Plumbing Calculators
Pipe volume, drain sizing, water heater selection, and irrigation design tools — built on the Uniform Plumbing Code (UPC) and International Plumbing Code (IPC) reference tables. Currently in development.
Plumbing math hides behind almost every repair and remodel decision. Will a tankless water heater keep up with two simultaneous showers? What size PEX trunk line feeds three bathroom groups? How much water sits in 200 feet of 3/4-inch pipe waiting to be drawn off before hot water reaches the tap? These questions have exact mathematical answers, and the calculators we're building for this category will give them to you instantly — the same way our masonry, drywall, and paint calculators already do.
The roadmap starts with a Water Heater Sizing Calculator that handles both tank-style (First Hour Rating method) and tankless (GPM at temperature rise) sizing for residential applications. After that comes a Pipe Volume Calculator for PEX, copper, CPVC, and PVC; a Drain Waste Vent (DWV) Sizing Calculator based on DFU (drainage fixture unit) loading; and an Irrigation Design Calculator for sprinkler head counts and zone sizing.
Safety Notice
Plumbing work requires a permit and inspection in most jurisdictions. Gas line work, water heater replacements, and any modification to drain-waste-vent systems should be performed by a licensed plumber. Calculators provide reference math only — they do not replace code expertise or local inspection requirements.
Plumbing Calculators — In Development
We're building plumbing calculators with the same rigor as our masonry, carpentry, painting, and landscaping tools — verified against NEC and UPC standards, with full formula transparency and worked examples. The roadmap targets release in Q4 2026.
Expected: Q4 2026
While we build the interactive plumbing calculators, here's a reference for the math most residential plumbing decisions require.
Standard Pipe Sizes & Volumes
- 1/2" PEX: 0.0048 gal/ft internal volume. Max 3 fixture units on hot side.
- 3/4" PEX: 0.0115 gal/ft internal volume. Typical residential hot trunk line.
- 1" PEX: 0.0207 gal/ft internal volume. Whole-house cold trunk.
- 1/2" copper (Type L): 0.0097 gal/ft. Slightly more volume than PEX.
- 3/4" copper (Type L): 0.0225 gal/ft.
- 1" copper (Type L): 0.0400 gal/ft.
- 3" PVC DWV: 0.306 gal/ft. Minimum main stack for typical residence.
- 4" PVC DWV: 0.545 gal/ft. Standard residential sewer lateral.
Water Heater Sizing Reference
- 1 bathroom, 2 people: 30-gallon tank or 3.5 GPM tankless.
- 1.5 bathrooms, 3 people: 40-gallon tank or 4.5 GPM tankless.
- 2 bathrooms, 4 people: 50-gallon tank or 6.5 GPM tankless.
- 3 bathrooms, 5+ people: 75-gallon tank or 8+ GPM tankless (or 2 units in parallel).
- Tankless GPM rule: Match concurrent peak demand (two showers + sink = ~4.5 GPM at 70°F rise).
- First Hour Rating (FHR): Tank capacity × 0.7 + recovery rate. A 50-gallon tank with 40 GPH recovery has FHR = 75 gallons.
Drainage Fixture Unit (DFU) Loading
- Bathroom group (toilet + sink + shower): 4-6 DFU depending on toilet flush.
- Toilet (1.6 GPF): 3 DFU. Toilet (1.28 GPF): 2.5 DFU.
- Lavatory sink: 1 DFU.
- Shower/tub: 2 DFU.
- Kitchen sink: 2 DFU.
- Clothes washer: 3 DFU.
- Dishwasher: 2 DFU.
- 3" drain max: 20 DFU (vertical), 12 DFU (horizontal).
- 4" drain max: 160 DFU (vertical), 70 DFU (horizontal).
Hot Water Recirculation Volume Formula
To calculate how long it takes for hot water to reach a fixture, divide the pipe volume by the flow rate:
wait_time_seconds = (pipe_volume_gallons × 60) / flow_rate_gpm
// 100 ft of 3/4" PEX = 1.15 gal
// At 2 GPM flow: (1.15 × 60) / 2 = 34.5 seconds wait
Pro Tip
When sizing a tankless water heater, always size for the coldest groundwater temperature in your region. Northern US groundwater hits 40°F in winter; a unit rated 5 GPM at 70°F rise delivers only 3 GPM at 80°F rise — barely enough for a single shower. Use the calculator (when ready) with your local groundwater data.