Free Plumbing Calculators — IPC Supply, Drainage & Cost
Size a system end to end without opening the code book. On the pressure side: fixture units into peak demand, then the pipe that carries it — checked against the velocity ceiling and the pressure budget, because a pipe that passes one can still fail the other. On the gravity side: drainage fixture units, the drain the code allows, the fall it needs, and the vent that keeps the traps sealed. Then size the water heater and price the job. Every tool is free, instant, and shows which rule governed its answer.
Code baseline: IPC 2021. The IPC is a model code. Roughly fifteen states use the UPC or a derivative such as California's CPC instead, where fixture-unit values and sizing tables differ — and local amendments override either one. Results are for estimating and planning; a licensed plumber and your AHJ have final say on anything installed.
Water Supply & Pipe Sizing
Work out the demand, then size the pipe that carries it. Start at fixture units and finish with a nominal size that satisfies both velocity and pressure.
Water Supply Fixture Unit Calculator
Add up the water supply fixture units for a building and convert them to probable peak demand in gpm — the number every supply pipe is sized from.
Answers: "How many fixture units is my house?"
Open calculatorWater Pipe Size Calculator
Size a water supply line from peak demand, available pressure, and run length — checked against both the velocity ceiling and the friction budget.
Answers: "What size water line do I need?"
Open calculatorPipe Velocity Calculator
Water velocity in ft/s for any flow and pipe size, against the 8 ft/s cold and 5 ft/s hot limits that keep a system quiet and erosion-free.
Answers: "How fast is water moving in a 3/4 inch pipe?"
Open calculatorPipe Friction Loss Calculator
Hazen-Williams pressure loss per 100 ft for copper, PEX, CPVC, PVC, and galvanized steel, plus the pressure left over for the run.
Answers: "How much pressure do I lose over 100 feet?"
Open calculatorDrainage, Waste & Vent
Gravity side of the system. Total the drainage fixture units, size the drain, set the fall, and vent it so the traps hold their seal — then deal with what leaves the roof, whether you shed it or keep it.
Drainage Fixture Unit Calculator
Total the drainage fixture units for any set of fixtures using IPC Table 709.1, with the minimum trap size for each one.
Answers: "How many DFU is a bathroom group?"
Open calculatorDrain Pipe Size Calculator
Size a building drain, sewer, horizontal branch, or stack from its DFU load using IPC Tables 710.1(1) and 710.1(2).
Answers: "What size drain pipe do I need?"
Open calculatorDrain Pipe Slope Calculator
Minimum code slope by pipe size, total fall over any run, and the percent grade — plus what happens when a drain is laid too flat or too steep.
Answers: "What slope does a drain pipe need?"
Open calculatorVent Pipe Size Calculator
Individual and branch vent size per IPC 906.2, with the Table 909.1 maximum distance from the trap weir to the vent fitting.
Answers: "What size vent pipe do I need?"
Open calculatorStorm & Roof Drainage Calculator
Design runoff in gpm from roof area and rainfall rate, with a Manning capacity check on the horizontal storm drain.
Answers: "How much water comes off my roof in a storm?"
Open calculatorSeptic Tank Size Calculator
Daily design flow, tank capacity and leach field area from bedroom count and soil percolation rate — with every local variable left editable.
Answers: "What size septic tank do I need?"
Open calculatorGrease Trap Size Calculator
Size an under-sink hydromechanical interceptor in gpm from the sink, or a gravity grease interceptor in gallons from the kitchen — two devices, two methods.
Answers: "What size grease trap do I need?"
Open calculatorRainwater Harvesting & Grey Water Calculator
Cistern size from catchment area, rainfall and dry-spell length, plus grey water yield and the landscape it irrigates — two sources, two sizing rules.
Answers: "What size rainwater tank do I need?"
Open calculatorPressure & Pump Head
Pressure is head and head is pressure. Convert between them, find what is left at the top fixture, and size a pump against the total it has to lift.
