Measurement Tip: Use a flexible tape measure. Wear the undergarments you plan to wear with the finished garment.
Measure around the chest, directly under the armpits and above the full bust.
Measure horizontally around the fullest part of the bust.
A Cup (1″ difference)
B Cup (Standard Commercial – 2″ diff)
C Cup (3″ difference)
D Cup (4″ difference)
Most commercial patterns (Big 4) are drafted for a B cup. Indie patterns vary.
Your Calculated Cup Size:–
Difference from Pattern:–
Total Adjustment Needed:–
Adjustment Per Pattern Piece:–
function calculateBustAdjustment() {
// Get input values
var highBust = parseFloat(document.getElementById('highBust').value);
var fullBust = parseFloat(document.getElementById('fullBust').value);
var patternDiff = parseFloat(document.getElementById('patternCup').value);
// Validation
if (isNaN(highBust) || isNaN(fullBust) || highBust <= 0 || fullBust = fullBust) {
// While medically possible, for sewing patterns usually full bust is larger or equal.
// We will proceed but warn if difference is negative.
}
// Calculate User's Bust Difference
var userDiff = fullBust – highBust;
// Determine User's Sewing Cup Size
var cupLetter = "";
if (userDiff < 0.5) cupLetter = "AA";
else if (userDiff < 1.5) cupLetter = "A";
else if (userDiff < 2.5) cupLetter = "B";
else if (userDiff < 3.5) cupLetter = "C";
else if (userDiff < 4.5) cupLetter = "D";
else if (userDiff < 5.5) cupLetter = "DD/E";
else if (userDiff 0 ? "+" + totalAdjustment.toFixed(2) + "\"" : totalAdjustment.toFixed(2) + "\"";
document.getElementById('calcDiff').innerText = diffText;
// Show Total Adjustment
document.getElementById('calcTotalAdjust').innerText = Math.abs(totalAdjustment).toFixed(2) + "\"";
// Show Half Adjustment (What applies to the paper pattern)
document.getElementById('calcHalfAdjust').innerText = Math.abs(halfAdjustment).toFixed(2) + "\"";
// Determine Advice
var adviceBox = document.getElementById('adjustmentType');
if (Math.abs(totalAdjustment) 0) {
adviceBox.innerText = "You need a Full Bust Adjustment (FBA). Add " + Math.abs(halfAdjustment).toFixed(2) + "\" to your pattern piece.";
adviceBox.style.backgroundColor = "#e8f5e9";
adviceBox.style.color = "#1b5e20";
} else {
adviceBox.innerText = "You need a Small Bust Adjustment (SBA). Remove " + Math.abs(halfAdjustment).toFixed(2) + "\" from your pattern piece.";
adviceBox.style.backgroundColor = "#fff3e0";
adviceBox.style.color = "#e65100";
}
}
Mastering the Full Bust Adjustment (FBA)
Achieving the perfect fit in sewing often comes down to how the fabric drapes over the curves of the body. The most common fitting issue for sewists is the bust area. Commercial sewing patterns are typically drafted for a standard B-cup (a 2-inch difference between high bust and full bust), which rarely matches the diversity of real human bodies. This calculator helps you determine exactly how much bust allowance you need to add or remove to achieve a tailored fit.
Why High Bust Matters
Many beginners choose their pattern size based solely on their Full Bust measurement. However, if your cup size is larger than a B, choosing a size based on full bust will result in the pattern being too large in the shoulders and neckline. The correct method is to:
Choose your pattern size based on your High Bust measurement.
Perform a Full Bust Adjustment (FBA) to add the necessary room strictly in the bust area.
How to Measure for Bust Allowance
High Bust: Wrap the tape measure around your back, under your armpits, and above your actual breast tissue. Keep the tape snug but not tight.
Full Bust: Measure around the fullest part of your bust, keeping the tape parallel to the floor.
Interpreting Your Results
The calculator above provides two critical numbers: the Total Adjustment and the Adjustment Per Pattern Piece.
Since sewing patterns typically represent half of the front bodice (placed on the fold or buttoned), you only apply half of the total difference to the paper pattern. For example, if your bust is 4 inches larger than the pattern standard, the total needed is 4 inches, but you will only add 2 inches to the actual paper pattern piece.
FBA vs. SBA
Full Bust Adjustment (FBA): Required when your bust measurement is larger than the pattern's drafted cup size. You will slash and spread the pattern to add width and length.
Small Bust Adjustment (SBA): Required when your bust measurement is smaller than the pattern's drafted cup size. You will overlap the pattern pieces to reduce fullness.
Using this calculator ensures you aren't guessing with your expensive fashion fabric. Always make a muslin (toile) after altering your paper pattern to verify the fit before cutting into your final material.