Baby Length Percentile Calculator

Recumbent length is the distance from the top of the head to the heels while a baby lies down. Standing height is measured upright. Recumbent length is generally used before age two, and it is the measurement used by the WHO birth–24 month standard on this page. Sharing a unit such as centimetres does not make the two measurements interchangeable.

Use an infant length board with help from someone familiar with the method: position the head against the headboard, align the body, and bring the footboard to the heels. Movement, bent knees, and a tilted footboard can change the reading. Do not force the limbs straight or treat a soft tape following the body’s curves as a standard length measurement. Ask a child health professional to demonstrate if you are unsure.

• Enter recumbent length, rather than a standing measurement taken in shoes.
• Repeat with the same equipment and method; check positioning when readings differ.
• This tool does not add a fixed amount to standing height to convert it into length.

Select boys or girls and enter the age when the length was measured, including for older records. The tool covers birth through exactly 24 months. Decimal months are allowed: 6.5 means six and a half months, not six months and five days.

Choose cm or inches, enter lying-down length and calculate. Accepted lengths are 30–120 cm, an input limit rather than an age-specific normal range. One inch equals exactly 2.54 cm. The reference median and chart use cm.

• Reference sex, age in months, and length are required; a name and date of birth are not.
• After changing age, length, or reference sex, calculate again to see the corresponding result.
• For a measurement of 67.6 cm at six months, enter age 6 and length 67.6, rather than age 0.5 years.
• Fractional months interpolate adjacent monthly parameters; this page does not derive exact age in days from a birth date.

The source is the official WHO Child Growth Standards recumbent length tables for boys and girls from birth to two years. Each reference sex has 25 monthly LMS records covering months 0–24. M is the median, S describes dispersion, and L is the Box–Cox power parameter used to account for distribution shape. The calculation first finds a Z score, then converts it to a percentile using the standard normal cumulative distribution function, Φ.

At a whole month, the tool uses that month’s L, M, and S. At a fractional month it linearly interpolates each parameter between the neighbouring months. For example, 6.5 months uses the midpoint of the month 6 and month 7 parameters before calculating the result; it does not average two percentiles. This is a monthly-table approximation and should not be treated as identical to WHO software using exact age in days.

For a boy at exactly 24 months, the WHO monthly record gives L = 1, M = 87.8161 cm, and S = 0.03479. Entering a length of exactly 87.8161 cm makes length divided by M equal to 1. The numerator becomes zero, so Z = 0 and the percentile is 50. These long decimals explain the calculation; they are not a precision requirement for measuring at home.

With that same reference record, Z = +2 corresponds to a length of about 93.9263 cm and a percentile of about 97.7. P97 and Z = +2 are slightly different: one is the 97th percentile, while the other is about the 97.7th. The chart on this page uses P3, P50, and P97.

A result of P25 means about 25% of lengths in the selected reference distribution, at the same age and reference sex, are lower and about 75% are higher. It does not mean a baby has reached 25% of adult height, and it is not a nutrition or health score. P50 is simply the middle of the distribution, rather than a target for every baby.

The Z score expresses distance from the centre on the reference distribution’s scale: zero corresponds to the median, negative values fall below it, and positive values above it. Percentile and Z score describe the same calculation in different ways. The tails are displayed as below 0.1% or above 99.9%, with an input-check reminder when |Z| > 3. Tiny differences in extreme tail percentiles should not be read as precise assessments of an individual.

Find age on the horizontal axis and length in cm on the vertical axis. The solid curve is P50; dashed curves are P3 and P97. The dark dot is your submitted measurement. The chart stays in cm even if you enter inches.

Between P50 and P97 means above the median and below the 97th percentile. Read the calculated percentile on the result card, or expand the monthly table for curve values. These lines do not forecast a baby’s growth, and P3/P97 are not stand-alone health cutoffs.

• Choosing boys or girls changes the reference curves; interpret the result against the current selection.
• The chart shows the current measurement only. Optional browser history can save dated length records separately; it does not add a personal growth curve to this chart.
• The values table lists P3, P50, and P97 at whole months for an alternative way to read the chart.

Keep the date, age, original length, unit and measuring method in your own record. You can also enable optional browser history to save the dated length and method; age and percentile are not stored there. Use the same reference for later comparisons. As the reference changes with age, a baby can grow longer while their percentile falls.

If a result differs greatly from earlier records, first check the age, decimal point, centimetres versus inches, and whether standing height was mixed with recumbent length. If a marked change persists after remeasurement, or you also have concerns about feeding or development, take the full records to a child health professional. Do not change feeding or use height products solely to chase P50 or P97 on a webpage.

• A useful record includes: date | age at measurement | recumbent length and unit | equipment or technique notes.
• For the numerical difference between two records, use the growth velocity calculator separately; it does not classify an infant’s rate against age-specific normal values.
• This tool calculates length-for-age only, without weight, weight-for-length, or nutritional assessment.

This calculator does not automatically correct age for gestational age and does not include a preterm-specific chart. For a baby born prematurely or with a particular health condition, a clinician should determine the appropriate age basis and growth reference. Other software may use exact age in days, corrected age, or a different standard; different results from the same length do not by themselves prove a calculation error.

The endpoint at exactly 24 months on this page remains WHO recumbent length. The site’s age 2–20 height and weight percentile tool uses CDC standing height. Changing to that page changes both measurement posture and reference population, so percentiles need not join smoothly. Choose the reference according to the actual measurement and professional guidance, rather than interpreting a chart-switch difference as sudden growth or a growth stop.