Plumbing: Calculators & Sizing Guides
8 calculators · 5 diagrams · 5 guides
Plumbing is the trade where the sizing questions arrive before the project does. A water softener is useless if the grain capacity is wrong for your hardness. A basement bathroom fails inspection if the drain slope is off by an eighth of an inch per foot. A well system short-cycles itself to death if the pressure tank is undersized for the pump. And almost every one of those numbers has a published, citable answer — in the International Plumbing Code, the Uniform Plumbing Code, or a manufacturer sizing method — that the average product page never mentions. The calculators in this hub put those published numbers to work on your actual project.
Two things make this hub different from the plumbing calculators scattered around the web. First, the code toggle: the IPC and UPC genuinely disagree on core values — a bathtub counts 1.4 water supply fixture units under the IPC and 4.0 under the UPC — so tools here name the code edition behind every number instead of silently averaging. Second, honesty about what is an authoritative code value versus a manufacturer estimate: recovery rates, groundwater temperatures, and pump curves vary by product, and the results say so rather than pretending one number fits all.
Every plumbing tool here follows the same rules as the rest of the site: the math comes from published code tables and manufacturer methods with sources named, results are sizing references only, and the same advice applies throughout — most plumbing work needs a permit and inspection, water heater replacement requires a permit almost everywhere, and your local plumbing inspector has the final word. Water heater sizing, supply pipe sizing, sump pumps, and septic tanks are all on the roadmap for this hub. Free, no signup.
Plumbing calculators
- Water Softener CalculatorFree water softener sizing calculator — grain capacity from household size and hardness, with ppm conversion, iron correction, and WQA hardness classes.
- Water Heater Size CalculatorFree water heater size calculator using the DOE first-hour-rating method — count your busiest hour's hot-water uses and get the FHR to shop for.
- Tankless Water Heater Size CalculatorFree tankless water heater sizing calculator — peak GPM and temperature rise to the flow rate and BTU/kW you need, with WaterSense and Title 20 presets.
- Sump Pump CalculatorFree sump pump sizing calculator — Total Dynamic Head from static lift, pipe run, and fittings, so you can match the duty point to a pump curve.
- Water Supply Pipe Size CalculatorFree water supply pipe sizing calculator — total water supply fixture units and minimum main size, with the real IPC-vs-UPC fixture-unit difference.
- Septic Tank Size CalculatorFree septic tank size calculator — minimum tank capacity by state and bedroom count from state code, with garbage-disposal adders and formula-state rules.
- Drain Slope CalculatorFree drain slope calculator — minimum fall per foot by pipe size under IPC 704.1 and UPC 708.0, total drop over your run, and DFU capacity checks.
- Well Pressure Tank CalculatorFree well pressure tank calculator — Boyle's Law drawdown from pump GPM and switch settings, acceptance factor, and the tank size that stops short-cycling.
Guides & references
Plumbing · 5 diagrams
- Plumbing
Drain slope by pipe size — smaller pipe needs steeper fall
Smaller pipe needs steeper slope: 1/4" per foot for 2½" and under, 1/8" per foot for 3–6" (IPC 704.1). Too flat strands solids; too steep outruns them.
- Plumbing
Pressure tank drawdown — only about a quarter is usable water
Only the water between cut-in and cut-out is usable — about 28% of the tank on a 40/60 switch. Boyle’s Law: the air cushion does the pushing.
- Plumbing
Sump pump Total Dynamic Head — static lift plus pipe friction
Total Dynamic Head = static lift + pipe friction. Match your flow at that head to a pump curve — the box’s “max GPM” is at zero head.
- Plumbing
A septic system — the tank is only half of it
The tank is only half the system: solids settle and digest in the tank, then liquid disperses through the drainfield — sized separately by a perc test.
- Plumbing
High pressure, the PRV, and the expansion tank
Over 80 psi, a PRV is required to reduce it (IPC 604.8) — and because that closes the system, a thermal-expansion tank is required too (IPC 607.3).
Frequently Asked Questions
What size water softener do I need?
Multiply the people in your household by daily water use (about 82 gallons per person per day, per EPA WaterSense figures) and your water hardness in grains per gallon. That daily grain load, times the days between regenerations plus a safety buffer, gives the capacity class to buy. The water softener calculator runs the full math, including the iron correction most sizing charts skip.
What is the correct slope for a drain pipe?
Under the IPC, pipes 2-1/2 inches and smaller need 1/4 inch of fall per foot, 3-to-6-inch pipes need 1/8 inch per foot, and 8-inch and larger need 1/16 inch per foot. The UPC is stricter — 1/4 inch per foot for all sizes by default, with 1/8 inch allowed on 4-inch-plus pipe only with your inspector’s approval. Too much slope can be as bad as too little: liquids outrun solids and leave them behind in the pipe.
Do IPC and UPC really give different answers?
Yes, materially. The two model codes assign different fixture-unit values to the same fixtures — a clothes washer is 1.4 water supply fixture units under the IPC and 4.0 under the UPC — which changes pipe sizing outcomes. Roughly half the states follow each lineage, so the first question for any supply or drain calculation is which code your state adopted. Tools in this hub carry a code toggle instead of picking one silently.
Can homeowners do their own plumbing work?
It follows the same pattern as electrical: most states let an owner-occupant pull a permit and do their own plumbing on their own single-family home, with required inspections — but a permit is almost always still required, and water heater replacement specifically requires a permit nearly everywhere, including like-for-like swaps in California. Some states exclude new construction from the homeowner exemption. Confirm with your local building department before starting.
Why does my well pump turn on and off so often?
Short-cycling usually means the pressure tank is undersized or has lost its air charge. The tank’s usable drawdown between the cut-in and cut-out pressures is what spares the pump — a 40/60 switch setup only makes about 28% of the tank volume usable, so a 10 GPM pump needs roughly a 36-gallon tank to guarantee a one-minute minimum run time. A well pressure tank calculator doing the Boyle’s-law math for your exact switch settings is next on this hub’s roadmap.