Calculate and track your baby's growth velocity accurately. Monitor daily grams gained, percentile trends, and overall health progress with our professional baby weight gain calculator.
Metric (kg/grams)
Imperial (lbs/oz)
Enter the baby's birthday.
Please enter a valid birth date.
Weight at birth.
Please enter a positive weight.
Defaults to today.
Current date must be after birth date.
Weight measured today.
Please enter a positive weight.
Average Daily Weight Gain
— g/day
(– oz/day)
Formula: (Current Weight – Birth Weight) ÷ Age in Days
—Total Gain
—Age (Days)
—% Change from Birth
Projected weight if current daily gain rate continues for next 4 weeks.
Timeline
Projected Weight (Metric)
Projected Weight (Imperial)
Total Gain Projected
Enter data to see projections
What is a Baby Weight Gain Calculator?
A baby weight gain calculator is a specialized tool designed for parents, pediatricians, and lactation consultants to monitor an infant's growth velocity. Unlike a simple scale, this calculator analyzes the rate of change in your baby's mass over a specific period, providing critical insights into nutritional health and development.
Ensuring an infant is gaining weight at a healthy pace is one of the primary indicators of well-being in the first year of life. This tool helps quantify that progress, distinguishing between normal physiological weight loss immediately after birth and the rapid growth phases that follow.
This tool is essential for parents concerned about breastfeeding adequacy, formula intake, or recovery from illness. However, it is not a substitute for professional medical advice. Always consult with a healthcare provider regarding specific growth concerns.
Baby Weight Gain Calculator Formula
The core mathematics behind the baby weight gain calculator involves determining the "growth velocity," typically expressed in grams per day (g/day). This metric smooths out day-to-day fluctuations (due to feeding or elimination) to show the overall trend.
The Formula:
Average Daily Gain = (Current Weight – Birth Weight) / (Current Date – Birth Date)
To calculate the Percentage Change, which helps identify the initial weight loss phase (commonly up to 10% in the first week):
Scenario: Baby Liam was born weighing 3.5 kg. At 14 days old, he weighs 3.6 kg. His parents want to know if he has regained his birth weight and what his daily gain is.
Interpretation: While the daily average over 14 days is low (~7g/day), the critical factor is that he has surpassed his birth weight. This indicates he lost weight initially (normal) and is now on the upswing.
Example 2: 2-Month Old Growth Spurt
Scenario: Baby Emma was 3.2 kg at birth. Now, at 60 days old, she weighs 5.1 kg.
Total Gain: 5.1 kg – 3.2 kg = 1.9 kg (1900 g)
Time Elapsed: 60 days
Calculation: 1900 / 60 = 31.6 g/day.
Interpretation: A gain of ~31g per day is excellent and aligns with the upper end of the WHO standards for infants aged 0-3 months, suggesting robust health and effective feeding.
How to Use This Baby Weight Gain Calculator
Getting accurate results requires precise inputs. Follow these steps to utilize the baby weight gain calculator effectively:
Select Your Units: Toggle between Metric (kg/g) and Imperial (lbs/oz) using the dropdown menu at the top.
Enter Birth Details: Input the exact date of birth and the recorded birth weight. If using imperial, ensure ounces are accurate.
Enter Current Details: Input today's date (or the date of the weigh-in) and the current weight. Ensure the scale was tared correctly (zeroed out) before weighing the baby.
Analyze Results:
Look at the Average Daily Gain. Compare this to standard guidelines (typically 20-30g/day for newborns).
Check the Total Gain to see absolute growth.
Use the chart to visualize the trajectory. A steep upward line generally indicates rapid growth.
Use the "Copy Results" button to save the data for your pediatrician appointments.
Key Factors That Affect Baby Weight Gain Results
Several variables influence the numbers you see on a baby weight gain calculator. Understanding these provides context to the raw data.
Feeding Method: Breastfed babies and formula-fed babies grow at different rates. Formula-fed infants may gain weight slightly faster after the first 3 months compared to breastfed infants.
Genetics: Parental size plays a significant role. Larger parents often have larger babies who may gain weight in absolute terms faster than smaller babies.