Water Pressure Calculator
Convert psi to feet of head and back, subtract the elevation loss to the highest fixture, and check whether IPC 604.8 requires a pressure-reducing valve.
Answers: "How much pressure do I lose going up a storey?"
Open calculatorPump Head Calculator
Total dynamic head for a sump, well, or booster pump — static lift, friction head, pressure head, and velocity head added up properly.
Answers: "What head does my pump need?"
Open calculatorWell Pressure Tank Calculator
Drawdown, minimum pump run time, and the tank that buys it — plus why a 20-gallon shell only ever gives up about six gallons.
Answers: "What size pressure tank do I need?"
Open calculatorWater Heating
Storage and tankless sized the way they actually fail — on peak-hour demand for a tank, and on temperature rise for a tankless.
Water Heater Size Calculator
Peak-hour hot water demand against first-hour rating and recovery rate, to land on the tank size that actually keeps up.
Answers: "What size water heater do I need?"
Open calculatorTankless Water Heater Calculator
The flow a tankless unit can hold at your temperature rise, and the BTU input a target flow needs — the calculation cold climates get wrong.
Answers: "What size tankless water heater do I need?"
Open calculatorThermal Expansion Tank Calculator
Size the expansion tank a closed system needs, from the water's own thermal expansion and the pressure window the tank has to work inside.
Answers: "What size expansion tank do I need?"
Open calculatorMixing Valve Calculator
Split a tempered flow into its hot and cold parts, and see how much further storing hot and tempering down stretches a water heater.
Answers: "How much hot water does a 120 degree mix actually use?"
Open calculatorVolume & Conversions
The utility drawer. How much water a run of pipe holds, what a leak is quietly costing, and every unit a plumbing job gets quoted in, converted both ways.
Pipe Volume Calculator
Gallons, litres and weight held in any run of pipe — and how long the tap runs before the water in it has been pushed out.
Answers: "How many gallons are in my pipe?"
Open calculatorLeak & Water Waste Calculator
Turn a drip rate, a container fill test or a water meter reading into gallons per year and what it costs — including the energy on a hot-water leak that most calculators ignore.
Answers: "What is this leak actually costing me?"
Open calculatorPlumbing Unit Converter
Flow, pressure, head, volume, diameter, velocity and temperature converted between US and metric units, with the exact factors.
Answers: "How many litres per minute is 10 gpm?"
Open calculatorCost & Business
What the job is worth and what it costs you to show up. Job pricing, the two biggest replacement tickets, and the rate that has to cover everything.
Plumbing Estimate Calculator
Price a plumbing job from labour hours, fixtures, and material, with overhead and the markup-versus-margin difference shown side by side.
Answers: "How do I price a plumbing job?"
Open calculatorRepipe Cost Calculator
Whole-house repipe cost by floor area, pipe material, wall access, and storeys — the access factor moves the total more than the pipe does.
Answers: "How much does it cost to repipe a house?"
Open calculatorWater Heater Replacement Cost Calculator
Installed cost to replace a water heater — tank or tankless, gas or electric — including the code upgrades that catch people out.
Answers: "How much does a new water heater cost installed?"
Open calculatorPlumbing Labor Rate Calculator
Turn a plumber's wage into the hourly rate you have to bill — payroll burden, overhead, and billable utilisation, which matters most of all.
Answers: "What should I charge per hour as a plumber?"
Open calculatorRunning a gas line?
Gas piping reads the same IFGC table whether a plumber or an HVAC tech sizes it, so rather than keep two copies the Gas Line Sizing Calculator is shared. Feed it the total appliance load in BTU/h and the longest run and it returns the pipe size and its capacity. There are 43 HVAC calculators and 39 electrical calculators alongside these — useful when a water heater needs a circuit or a boiler needs a load calculation.
More trades on the way
The plumbing set is complete — every calculator and every guide this category planned is built, from pipe sizing through to permits and estimating. Calculators for these trades are also planned:
Coming soon
Which calculator do I need?
