Fixture Unit Calculator

Count every fixture in your plumbing system including toilets, sinks, showers, tubs, washing machines, dishwashers, and hose bibs. Include both existing fixtures and any planned additions. Specify whether toilets use tank or flush valve mechanisms as these have different FU values.

Use the Add Fixture button to create entries for each fixture type. Select the appropriate fixture from the dropdown menu, which automatically assigns correct supply and drainage FU values from IPC tables. Enter the quantity if you have multiple identical fixtures like several bathroom sinks.

Check that each fixture shows correct supply and drainage FU values. The calculator displays per-fixture values and total multiplication for quantities. Supply and drainage FUs differ for most fixtures, so both numbers matter for complete system sizing and code compliance.

Click Calculate System Requirements to total all fixture units. The tool sums supply FUs separately from drainage FUs, then applies Hunter's Curve approximation to estimate actual peak GPM demand. This conversion accounts for simultaneous usage probability rather than assuming maximum flow from all fixtures at once.

Use the recommended minimum pipe sizes for your water supply main and building drain. These values come from IPC sizing tables matched to your total FU load. Remember these are minimums for the main runs - branch lines and individual fixture supplies may use smaller diameters per code.

Fixture units prevent both undersizing and oversizing plumbing systems. Adding up maximum GPM flows for all fixtures assumes everything runs simultaneously, which never happens in real use and leads to wastefully oversized pipes. FU calculations apply statistical probability that only some fixtures operate at once, determining realistic peak demand. This ensures adequate capacity during actual usage patterns without spending extra on unnecessarily large pipes and fittings.

Building codes require fixture unit calculations for permit approval on new construction and major renovations. Inspectors verify that pipe sizes match FU loads according to IPC or UPC tables. Submitting plans without proper FU calculations results in permit delays or rejections. Using standardized FU values also provides legal protection, demonstrating you followed accepted engineering practices rather than guessing at pipe sizes or copying existing installations.

Proper FU-based sizing saves money on materials and labor. Oversized pipes cost more to purchase and require larger fittings, hangers, and penetrations through framing. A 2-inch main line costs nearly double what 1-1/2-inch costs when FU calculations show the smaller size handles your actual load. Conversely, undersizing causes expensive callbacks to replace inadequate pipes when pressure drops and slow drainage problems emerge after construction completes.

FU totals reveal how much capacity remains for future fixture additions. If your current load totals 35 supply FU with 1-1/2-inch main rated for 40 FU, you can only add 5 more FU before needing larger piping. This helps plan bathroom additions or kitchen remodels by showing whether existing infrastructure can support new fixtures or if main line replacement becomes necessary. Knowing capacity limits prevents expensive surprises during later renovation projects.