Bath tub Drainage Fixture Units (DFU) value is specified as how many DFU?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (B)

DFU values are a drafting rule for sizing drainage systems, not exact flow rates. They assign a standardized demand to each fixture so you can sum them to determine pipe size and venting requirements. A bathtub is given two DFU because its drainage demand, in code tables, is equivalent to two small fixtures rather than just one. So when you’re sizing a line, a single tub contributes 2 DFU to the total. The other numbers correspond to different fixtures (for example, a lavatory is 1 DFU, larger loads would be shown by higher DFU values).