Match your question to the right tool. A supply job runs Fixture units → Demand → Pipe size. A drainage job runs DFU → Drain size → Slope → Vent. If you already know the gpm, start at pipe size; if you already know the DFU, start at drain size.
| You're asking… | Use this tool | The rule it's built on |
|---|---|---|
| How many fixture units is this building? | Water Supply Fixture Units | Add the WSFU weights, then read peak gpm off the demand curve |
| What size water line do I need? | Water Pipe Size | Smallest size that passes BOTH the velocity cap and the friction budget |
| Is my pipe going to be noisy? | Pipe Velocity | V = 0.4085 x gpm / ID squared; keep under 8 ft/s cold, 5 ft/s hot |
| How much pressure will the run cost me? | Friction Loss | Hazen-Williams psi per 100 ft, then multiply by developed length |
| How many DFU do these fixtures add up to? | Drainage Fixture Units | IPC Table 709.1 — a bathroom group is 5 DFU at 1.6 gpf |
| What size drain or sewer do I need? | Drain Pipe Size | DFU into Table 710.1(1) for drains, 710.1(2) for branches and stacks |
| How much fall does this drain need? | Drain Pipe Slope | 1/4 in/ft up to 2-1/2 in, 1/8 in/ft for 3 to 6 in, 1/16 in/ft at 8 in |
| What size vent, and how far can the trap sit? | Vent Pipe Size | Half the drain diameter, minimum 1-1/4 in; trap arm capped by Table 909.1 |
| Do I need a pressure-reducing valve? | Water Pressure | 0.433 psi per foot of lift; IPC 604.8 requires a PRV above 80 psi |
| What head does the pump have to make? | Pump Head | TDH = static lift + friction + pressure + velocity head |
| What size water heater do I need? | Water Heater Size | Peak-hour demand against first-hour rating, not against tank gallons |
| Will a tankless keep up in winter? | Tankless Water Heater | gpm = BTU/hr x efficiency / (500 x temperature rise) |
| What should I quote for this job? | Plumbing Estimate | Direct cost + overhead, then divide by (1 - margin), never multiply |
| What does a whole-house repipe cost? | Repipe Cost | Dollars per square foot, driven mostly by how open the walls are |
| What does a water heater swap cost? | Water Heater Replacement Cost | Unit + labour + the code upgrades the inspector will want |
| What hourly rate do I need to charge? | Plumbing Labor Rate | Wage + burden + overhead, divided by BILLABLE hours, then by margin |
| What size expansion tank does this heater need? | Thermal Expansion Tank | System volume x expansion / (1 - precharge / max), both absolute |
| How much hot water does a tempered mix use? | Mixing Valve | Hot fraction = (mix - cold) / (hot - cold) |
| How many gallons does this pipe hold? | Pipe Volume | gal per ft = 0.0408 x ID squared, from pi/4 x d squared x 12 / 231 |
| What is this leak actually costing me? | Leak & Water Waste | Drips to gallons at 15,140 per gallon, then water, sewer and the energy on a hot leak |
| How many litres per minute is that in gpm? | Plumbing Unit Converter | Flow, pressure, head, volume, diameter, velocity and temperature |
| How much runoff comes off this roof? | Storm & Roof Drainage | gpm = roof area x rainfall rate / 96.25, derived not quoted |
| What size septic tank and leach field? | Septic Tank Size | Bedrooms x daily flow, 2-day retention, field = flow / soil rate |
| What size grease trap does this kitchen need? | Grease Trap Size | Sink volume into gpm, or seats and meals into gallons |
| What size well pressure tank do I need? | Well Pressure Tank | Pump gpm x run minutes, divided by the Boyle's-law drawdown |
| What size rainwater tank do I need? | Rainwater Harvesting & Grey Water | The smaller of the dry spell and what the roof can refill |
Every plumbing formula on one page
Rather than repeat them here, every formula behind these tools lives on one reference page — each with what every variable means, the values you should expect back, and the IPC section it comes from. Four more reference pages sit alongside it: the same formulas written for Excel, the Google Sheets build of the same thing, the vocabulary, and the one subject where getting the device wrong is a health risk rather than a callback.