Initial Weight Loss: Almost all newborns lose fluid weight in the first 3-5 days. A calculation done on Day 4 might show a negative gain, which is physiologically normal up to a 10% loss.
Illness or Issues: Reflux, minor infections, or tongue-ties can temporarily slow weight gain.
Output (Diapers): Weight gain is the net result of Intake minus Output. Frequent heavy wet diapers indicate good intake, even if the scale moves slowly.
Measurement Error: Using different scales (e.g., one at the doctor's office, one at home) can introduce discrepancies. Always try to use the same scale under similar conditions (e.g., naked weight).
Frequently Asked Questions (FAQ)
What is considered normal weight gain for a newborn?
According to general guidelines, infants typically gain 140–200 grams (5–7 ounces) per week during the first 3 months. This averages to about 20–30 grams per day.
When should a baby regain their birth weight?
Most healthy full-term infants regain their birth weight by 10 to 14 days of age. If your baby has not regained birth weight by 2 weeks, consult a pediatrician.
Why does the baby weight gain calculator show a negative number?
If the baby is less than a week old, a negative number reflects physiological weight loss (loss of excess fluid). If the baby is older and losing weight, medical attention is required immediately.
Can a baby gain too much weight?
While rapid weight gain is generally positive in infancy, excessive gain relative to length (crossing multiple percentile lines rapidly) can be discussed with a doctor. However, breastfed babies are rarely considered "overweight."
How often should I weigh my baby?
Unless advised otherwise by a doctor, weighing once a week or once a month is sufficient. Daily weighing can be misleading due to fluid fluctuations.
Does this calculator work for premature babies?
This calculator provides raw math based on dates. Premature babies have different growth charts (Fenton charts) and "adjusted age" should be considered for developmental milestones.
How does solid food affect weight gain?
Starting solids (around 6 months) changes the diet composition. Weight gain often slows slightly as the baby becomes more active (crawling/walking), dropping to ~85-140g per week.
Is the baby weight gain calculator accurate?
The math is precise based on your inputs. However, the accuracy depends on the quality of the scale used to measure the weight. Household scales are often less accurate than medical scales.
Related Tools and Internal Resources
Explore more tools to support your parenting journey:
// Global variable for chart instance
var chartInstance = null;
// — Helper Functions —
function getVal(id) {
var el = document.getElementById(id);
return el ? parseFloat(el.value) : NaN;
}
function setHtml(id, val) {
document.getElementById(id).innerHTML = val;
}
// Convert Lbs/Oz to Kg
function imperialToKg(lbs, oz) {
var totalLbs = lbs + (oz / 16);
return totalLbs * 0.453592;
}
// Convert Kg to Lbs/Oz string
function kgToImperialStr(kg) {
var totalLbs = kg * 2.20462;
var lbs = Math.floor(totalLbs);
var oz = (totalLbs – lbs) * 16;
return lbs + " lb " + oz.toFixed(1) + " oz";
}
// — Main Logic —
function toggleUnits() {
var system = document.getElementById('unitSystem').value;
var lblBirth = document.getElementById('lbl-birthWeight');
var lblCurr = document.getElementById('lbl-currentWeight');
var metricBirth = document.getElementById('metric-birth-input');
var impBirth = document.getElementById('imperial-birth-input');
var metricCurr = document.getElementById('metric-current-input');
var impCurr = document.getElementById('imperial-current-input');
if (system === 'metric') {
lblBirth.textContent = "Birth Weight (kg)";
lblCurr.textContent = "Current Weight (kg)";
metricBirth.style.display = 'block';
impBirth.style.display = 'none';
metricCurr.style.display = 'block';
impCurr.style.display = 'none';
} else {
lblBirth.textContent = "Birth Weight (Lbs/Oz)";
lblCurr.textContent = "Current Weight (Lbs/Oz)";
metricBirth.style.display = 'none';
impBirth.style.display = 'flex';
metricCurr.style.display = 'none';
impCurr.style.display = 'flex';
}
calculateResults();
}
function calculateResults() {
// 1. Get Inputs
var system = document.getElementById('unitSystem').value;
var birthDateStr = document.getElementById('birthDate').value;
var currentDateStr = document.getElementById('currentDate').value;
var birthWeight = 0;
var currentWeight = 0;
if (system === 'metric') {
birthWeight = getVal('birthWeightKg');
currentWeight = getVal('currentWeightKg');
} else {
birthWeight = imperialToKg(getVal('birthWeightLbs') || 0, getVal('birthWeightOz') || 0);
currentWeight = imperialToKg(getVal('currentWeightLbs') || 0, getVal('currentWeightOz') || 0);
}
// Reset errors
document.getElementById('err-birthDate').style.display = 'none';
document.getElementById('err-currentDate').style.display = 'none';
document.getElementById('err-birthWeight').style.display = 'none';
document.getElementById('err-currentWeight').style.display = 'none';
// Validate
var hasError = false;
var bDate = new Date(birthDateStr);
var cDate = new Date(currentDateStr);
if (!birthDateStr) { hasError = true; }
if (!currentDateStr) { hasError = true; }
if (birthDateStr && currentDateStr) {
if (cDate < bDate) {
document.getElementById('err-currentDate').style.display = 'block';
hasError = true;
}
}
if (isNaN(birthWeight) || birthWeight <= 0) {
// Only show error if input is not empty (user typing)
// But for calculation, we stop.
