Pattern Adjustment Ratio (P.A.R.) Calculator for Stretch Fabrics and Pattern Drafting
Use our free Pattern Adjustment Ratio (P.A.R.) calculator to scale your sewing patterns measurments for stretch fabrics. Enter your body measurements and desired stretch values below to get instant results.
When drafting a stretch garment, you’ll calculate two P.A.R. values: one for Width and one for Length.
Note: P.A.R. is the same for inches and centimeters — the units cancel out, so use whichever you prefer.
P.A.R. = Fabric Stretch Measurment ÷ Body Measurment
P.A.R. Width Calculator
P.A.R. Length Calculator
How to Find Your Desired Stretch
Before using the calculator, you need to test your fabric and record the desired stretch. This is the measurement of the fabric when stretched to a comfortable tension.
Width (Circumference)
- Measure your body’s bust circumference. Wrap a tape measure around the fullest part of your chest, just under the armpits. Write this down — this is your body bust measurement. Example: 43 inches.
- Cut a test strip of your stretch fabric. It should be about 6 inches tall (up and down) by the length of your bust circumference measurement. Example: 6 inches × 43 inches.
- Make sure the direction with the greatest stretch runs horizontally across the strip.
- Pin one edge of the fabric at your side seam, then wrap it around your bust, front and back.
- Pull the fabric until it feels snug but still comfortable. This is your desired tension.
- Mark and pin the point where the free edge of the fabric meets the side seam pin.
- Remove the fabric and measure the distance between the pinned edge and your tension pin, without stretching. This is your desired bust stretch. Example: 33 inches.
Length (Vertical)
- Measure your back length from the center back of your neck to your natural waistline. Write this down. This is your body back length measurement. Example: 18 inches.
- Cut a test strip of your stretch fabric. It should be about 6 inches wide (side to side) by the length of your back measurement. Example: 6 inches × 18 inches.
- Orient the strip so the least stretchy direction runs vertically from neck to waist. (The garment should stretch more side-to-side than up-and-down.)
- Pin the top of the strip at the back neck point, then gently pull it down toward the waist.
- Pull only until the fabric feels smooth and comfortable. Do not pull to maximum stretch. This is your desired tension for length.
- Mark the point on the fabric where it reaches the waist at that comfortable tension.
- Remove the fabric and measure the distance from the neck pin to the waist mark without stretching. This is your desired back length stretch. Example: 13 inches.
FAQs - Pattern Adjustment Ratio (P.A.R.)
What is Pattern Adjustment Ratio (P.A.R.)?
The Pattern Adjustment Ratio (P.A.R.) is the percentage used to scale sewing patterns when working with stretch fabrics. Instead of drafting patterns at full body size, you apply a ratio that accounts for fabric stretch to ensure a perfect fit.
How do I calculate P.A.R.?
The formula is simple: P.A.R. = Desired Measurement ÷ Body Measurement
Example:
- Body bust circumference = 43.5"
- Desired stretch bust = 33"
- P.A.R. = 33 ÷ 43.5 = 0.76 (76%)
How do I apply P.A.R. to patterns?
Once you know your P.A.R., apply it to horizontal and vertical measurements of your pattern.
B–C = (Bust Circumference ÷ 2) × P.A.R.
Example: (43.5 ÷ 2) × 0.76 = 16.5"
Why should I use P.A.R.?
- Ensures stretch garments fit correctly without gaping or sagging.
- Adjusts both width and length for true-to-body proportions.
- Saves time compared to trial-and-error fitting.
What are the average P.A.R. values for different stretch fabrics?
The ratio depends on your fabric’s stretch percentage — there isn’t one universal number. But here are the common ranges used in practice:
- Stable knit / light stretch (10–20% stretch): P.A.R. ≈ 95–98% of body measurement (very close to body size).
- Medium stretch (25–40% stretch): P.A.R. ≈ 90–95% of body measurement.
- High stretch (50–75% stretch, like spandex/lycra): P.A.R. ≈ 80–90% of body measurement.
- Super stretch (80%+ stretch, dancewear/swimwear): P.A.R. ≈ 70–80% of body measurement.
👉 In other words, most average P.A.R. values for fashion and costume sewing fall between 80%–95%, depending on the fabric.
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