Shower pressure drops when a toilet flushes because the toilet's fill valve pulls cold water from the same lines. Your shower valve, which code requires to balance pressure or temperature, then cuts back the hot side so you don't get scalded. A small dip is normal. A big one points to undersized or scaled pipes or low pressure.
What's happening in the pipes
A toilet's tank fills fast. The 2024 International Plumbing Code (IPC) sizes water piping around what each fixture needs at peak demand, and its design table lists a close-coupled tank toilet at 3 gallons per minute and a one-piece tank toilet at 6 gallons per minute, both at 20 psi. A shower is listed at 2.5 gallons per minute. So when a toilet refills, it can briefly ask for more water than the shower is using.
If both fixtures share a cold water branch, the pipe has to carry both flows at once. Every pipe loses some pressure to friction as water moves through it, and more flow means more loss. The shower's cold supply sags while the toilet fills, then recovers when the fill valve shuts. Our page on why water pressure drops when two fixtures run at once explains static versus flow pressure in more detail.
### Why the hot side drops too
This is where the shower valve comes in. Since the cold side just lost pressure, a plain two-handle valve would suddenly deliver a mix with more hot water in it. That's the old "someone flushed and I got burned" problem.
The plumbing code closes that door. IPC 412.3 says individual shower and tub-shower valves "shall be balanced-pressure, thermostatic or combination balanced-pressure/thermostatic valves" that meet ASSE 1016 or a matching ASME/CSA standard. ASSE describes those as automatic compensating valves that "control the water temperature" at the shower. The code also requires a limit stop so the valve's maximum setting delivers water no hotter than 120°F.
A pressure-balance valve senses the drop on the cold side and throttles the hot side to match, so the mix stays about the same temperature. You stay safe, but the total flow drops for a moment. A thermostatic valve mixes to a set temperature using a temperature-sensing element. Either way, a weaker spray during a flush is the valve doing its job.
When it's normal and when it isn't
A brief, gentle dip that recovers in a few seconds is normal in most homes. Here's what isn't:
| What you notice | What it points to |
|---|---|
| Spray drops a lot, or nearly stops | Undersized or scaled supply pipes, or low incoming pressure |
| Water turns hot or cold for a moment | An older valve without pressure balancing, or a worn cartridge |
| It happens with any fixture, not just the toilet | The whole system is short on pressure or flow |
| It started recently | A failing pressure reducing valve, a partly closed valve or a clogged showerhead |
| Only one bathroom does it | That bathroom's branch or its shower valve |
### Common causes of a big drop
- Scaled or galvanized pipe. Phoenix's water services page lists "mineral deposits in galvanized plumbing" among the causes of reduced pressure. Scale narrows the inside of old steel pipe, so it can't carry two fixtures at once.
- Undersized branches. The IPC requires pipes to be sized so fixtures get their minimum flow "under conditions of peak demand." An older home, or a remodel that added a bathroom to an existing line, may fall short.
- Low incoming pressure. If static pressure is low to begin with, there's less to spare when a second fixture opens. Our page on what your home water pressure should be gives the target range.
- A failing pressure reducing valve. A PRV that's sticking or has a clogged strainer can choke flow when demand jumps. See the signs a pressure reducing valve is going bad.
- A clogged showerhead or cartridge. Phoenix's hard water builds scale. Our page on why shower water pressure is low covers the shower-side fixes.
### When temperature changes instead of pressure
If the water goes hot or cold during a flush, treat that as a safety problem, not a comfort one. It usually means the valve is older than the code's balancing rule or its cartridge is worn. Replacing the cartridge or the valve with one that meets ASSE 1016 fixes it, and the valve's limit stop should be set so the maximum is no hotter than 120°F.
What about commercial buildings?
The same thing happens at a bigger scale. Many commercial restrooms use flushometer valves instead of tanks, and those draw far more water in a short burst. The IPC's design table lists a blowout flushometer toilet at 25 gallons per minute and 45 psi, a siphonic flushometer at 25 gallons per minute and 35 psi, and a flushometer urinal at 12 gallons per minute and 25 psi. Put a locker room shower on a branch shared with a bank of flushometers, and every flush pulls hard on that line.
That's why the code treats commercial showers carefully. Gang showers fed by one tempered water pipe need either a listed central automatic temperature control mixing valve, or a balancing or thermostatic valve at each head (IPC 412.4). In gyms, hotels, schools and apartment buildings, the fix for a shower that sags or swings during flushes is usually on the design side: branch sizing, a dedicated line for the showers, or a booster where street pressure can't keep up. The code requires a booster system wherever street pressure is "insufficient to provide flow pressures at fixture outlets" (IPC 604.7).
How to fix it
Start with the cheap checks, then move to the pipes.
- 1Test your static pressure with a gauge at a hose bibb. A low reading leaves little margin for two fixtures at once; over 80 psi means a PRV is needed or failing. Our page on whether you need a pressure reducing valve explains the 80 psi rule.
- 2Clean or replace the showerhead and check the shower cartridge.
- 3Check the valves on the main line and under the toilet. A supply stop left half closed changes how fast the toilet fills, and a main valve left partly closed limits everything. Phoenix lists "a partially closed house valve" as a common cause of low pressure.
- 4Look at the pipe. If the home has galvanized steel, scale is a likely culprit, and repiping the affected branches is the lasting fix.
- 5Consider how the lines are laid out. The IPC allows parallel distribution, where a manifold sends its own line to each fixture (IPC 604.10). When the shower and toilet each have their own line from the manifold, they no longer share a branch, so one flush takes less from the shower.
A quick dip is fine. A big drop or a temperature swing is worth fixing. HQ Plumbing and Air can test your pressure, replace shower valves and cartridges and repipe scaled lines, with free estimates and upfront pricing.