if (document.getElementById('birthWeightKg').value !== "") {
// document.getElementById('err-birthWeight').style.display = 'block';
}
// For initial load we just return
if(birthWeight 0 ? '+' : ") + weightDiffGrams.toFixed(0) + ' g');
setHtml('result-age', daysDiff + ' days');
setHtml('result-percentChange', percentChange.toFixed(1) + '%');
// Update Projection Table
updateProjectionTable(currentWeight, dailyGain, daysDiff);
// Update Chart
drawChart(birthWeight, currentWeight, daysDiff);
}
function updateProjectionTable(currentKg, dailyGainGrams, currentAgeDays) {
var tbody = document.getElementById('projectionBody');
tbody.innerHTML = ";
var weeksToProject = 4;
for (var i = 1; i <= weeksToProject; i++) {
var daysToAdd = i * 7;
var addedWeightKg = (dailyGainGrams * daysToAdd) / 1000;
var projectedKg = currentKg + addedWeightKg;
var totalGainGrams = (projectedKg – (currentKg – ((dailyGainGrams*currentAgeDays)/1000))) * 1000; // Recalc from birth roughly or from now? Let's do from now.
// Actually request says "Total Gain Projected" usually implies accumulated.
var tr = document.createElement('tr');
var tdTime = document.createElement('td');
tdTime.innerText = '+' + i + ' Week(s)';
var tdMetric = document.createElement('td');
tdMetric.innerText = projectedKg.toFixed(2) + ' kg';
var tdImp = document.createElement('td');
tdImp.innerText = kgToImperialStr(projectedKg);
var tdGain = document.createElement('td');
tdGain.innerText = '+' + (dailyGainGrams * daysToAdd).toFixed(0) + ' g (from today)';
tr.appendChild(tdTime);
tr.appendChild(tdMetric);
tr.appendChild(tdImp);
tr.appendChild(tdGain);
tbody.appendChild(tr);
}
}
function drawChart(birthKg, currentKg, days) {
var canvas = document.getElementById('growthChart');
var ctx = canvas.getContext('2d');
// Handle HiDPI
var dpr = window.devicePixelRatio || 1;
var rect = canvas.getBoundingClientRect();
canvas.width = rect.width * dpr;
canvas.height = rect.height * dpr;
ctx.scale(dpr, dpr);
var width = rect.width;
var height = rect.height;
// Clear
ctx.clearRect(0, 0, width, height);
// Padding
var padding = 40;
var chartW = width – (padding * 2);
var chartH = height – (padding * 2);
// Data Points
// Point 0: (0, birthKg)
// Point 1: (days, currentKg)
// Y Axis Scale (Weight)
// Min should be min(birth, current) – buffer
// Max should be max(birth, current) + buffer
var minW = Math.min(birthKg, currentKg) * 0.95;
var maxW = Math.max(birthKg, currentKg) * 1.05;
var rangeW = maxW – minW;
// X Axis Scale (Days)
var maxDays = Math.max(days, 1);
// Draw Axes
ctx.beginPath();
ctx.strokeStyle = '#dee2e6';
ctx.lineWidth = 1;
// Y Axis
ctx.moveTo(padding, padding);
ctx.lineTo(padding, height – padding);
// X Axis
ctx.lineTo(width – padding, height – padding);
ctx.stroke();
// Helper: Map Data to Coords
function getX(d) {
return padding + (d / maxDays) * chartW;
}
function getY(w) {
return (height – padding) – ((w – minW) / rangeW) * chartH;
}
// Draw Actual Growth Line
ctx.beginPath();
ctx.strokeStyle = '#004a99';
ctx.lineWidth = 3;