Plumbing Formula Reference Water supply fixture units to gpm, pipe velocity, Hazen-Williams friction loss, available pressure, developed length, drainage fixture units, minimum slope, Manning's equation, vent and trap arm sizing, pressure and head, total dynamic head, recovery rate, first-hour rating, tankless flow, mixing-valve ratio, and gas load in CFH. Open the reference →These reproduce IPC tables and standard hydraulic formulas, but they cannot see your existing piping, water quality, fixture selection, or local amendments. Confirm final design with a licensed plumber and the edition of the code your jurisdiction actually enforces.
Frequently asked questions
About the calculators, which code edition they follow, and what's next.
Are these plumbing calculators free?
Yes. Every one of them runs in your browser, needs no sign-up, and stores nothing. They exist because we build estimating software for trades and would rather be useful to plumbers before we ever ask for anything.
Which calculator should I start with?
For a water supply job the order is fixture units → peak demand → pipe size, so start with the Water Supply Fixture Unit Calculator. For a drainage job it is DFU → drain size → slope → vent, starting with the Drainage Fixture Unit Calculator. The "Which calculator do I need?" table above maps a question to a tool directly.
Which plumbing code are these based on?
The 2021 International Plumbing Code, and every page names the table or section it uses. That matters because the IPC is a model code — roughly fifteen states use the UPC or a UPC derivative such as California's CPC instead, and fixture-unit values and sizing tables differ between them. Confirm the edition your jurisdiction actually enforces before you pull a permit. The Plumbing Formula Reference lists every formula with its citation.
How accurate are they compared with a full design?
Accurate enough to size a residential or light-commercial job and to sanity-check someone else's numbers, and they show their working so you can see exactly what governed. They are not a substitute for a stamped design on a large or unusual building, and they deliberately say so where the method has limits — the vent calculator, for instance, handles individual and branch vents but hands multi-storey vent stacks back to IPC Table 906.1.
Do you have calculators for other trades?
Yes — there are free HVAC calculators and electrical calculators built the same way, on ACCA and NEC data respectively. Carpentry, painting, and cleaning are still to come.
Why does gas piping sit under HVAC?
Because it is the same IFGC table either trade reads. Rather than build a second copy, the Gas Line Sizing Calculator is shared, and the gas-load formula on the plumbing formula reference links straight to it.
Trusted Plumbing Standards & Resources
Every fixture unit value, sizing table, pipe dimension, and velocity limit on these pages traces back to a published standard. These are the organisations that write them and the data behind the sizing figures.
The model code these calculators are built on. Chapter 6 covers water supply, Chapter 7 sanitary drainage, Chapter 9 vents, and Appendix E water pipe sizing.
The other model code. Roughly fifteen states use the UPC or a derivative such as California's CPC, and its fixture-unit values and sizing tables differ from the IPC's.
Source of the velocity design limits and the Hunter's-curve demand method. The IPC requires a system free of noise and erosion without naming a number; ASPE names it.
Pipe dimensional standards: B88 for copper tube, F876 for PEX, D1785 for Schedule 40. Every inside diameter here is computed from these, not transcribed.
The first-hour-rating worksheet and per-use hot water figures behind the peak-hour demand model in the water heater calculator.
Copper tube handbook data — wall thicknesses, friction-loss charts, and the erosion-corrosion velocity guidance that sets the 5 ft/s hot-water limit.
Calculators size the system. TradesQuote prices the job.
Once the pipe size, the fall, and the vent are settled, the real work is the quote. Describe the job or upload photos, and TradesQuote's AI builds a detailed line-item estimate — fixtures, pipe, fittings, and labor with quantities, unit prices, and totals — validated by a built-in quality control agent and ready for your client to sign online.
AI line-item estimates
Quantities, unit prices, and totals generated instantly.
Knowledge base
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