Water Supply Pipe Size Calculator
This free water supply pipe size calculator sizes a house main the way the plumbing code does — by Water Supply Fixture Units. Each fixture is weighted by how much it draws and how often, those weights are summed, and the total maps to a minimum pipe diameter. The twist most online calculators hide is that the two US model codes disagree, and not slightly: a bathtub counts 1.4 fixture units under the International Plumbing Code and 4.0 under the Uniform Plumbing Code. Your state adopted one or the other, and it can swing your total by 40%.
Count your fixtures, choose your code, and enter the developed length — the actual pipe distance from the meter to the farthest fixture, which matters as much as the fixture count because longer runs lose pressure. The calculator returns your total fixture units and the smallest main that carries them at that length, using the UPC/CPC prescriptive sizing table at a typical 46–60 psi with a ¾-inch meter.
One honest caveat the results make plain: the IPC has no single fixture-unit-to-diameter table. IPC jurisdictions size supply by a performance method (Appendix E — fixture units to GPM to friction loss), so on the IPC setting this prescriptive answer is a conservative cross-check, not the code calculation. Low street pressure, a small meter, or a big elevation change all push toward a larger main — verify your static pressure with a gauge. Free, no signup.
Water Supply Pipe Size Calculator
What size supply main does your house need? Count your fixtures for the total water supply fixture units, then get the minimum pipe size — with the real IPC-vs-UPC fixture-unit difference built in. Free, no signup.
Fixtures on the supply
The UPC counts a bathtub and clothes washer at 4.0 WSFU each; the IPC counts them at 1.4 — a real code difference, so pick the code your state adopted.
Calculation Formulas
Water Supply Fixture Units weight each fixture by its likely simultaneous draw. The per-fixture values differ between the IPC and UPC.
Example:
A 3-bath home ≈ 29 WSFU under UPC values.
The two model codes assign very different values to the same fixtures — bathtubs and clothes washers most dramatically — so the code your state adopted changes the total.
Example:
The same house totals ~21 WSFU (IPC) vs ~29 (UPC).
Outdoor faucets are counted with a diminishing rule rather than a flat per-unit value.
Example:
2 hose bibbs = 2.5 + 1.0 = 3.5 WSFU.
The UPC/CPC prescriptive table gives max WSFU per pipe size at 46–60 psi and a ¾" meter, decreasing with developed length.
Example:
29 WSFU at ~90 ft → 1" main (carries ~36 WSFU there).
Standard Constants
| Constant | Value | Description |
|---|---|---|
| WC (toilet) | 2.2 WSFU (IPC) / 2.5 (UPC) | Gravity tank, 1.6 gpf. |
| Bathtub & clothes washer | 1.4 (IPC) / 4.0 (UPC) | The largest code divergence — nearly 3×. |
| 1" main capacity | 39 WSFU (short) → 19 (300 ft) | UPC/CPC prescriptive, 46–60 psi, ¾" meter. |
| ¾" main capacity | 20 WSFU (short) → 8 (300 ft) | Why most whole-house mains step up to 1". |
| Worked example | 3-bath, ~29 WSFU → 1" | The research anchor this calculator reproduces. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
Uniform Plumbing Code(UPC/CPC Table 610.3 + water-pipe-sizing chart)
View StandardThe IAPMO model code — provides both the fixture-unit values and the prescriptive WSFU→diameter table this calculator sizes from.
Key Requirements:
- •WSFU by fixture
- •Prescriptive pipe capacity by length
International Plumbing Code(IPC Appendix E (E103.3(2)))
View StandardThe ICC model code assigns different WSFU values and sizes supply by a PERFORMANCE method (WSFU→GPM→friction), not a single table.
Key Requirements:
- •Appendix E is informative unless locally adopted
- •Performance-based sizing
Pressure & meter assumptions(46–60 psi, ¾" meter)
View StandardThe prescriptive table's baseline conditions — low street pressure, a small meter, or high elevation change all push toward a larger main.