ctx.moveTo(getX(0), getY(birthKg));
ctx.lineTo(getX(days), getY(currentKg));
ctx.stroke();
// Draw Points
ctx.fillStyle = '#004a99';
ctx.beginPath();
ctx.arc(getX(0), getY(birthKg), 5, 0, Math.PI * 2);
ctx.fill();
ctx.beginPath();
ctx.arc(getX(days), getY(currentKg), 5, 0, Math.PI * 2);
ctx.fill();
// Reference Line (Standard Gain ~25g/day visual guide)
// This is just a visual reference starting from birth
var refGainKg = (25 * days) / 1000;
var refEndW = birthKg + refGainKg;
if (refEndW = minW) {
ctx.beginPath();
ctx.strokeStyle = '#28a745'; // Green for 'Standard'
ctx.setLineDash([5, 5]);
ctx.lineWidth = 2;
ctx.moveTo(getX(0), getY(birthKg));
ctx.lineTo(getX(days), getY(refEndW));
ctx.stroke();
ctx.setLineDash([]);
// Legend for Reference
ctx.fillStyle = '#28a745′;
ctx.font = '12px Arial';
ctx.fillText("Std Growth (~25g/day)", getX(days/2), getY(birthKg + (refGainKg/2)) – 10);
}
// Labels
ctx.fillStyle = '#212529′;
ctx.font = '12px Arial';
ctx.textAlign = 'center';
// X Labels
ctx.fillText("Birth", padding, height – padding + 15);
ctx.fillText("Day " + days, width – padding, height – padding + 15);
// Y Labels
ctx.textAlign = 'right';
ctx.fillText(maxW.toFixed(2) + "kg", padding – 5, padding + 10);
ctx.fillText(minW.toFixed(2) + "kg", padding – 5, height – padding);
}
function resetCalculator() {
document.getElementById('birthDate').value = ";
document.getElementById('currentDate').valueAsDate = new Date();
document.getElementById('birthWeightKg').value = ";
document.getElementById('birthWeightLbs').value = ";
document.getElementById('birthWeightOz').value = ";
document.getElementById('currentWeightKg').value = ";
document.getElementById('currentWeightLbs').value = ";
document.getElementById('currentWeightOz').value = ";
setHtml('result-dailyGain', '– g/day');
setHtml('result-dailyGain-alt', '(– oz/day)');
setHtml('result-totalGain', '–');
setHtml('result-age', '–');
setHtml('result-percentChange', '–');
var canvas = document.getElementById('growthChart');
var ctx = canvas.getContext('2d');
ctx.clearRect(0, 0, canvas.width, canvas.height);
document.getElementById('projectionBody').innerHTML = '
Enter data to see projections
';
}
function copyResults() {
var gain = document.getElementById('result-dailyGain').innerText;
var total = document.getElementById('result-totalGain').innerText;
var age = document.getElementById('result-age').innerText;
var text = "Baby Weight Gain Calculator Results:\n";
text += "Age: " + age + "\n";
text += "Total Gain: " + total + "\n";
text += "Daily Average: " + gain + "\n";
var dummy = document.createElement("textarea");
document.body.appendChild(dummy);
dummy.value = text;
dummy.select();
document.execCommand("copy");
document.body.removeChild(dummy);
var btn = document.querySelector('.btn-primary');
var original = btn.innerText;
btn.innerText = "Copied!";
setTimeout(function(){ btn.innerText = original; }, 2000);
}
// Initialize defaults
window.onload = function() {
document.getElementById('currentDate').valueAsDate = new Date();
toggleUnits();
};