In short: a standard 1/2 inch hose bib, fully open at 50 psi, flows at about 18 GPM (68 L/min) straight out of the spigot. A smaller frost-free valve delivers closer to 11 GPM, while a full-port 3/4 inch ball-valve bib can push past 90 GPM. Flow rises with the square root of pressure and scales with how far the valve is open — use the calculator above for your own numbers.
These figures are planning aids based on the orifice flow equation. For irrigation zone sizing, fire-flow testing or code-driven work, confirm with a licensed engineer or your local water utility.
How to calculate hose bib flow rate
An open hose bib behaves like a small orifice: water under pressure squeezes through the valve's internal opening and out into open air. The flow depends on three things — how much pressure is pushing the water, how big the opening is, and how efficiently that opening lets water through (its discharge coefficient). The governing relationship is the orifice flow equation:
This calculator converts your pressure to pascals, works out the opening's area from the bib size and how far the valve is turned, and solves for flow in cubic meters per second before converting to GPM, liters per minute and the other units below.
Worked example. A standard 1/2" sillcock (effective opening 0.110 in², Cd 0.61) fully open at 50 psi: convert 50 psi to 344,738 Pa, take √(2 × 344,738 ÷ 998.2) = 26.28 m/s, then multiply 0.61 × 0.0000713 m² × 26.28 m/s = 0.00114 m³/s, which converts to about 18.1 GPM.
Exit velocity doesn't care about the opening size. The √(2ΔP/ρ) term depends only on pressure, not on the valve's diameter — a consequence of Torricelli's theorem. A bigger opening moves more total volume of water, but every opening at the same pressure sends water out just as fast, around 86 ft/s at 50 psi.
Why pressure and valve opening change the answer
Flow rises with the square root of pressure, not in a straight line. Doubling the pressure at the bib — say from 25 psi to 50 psi — only increases flow by about 41 percent, because the formula uses √ΔP. To actually double the flow rate through the same opening, you need roughly four times the pressure. That's why pressure boosting has diminishing returns and why very high pressure rarely feels proportionally stronger.
Opening the valve, by contrast, has a more direct effect. This calculator scales the effective opening area in proportion to how far the valve is turned, so at 50 percent open you get roughly half the fully-open flow, and at 25 percent open roughly a quarter. Real valves — especially globe-style sillcocks — often throttle even more sharply than this near the fully-closed position, so treat the low end of the opening range as an approximation.
Flow rate by pressure (standard sillcock, fully open)
Flow from a standard 1/2" sillcock, fully open, across the typical residential pressure range.
| Pressure | Flow (GPM) | Flow (L/min) |
|---|---|---|
| 20 psi | 11.4 | 43.4 |
| 30 psi | 14.0 | 53.1 |
| 40 psi | 16.2 | 61.3 |
| 50 psi (typical) | 18.1 | 68.5 |
| 60 psi | 19.8 | 75.1 |
| 70 psi | 21.4 | 81.1 |
| 80 psi (max residential) | 22.9 | 86.7 |
| 100 psi | 25.6 | 96.9 |
Quick reference: flow by bib type at 50 psi
How the fully-open flow rate compares across common outdoor bib and valve types, all at 50 psi supply pressure.
| Bib type | Effective opening | Flow (GPM) |
|---|---|---|
| Frost-free anti-siphon | 0.077 in² | 11.3 |
| Standard 1/2" sillcock | 0.110 in² | 18.1 |
| 3/4" ball-valve hose bib (full port) | 0.442 in² | 95.0 |
Worked examples
Filling a 5-gallon bucket
~17 seconds
At 50 psi from a fully-open standard sillcock (18.1 GPM), a 5-gallon bucket fills in about 16.6 seconds — quick enough that hose bib flow rate rarely matters for a simple bucket fill.
Frost-free sillcock at 40 psi
10.1 GPM
A frost-free anti-siphon valve's narrower internal path delivers about 10.1 GPM at a modest 40 psi — worth knowing before sizing a drip-irrigation manifold straight off the bib.
Throttled to half open
9.1 GPM
The same standard sillcock at 50 psi, turned to 50 percent open, drops to roughly 9.1 GPM — useful for estimating flow when a valve is used to meter water rather than fully opened.
What else affects the result
Upstream pipe sizing. This calculator treats the bib's own opening as the only restriction. In many homes, the supply pipe, water meter or a partly-closed shutoff valve upstream limits flow below what the bib's opening alone would allow — so treat the result as an upper bound for that valve, not a guarantee of delivered flow.
A hose attached. Once a garden hose is connected, its length and inside diameter usually become the dominant restriction, especially over 50 feet. See the Garden Hose Flow Rate Calculator for that case, or the Garden Hose Fill Time Calculator to estimate how long a container takes to fill through a hose.
Mineral buildup and corrosion. Older sillcocks accumulate scale and sediment that narrow the effective opening over years of use, quietly reducing flow well below a new valve's rating.
Water temperature. Density changes only slightly with temperature, so its effect on flow rate is under 1 percent across the normal range of household water temperatures — small enough to ignore for planning purposes.
Frequently asked questions
How many GPM does a hose bib flow at 50 psi?
A standard 1/2 inch hose bib, fully open at 50 psi, flows at roughly 18 GPM (about 68 liters per minute) directly from the spigot, based on the orifice flow equation. A smaller frost-free anti-siphon valve delivers less, around 11 GPM, while a full-port 3/4 inch ball-valve bib can exceed 90 GPM. Attaching a garden hose cuts these numbers down significantly because the hose itself becomes the main restriction.
Does attaching a garden hose reduce the flow rate?
Yes, substantially. This calculator estimates flow straight out of the open bib, where the valve opening is the only restriction. Once a hose is attached, friction along its length becomes the dominant factor, and flow drops well below the bib's own capacity, especially with a long or narrow hose. Use the Garden Hose Flow Rate Calculator to estimate flow through the hose itself.
Why doesn't flow double when pressure doubles?
Flow through an orifice follows a square-root relationship with pressure, not a straight line. Going from 25 psi to 50 psi, double the pressure, increases flow by only about 41 percent, not 100 percent. To double the flow rate through the same opening you need roughly four times the pressure.
What water pressure is normal at an outdoor hose bib?
Most homes run outdoor hose bib pressure between about 40 and 60 psi, matching the household supply. Utilities generally deliver water in the 40 to 80 psi range at the meter; pressure above 80 psi is usually cut back with a pressure-reducing valve to protect fixtures and appliances.
How much does the valve opening percentage matter?
A lot. This calculator scales the effective opening area with how far the valve is turned, as a straightforward approximation. Closing a standard sillcock to half open cuts flow roughly in half; closing it to a quarter open cuts flow to about a quarter of the fully-open rate. Near fully closed, real valves choke off flow even faster than this straight-line estimate suggests.
Sources & references
- International Organization for Standardization (ISO). ISO 5167 — Measurement of fluid flow by means of pressure differential devices.
- Torricelli's theorem — the classical relation for efflux velocity of a fluid through an orifice under a pressure head.
- U.S. Environmental Protection Agency, WaterSense Program. Flow-rate specifications for residential water fixtures.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)