A clear, standardised way to understand and compare the absorbency of period underwear.
FlowGrade© is our own absorbency rating, created so you can choose the right level of protection with confidence. Rather than vague promises, FlowGrade© measures how much liquid each pair can hold under consistent, repeatable test conditions, then turns that figure into a simple grade you can read at a glance.
Every Piper style is tested the same way, which means our Light, Moderate, Heavy and Super ratings actually mean something. You can compare across our range, and trust that the rating reflects real, measured performance, not marketing language.
FlowGrade© exists because absorbency claims in the period underwear industry have historically been inconsistent. We wanted our customers to have an honest, comparable reference point, so the protection you expect is the protection you get.
Everyone's flow is different.
FlowGrade© is a guide to how much fluid each product is designed to absorb, based on our testing. Your experience may vary depending on your body, your cycle and how you wear it, so use the rating as a guide and change your underwear as often as you need.
Each style is measured under the same repeatable conditions for an honest comparison.
Results are translated into simple grades, Light, Moderate, Heavy and Super.
The rating reflects real, measured absorbency, so you can choose with confidence.
Each grade corresponds to a measured volume of liquid absorbed under controlled test conditions. The table below shows the functional performance classification and its interpretation.
| FlowGrade© | Functional performance | Interpretation |
|---|---|---|
| FG1 | <2 mL | Very low |
| FG2 | 2–3.9 mL | Low |
| FG3 | 4–5.9 mL | Light |
| FG4 | 6–7.9 mL | Light–moderate |
| FG5 | 8–9.9 mL | Moderate |
| FG6 | 10 mL | Strong |
| FG7 | 11–14.9 mL | High |
| FG8 | 15–19.9 mL | Very high |
| FG9 | 20–24.9 mL | Exceptional |
| FG10 | ≥25 mL | Extreme |
For more detail on the FlowGrade© standard, including the methodology behind it, visit the dedicated FlowGrade website.
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