Key Requirements:
- •Verify static pressure at the meter
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
IPC vs UPC states
A 40% swing on the same house
Roughly half the states follow each lineage. Because the UPC counts bathtubs and washers at 4.0 WSFU vs the IPC's 1.4, the same home totals meaningfully more under the UPC — sometimes pushing the main up a size.
Regional Examples:
Street pressure
The table assumes 46–60 psi
Low municipal pressure (or a long uphill run) reduces how much a given pipe delivers, so a low-pressure area may need the next size up — or a booster.
Regional Examples:
Well vs municipal
Different sizing basis
On a private well the pressure tank and pump set the delivered pressure and flow — size the supply against the pump/tank system, not a street-pressure table.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Want to Learn More?
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Read the PEX vs Copper Pipe
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How to Use This Calculator
- Count every fixture the main supplies with the +/− steppers — toilets, sinks, showers, tubs, the dishwasher, clothes washer, and outdoor hose bibbs.
- Pick your plumbing code — IPC or UPC. Watch the per-fixture WSFU values change as you switch; the bathtub and clothes washer move the most.
- Enter the developed length: the total pipe distance from the water meter to the farthest fixture (not straight-line distance).
- Click Calculate for your total water supply fixture units and the minimum main diameter that carries them at that length.
- Confirm your static street pressure with a gauge on a hose bibb — low pressure or a long uphill run may require the next size up.
Developed length matters as much as fixtures
A pipe carries fewer fixture units the farther it has to push water — a 1-inch main handles about 39 WSFU on a short run but only about 19 at 300 feet. Measure the real pipe path to your farthest fixture, including the vertical rise, not the straight-line distance across the house. When in doubt between two sizes, size up: oversized supply is cheap insurance against low flow at the far fixtures.
Frequently Asked Questions
What size water line do I need for my house?
For most homes the main is 1 inch and drops to 3/4 inch for branch runs, but the honest answer depends on your fixture count and how far the water travels. Add up your water supply fixture units — a typical 3-bathroom house runs about 29 WSFU under UPC values — and match it to the pipe table at your developed length. At roughly 90 feet, 29 WSFU calls for a 1-inch main; a shorter run or fewer fixtures can use 3/4 inch, and a large home with a long run may need 1-1/4 inch.
What are water supply fixture units (WSFU)?
A weighting system that estimates each fixture's demand on the supply, accounting for both how much it uses and how likely it is to run at the same time as others. A toilet is about 2.2–2.5 WSFU, a shower 1.4–2.0, a bathtub 1.4–4.0 depending on code. You sum every fixture's units and size the pipe to the total, rather than adding up peak GPM (which would wildly oversize the pipe, since fixtures rarely all run at once).
Do the IPC and UPC really give different pipe sizes?
They can, because they assign different fixture-unit values to the same fixtures. The starkest example: a bathtub and a clothes washer each count 4.0 WSFU under the Uniform Plumbing Code but only 1.4 under the International Plumbing Code. For a house with several tubs and a washer, that gap can total 40% more fixture units under the UPC — enough to push the main up a size. Roughly half the states follow each code, so identifying yours is the first step.
Why does pipe length affect the size I need?
Because water loses pressure to friction as it travels, so a longer run delivers less. The sizing tables reflect this directly: a 1-inch main carries about 39 fixture units on a short run but only about 19 at 300 feet. 'Developed length' means the actual pipe path from the meter to the farthest fixture — including vertical rise — not the straight-line distance. A far-flung fixture or an uphill lot can require the next pipe size up even with a modest fixture count.
Should I use 3/4 inch or 1 inch for my main water line?
Compare your total fixture units to each pipe's capacity at your developed length. A 3/4-inch main handles about 20 WSFU on a short run but only 8 at 300 feet; a 1-inch main handles 39 and 19 respectively. Most whole-house mains with two or more bathrooms exceed 3/4-inch capacity once the run gets long, which is why 1-inch is the common residential main. When you're between sizes, size up — oversized supply costs little and prevents weak flow at the far fixtures when several run at once.