Every repipe bid starts with the same question: what does this house actually demand at peak? Fixture units answer it with math instead of habit — and the rate calculators handle the other half of the business, what to charge.
Fixture units encode how likely fixtures run simultaneously — a toilet is 3 FU, a sink 1. Total the FU, convert to probable peak GPM (a 2.5-bath home lands near 8–10 GPM), and size the line to keep velocity under 8 ft/s. A typical house needs a ¾-inch main.
Flushometer fixtures, low street pressure, long runs, multi-story buildings, and anything permitted for public or commercial use. Fixture-unit charts are the residential first pass; the local code and inspector have final say.
Start from the freelance rate floor: target income plus the van, insurance, tools, and licensing, divided by realistic billable hours (usually 50–65% of the week — the rest is estimates, driving, and callbacks). Most shops underprice the non-billable half of the week.