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, then enter three things the code tables need: your minimum static water pressure, the developed length (the actual pipe distance from the meter to the farthest fixture), and how high the top fixture sits above the meter. The calculator takes 0.5 psi off per foot of rise, picks the matching pressure section and the length column at or beyond your run, and reads the meter size and building supply pipe straight off the table — 2021 UPC Table 610.4, or 2021 IRC Table AP201.1 (the same table as IPC Table E104.1).
Two things to know. The IRC/IPC table lives in an appendix (IRC Appendix AP, IPC Appendix E) that only applies where your jurisdiction adopts it, and the IRC asks you to multiply your pipe length by 1.2 to cover fittings — the calculator does that for you. And pressure matters as much as length: the same 29-fixture-unit house that needs a 1-inch supply at 55 psi needs 1¼-inch at 40 psi. Use the lowest pressure of the day, not the best case. Free, no signup.
Water Supply Pipe Size Calculator
What size water meter and building supply pipe does your house need? Count your fixtures, enter your water pressure and pipe length, and get the size straight from the UPC or IRC/IPC code table. Free, no signup.
Fixtures on the supply
Count every fixture the main feeds. Fixture-unit values differ by code — the toggle below shows how much.
Fixture units differ by code: the UPC counts a bathtub and a clothes washer at 4.0 each, the IRC/IPC at 1.4. Pick the code your state or city adopted.
Pressure, length & meter
The code tables need all three. Low pressure or a long run pushes you to a bigger pipe.
Use the lowest pressure of the day (ask your water utility, or read a gauge on a hose bibb on a summer evening). Both codes take 0.5 psi off for every foot the highest fixture sits above the meter; a basement meter feeding a second-floor bath is typically 15–20 ft.
Why fixture units, not GPM
Each fixture's Water Supply Fixture Unit value already discounts how often it's actually open at once, so WSFU totals add up in a way raw flow rates never could. That total, plus the pipe's developed length and the pressure actually available at the meter, is what the code table reads together to pick a pipe size.
Calculation Formulas
Water supply fixture units weight each fixture by its likely simultaneous draw. Values come from UPC Table 610.3 (private) or IRC Table P2903.6 (combined).
Example:
A 3-bath house totals 29 WSFU under the UPC and 22.1 under the IRC.
UPC Table 610.3 note 8 reduces additional hose bibbs when sizing the building supply; IRC Table P2903.6 has no reduction.
Example:
2 hose bibbs = 3.5 WSFU (UPC) or 5.0 WSFU (IRC).
UPC 610.8 and IRC AP201.1 step 2 both adjust 0.5 psi per foot of elevation; IRC also deducts softeners, filters and backflow preventers.
Example:
55 psi with the top fixture 20 ft up → 45 psi.
IRC AP201.1 step 3 adds 20% for fittings. Both codes then use the table column equal to or longer than that length — no interpolation.
Example:
100 ft under the IRC → 120 ft → the 150 ft column.
Read down the chosen pressure section and length column; the row gives the meter and building supply size. Rows smaller than a ¾-inch building supply are skipped (UPC 610.4 note 2; IRC AP201.1 note a).
Example:
29 WSFU, 46–60 psi, 100 ft (UPC) → ¾" meter, 1" supply (carries 33).
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Toilet (1.6 gpf tank) | 2.5 WSFU (UPC) / 2.2 (IRC) | UPC Table 610.3; IRC Table P2903.6. |
| Bathtub & clothes washer | 4.0 (UPC) / 1.4 (IRC) | The largest difference between the two codes. |
| 1" supply, ¾" meter, UPC 46–60 psi | 39 WSFU at 60 ft → 19 at 300 ft | UPC Table 610.4. |
| 1" supply, ¾" meter, UPC 30–45 psi | 25 WSFU at 60 ft → 12 at 300 ft | Same pipe, lower pressure section. |
| ¾" supply, ¾" meter, IRC 50–60 psi | 9.5 WSFU (40–80 ft) | IRC Table AP201.1 — why most houses need 1". |
| Minimum building supply | ¾ inch | UPC Table 610.4 note 2; IRC P2903.7 and AP201.1 note a. |
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 2021(UPC Tables 610.3 & 610.4; Sections 610.7–610.8)
View StandardIAPMO model code. Table 610.3 assigns fixture units; Table 610.4 gives meter and building supply size by pressure section (30–45, 46–60, over 60 psi) and length (40–1,000 ft).
Key Requirements:
- •Use minimum static pressure after head loss
- •0.5 psi per foot of elevation (610.8)
- •Column equal to or longer than the developed length
- •Building supply not less than ¾ inch
International Residential Code 2021(IRC Table P2903.6; Appendix AP Table AP201.1)
View StandardP2903.6 assigns fixture units; Appendix AP sizes the meter and building supply by pressure section (30–39, 40–49, 50–60, over 60 psi) and length (40–500 ft). Appendix AP applies only where the jurisdiction adopts it.
Key Requirements:
- •Deduct 0.5 psi per foot of elevation and equipment losses (AP201.1 step 2)
- •Multiply developed length by 1.2 (step 3)
- •Column equal to or longer than that length
- •Water service not less than ¾ inch (P2903.7)
International Plumbing Code 2021(IPC Appendix E Tables E103.3(2) & E104.1)
View StandardTable E104.1 is identical to IRC Table AP201.1, and the private-dwelling values in E103.3(2) match IRC P2903.6.
