Infant Growth Velocity Calculator

Track Your Baby's Growth Rate Between Checkups

Infant Growth Velocity Formula

Formula

\[ \Delta d = d_2 - d_1 \] \[ v_W = \frac{W_2 - W_1}{\Delta d} \quad \text{(g/day)} \qquad v_{W,\,week} = 7 \cdot v_W \] \[ v_L = \frac{L_2 - L_1}{\Delta d / 30.4375} \quad \text{(cm/month)} \]

Where:

\(d_1, d_2\) — the dates of the first and second measurement (days)
\(\Delta d\) — the number of days between the two measurements
\(W_1, W_2\) — the baby's weight on each date, converted to grams (kg \(\times 1000\), lb \(\times 453.592\))
\(L_1, L_2\) — the head-to-heel length on each date, converted to centimeters (in \(\times 2.54\))
\(v_W\) — weight velocity, i.e. average weight change per day
\(v_L\) — length velocity, i.e. average length change per month (30.4375 days is the average month length)

These formulas express growth as a rate — how fast weight and length change over time — rather than as a single measurement. That makes it possible to compare a baby's growth across checkups of different lengths.

Infant Growth Velocity – Calculation Example

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Example: a baby weighed 3200 g and measured 50 cm on June 1, 2025. Four weeks later, on June 29, 2025, the baby weighed 3900 g and measured 51.5 cm.

\[ \Delta d = 28 \text{ days} \]

\[ \Delta W = 3900 - 3200 = 700 \text{ g} \]

\[ v_W = \frac{700}{28} = 25 \text{ g/day}, \quad v_{W,\,week} = 25 \times 7 = 175 \text{ g/week} \]

\[ v_L = \frac{51.5 - 50}{28 / 30.4375} = \frac{1.5}{0.9197} \approx 1.63 \text{ cm/month} \]

Result: the baby gained an average of 25 g/day (175 g/week) and grew about 1.63 cm/month over the 28-day interval.

{/example}

How to Use the Calculator

Enter the Date of first weighing — the earlier checkup or home weighing.
Type the baby's Weight on that date and pick the unit (g, kg or lb) from the dropdown.
If length was measured, fill in Length (optional) in cm or in; leave it empty if not measured.
Enter the Date of second weighing, the Weight (with its unit) and, optionally, the Length from the later measurement.
Read the results: time between measurements, total weight change, weight gain per day and per week, length gain per month, and a short interpretation against typical WHO-based ranges.

What Is Infant Growth Velocity?

Growth velocity is the speed at which a baby grows — grams per day for weight, centimeters per month for length. A single weighing only tells you how big the baby is now; velocity tells you whether growth is on track between visits. Two babies can sit at the same percentile while growing at very different rates, which is why pediatricians look at both the trend and the absolute value.

Velocity is especially useful in the first weeks and months, when weight changes are fast and small absolute differences matter. For healthy term infants, WHO growth standards suggest typical weight gains of about 25–30 g/day in months 0–3, 15–20 g/day in months 3–6, 10–13 g/day in months 6–9 and 7–9 g/day in months 9–12. Length typically increases by about 3–4 cm/month in months 0–3, then about 2 cm/month in months 3–6 and 1.2–1.5 cm/month in months 6–12.

When you interpret your result, compare it with the range for your baby's age bracket, not with a single fixed number. A rate of 20 g/day is excellent for a 5-month-old but would be low for a 2-week-old. Also remember that a healthy rate over one interval does not need to be maximal — steady, sustained growth along the baby's own curve is the goal.

Assumptions, Limitations and Practical Tips

Use intervals of at least 2–4 weeks. Over very short intervals, normal daily fluctuations (feeding, wet diapers, scales differences) can dominate the result and make it misleading.
Weigh consistently. Use the same scale when possible, at a similar time of day, without clothes or diaper. Differences between clinic and home scales can easily reach 50–100 g.
Length is harder to measure than weight. Length velocity from two measurements is only meaningful if both were taken carefully, head-to-heel, with the baby lying flat.
The calculator assumes term infants. For premature babies, growth targets depend on corrected age, so discuss the numbers with your pediatrician.
A negative result (weight loss) is common in the first days of life; see our Newborn Weight Loss Calculator for that stage. Later, small dips during illness can happen, but continued decline deserves a prompt check-in with your pediatrician.
This calculator does not replace growth charts or medical advice. Your pediatrician plots weight and length on WHO/CDC percentile curves at every visit — see also our Child Height and Weight chart for the bigger picture.
If you are unsure whether feeding volumes are adequate, the Infant Feeding Volume Calculator can help you check typical intake for your baby's weight.

Frequently Asked Questions

How do I calculate my baby's growth velocity by hand?

Subtract the earlier weight from the later one, then divide by the number of days between the two weighings. For example, a gain of 420 g over 14 days gives \( 420 / 14 = 30 \) g/day. Multiply by 7 for a weekly rate. For length, divide the change in centimeters by the number of months elapsed (days divided by 30.44). Always convert pounds to grams first: 1 lb = 453.592 g.

How much weight should a baby gain per week?

Healthy term babies typically gain about 175–210 g/week in the first 3 months, 105–140 g/week from 3 to 6 months, 70–90 g/week from 6 to 9 months, and 50–65 g/week from 9 to 12 months. These are averages, not strict cutoffs — a baby who is feeding well, alert and growing steadily along their own curve may be fine even outside them. Bring persistent low rates to your pediatrician.

Can I use this calculator for a premature baby?

Only with caution. The typical ranges shown assume full-term infants. For preemies, growth targets are usually based on corrected (adjusted) age — weeks or months counted from the original due date, not the birth date — and hospital follow-up programs often use different velocity targets, sometimes expressed in g/kg/day. Use the corrected age bracket when comparing, and discuss the numbers with your neonatologist or pediatrician.

Why is my result negative, and should I worry?

A negative value means the later weight was lower than the earlier one. Mild weight loss is normal in the first days after birth, when babies can lose 5–10% of birth weight before regaining it by about two weeks. Later on, small dips can happen during illness, teething or a feeding change. However, continued or repeated decline between visits deserves a prompt call to your pediatrician — don't wait for the next scheduled checkup.

How do I convert pounds and ounces to grams for the calculation?

Multiply pounds by 453.592, and ounces by 28.35, then add the two. A baby weighing 8 lb 4 oz equals \( 8 \times 453.592 + 4 \times 28.35 \approx 3799 \) g. For length, multiply inches by 2.54 to get centimeters. If you weigh in pounds and ounces at home, rounding to the nearest half ounce keeps the error small — each ounce is only about 28 g.

Is a one-week gap between weighings long enough?

It is technically possible, but not recommended. Normal daily variation — a full stomach, a wet diaper, a different scale, time of day — can shift a baby's weight by 50–100 g or more, which over just 7 days can distort the daily rate substantially. Intervals of 2–4 weeks or longer average out these fluctuations. The longer the interval, the more trustworthy the velocity figure.

What is the difference between growth velocity and percentile?

Percentile tells you how your baby's size compares with other babies of the same age at a single moment; velocity tells you how fast your baby is growing between two points in time. They answer different questions: a baby at the 20th percentile gaining steadily is usually healthier than a baby at the 60th whose growth has flattened. Pediatricians look at both together, because crossing percentile curves often shows up first as a change in velocity.