannot see. Percentiles describe position in a reference population, not health; growth patterns are for your pediatrician to read, and clinic measurements taken on proper equipment are the ones worth reading them from. The CDC’s growth chart pages publish the underlying charts and technical notes.

Keep going: plot a measurement, read the published reference tables, or browse the other guides.