Key Requirements:
- •Same procedure as IRC AP201.1
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.
UPC vs IRC/IPC states
Different fixture units and different tables
The UPC counts bathtubs and clothes washers at 4.0 fixture units versus 1.4 under the IRC/IPC, and its sizing table uses different pressure sections and length columns. The same house can come out a pipe size apart.
Regional Examples:
Street pressure
The table has separate pressure sections
Both codes size from the minimum static pressure available, less elevation and equipment losses. Pressure below 30 psi is outside both tables.
Regional Examples:
Well vs municipal
Different sizing basis
On a private well the pump and pressure tank set delivered pressure, and there is no utility meter. Use the pump cut-in pressure as the minimum pressure and size the supply with the pump and tank.
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
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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 — UPC or IRC/IPC. Watch the per-fixture WSFU values change as you switch; the bathtub and clothes washer move the most.
- Enter your minimum static water pressure at the meter — from your water utility, or a gauge on a hose bibb read at a low-pressure time of day.
- Enter the developed length (actual pipe distance from the meter to the farthest fixture), how many feet the highest fixture sits above the meter, and any pressure loss from a softener, filter, or backflow preventer.
- Leave the meter on "size it for me" for a new service, or pick the meter you already have.
- Click Calculate for your total fixture units, the meter and building supply pipe size, and the code table row it came from.
Length and pressure both shrink what a pipe can carry
In 2021 UPC Table 610.4, at 46–60 psi with a ¾-inch meter, a 1-inch building supply carries 39 fixture units up to 60 feet but only 19 at 300 feet. Drop to the 30–45 psi section and the same pipe carries 25 at 60 feet and 12 at 300 feet. Measure the real pipe path to your farthest fixture, not the straight-line distance, and always use the table column at or beyond your length — the code never lets you interpolate between columns.
Frequently Asked Questions
What size water line do I need for my house?
Most houses land on a 1-inch building supply, but the code answer depends on three things: fixture units, water pressure, and pipe length. A typical 3-bath house is about 29 water supply fixture units under the UPC. In 2021 UPC Table 610.4, at 46–60 psi and a 100-foot run, that fits a 1-inch supply on a ¾-inch meter (carries 33). At 30–45 psi the same house needs a 1¼-inch supply on a 1-inch meter, and at 300 feet even 46–60 psi pushes it to 1¼-inch. Building supply is never allowed below ¾ inch.
What are water supply fixture units (WSFU)?
A weighting system that estimates each fixture's demand on the supply, accounting for how much water it uses and how likely it is to run at the same time as others. Under 2021 UPC Table 610.3 a 1.6-gpf toilet is 2.5, a shower 2.0, and a bathtub 4.0; under 2021 IRC Table P2903.6 they are 2.2, 1.4, and 1.4. You add up every fixture and size the pipe to the total, rather than adding peak gallons per minute, which would badly oversize the pipe since fixtures rarely all run at once.
Do the UPC and IRC/IPC really give different pipe sizes?
Yes, in two ways. First, fixture units: a bathtub and a clothes washer each count 4.0 under UPC Table 610.3 but 1.4 under IRC Table P2903.6, so the same house totals far more under the UPC. Second, the sizing tables themselves differ — UPC Table 610.4 has three pressure sections and lengths out to 1,000 feet, while IRC Table AP201.1 (identical to IPC Table E104.1) has four pressure sections, stops at 500 feet, and requires multiplying your pipe length by 1.2 for fittings. Pick the code your state or city adopted.
How does water pressure change the pipe size?
Lower pressure means each pipe carries fewer fixture units, so both codes split their sizing table into pressure sections. Use the minimum static pressure at the meter, then subtract 0.5 psi for every foot the highest fixture sits above the meter (UPC 610.8; IRC AP201.1) plus any loss through a softener, filter, or backflow preventer. Example from UPC Table 610.4: a 1-inch supply on a ¾-inch meter at 100 feet carries 33 fixture units at 46–60 psi but only 21 at 30–45 psi. Below 30 psi the table doesn't apply — that needs a booster or an engineered design.
Why does pipe length affect the size I need?
Water loses pressure to friction as it travels, so a longer run delivers less. In UPC Table 610.4 at 46–60 psi with a ¾-inch meter, a 1-inch supply carries 39 fixture units up to 60 feet but only 19 at 300 feet. 'Developed length' is the actual pipe path from the meter to the farthest fixture, not the straight-line distance. Always use the table column equal to or longer than your length — a 110-foot run uses the 150-foot column. Under the IRC, multiply the length by 1.2 first to account for fittings.
Should I use 3/4 inch or 1 inch for my main water line?
Check your total fixture units against the table row. In 2021 UPC Table 610.4 at 46–60 psi, a ¾-inch supply on a ¾-inch meter carries 20 fixture units up to 60 feet and 8 at 300 feet; a 1-inch supply carries 39 and 19. Under 2021 IRC Table AP201.1 at 50–60 psi, ¾-inch tops out at 9.5 fixture units, so almost any house with more than one bathroom needs at least 1-inch. The water meter matters too: a ¾-inch meter caps a 1¼-inch supply at 39 fixture units under the UPC.