Quick Answer:

If pressure suddenly drops partway through a shower and you notice other fixtures weaken at the same time, treat it as a house-wide supply issue. Quickly confirm it by checking a nearby sink while the shower is low, watch whether the toilet starts refilling at the same moment, and if possible read a pressure gauge during the event. Common causes include a pressure-reducing valve (PRV) acting up, a clogged whole-house filter, or a temporary drop in the municipal supply when overall demand spikes.

Why This Happens

When water pressure falls across multiple fixtures at once it usually means the problem is upstream of the shower. Typical causes are:

• PRV chatter or failure — the pressure-reducing valve can stick, cycle, or lose its set point and let pressure drop suddenly.
• Clogged whole-house sediment filter — heavy debris or mineral buildup can restrict flow under peak use.
• City/utility supply sag — during high demand times the main can’t keep up, so everyone on the same line sees lower pressure.
• A significant leak or open tap elsewhere in the house — a running toilet or hose can pull down pressure.

For related supply problems see No water at any fixture suddenly and Water off only on cold or hot side.

Step-by-Step What to Do

Step 1 — Confirm it’s house-wide

• While the shower pressure is low, turn on a nearby sink faucet and watch the flow. If the sink drops at the same moment, it’s not the shower valve.
• Check the toilet(s): if a toilet begins a refill cycle or runs at the same time, that can confirm a system-wide event or an unrelated automatic refill drawing water.

Step 2 — Read a pressure gauge during the drop

• If you have a garden hose pressure gauge, attach it quickly to an outdoor hose bib and read the pressure while the problem is happening. Note the value and whether it fluctuates.
• If you don’t have a gauge, briefly watch an unmetered outdoor tap or the flow from a hose to compare before and during the event so you can describe it to a pro.

Step 3 — Look for obvious in-house causes

• Check for running toilets, automatic filling devices, or open hoses that might be using water at the same time.
• Inspect visible filters, like a whole-house sediment filter, for recent maintenance issues or clog warnings.

Step 4 — Suspect and test likely components

• PRV: a failing PRV often causes pressure to “chatter” or drop under demand. If you have one near the main shutoff, note noises or rapid pressure swings.
• Filter: if your house has a serviceable filter, check pressure before and after the filter (or bypass it briefly if safe and you know how) to see if flow returns.
• City service: call the water utility if you suspect a supply sag, especially if neighbors are affected or the gauge shows low pressure across the board.

Step 5 — Take temporary steps while you sort it

• Shut off unnecessary fixtures and appliances to reduce demand (washing machines, sprinklers, irrigation controllers).
• If the pressure drop affects hot water and cold independently, avoid adjusting the shower valve aggressively — sudden changes can create scald risk.

What Not to Do

• Don’t assume it’s your shower cartridge — confirm whether other fixtures drop at the same time before tearing parts apart.
• Don’t open the main water valve or tamper with the meter box if you’re unsure; that can cause leaks or legal issues with the utility.
• Don’t ignore repeated events. Intermittent wide-area drops often point to failing equipment or municipal issues that can get worse.

When to Call a Professional

• If the pressure gauge shows repeated large swings or you hear machinery-like chatter at a valve, call a licensed plumber to inspect the PRV and filter setup.
• If you can’t identify an in-house cause and multiple fixtures remain weak, contact the water utility and a plumber — the issue could be on the city side or at your service connection.
• Call immediately if you find water pouring from any hidden area (walls, ceilings, basement) during the event — that indicates a serious leak.

Safety Notes

• Be cautious of scalding: sudden cold-side drops can make shower water abruptly hotter. Test temperature slowly before stepping under water.
• Do not attempt to work on the municipal side of the service line or the meter—only the utility may legally and safely do that.
• If you bypass filters or valves to test flow, make sure you know the correct parts and have the right tools; accidental damage can cause leaks.

Common Homeowner Questions

• Q: Could a single appliance cause a house-wide drop?
  A: Yes — a major leak, running irrigation system, or a stuck toilet can reduce pressure for all fixtures.
• Q: How quickly should a plumber respond to intermittent pressure drops?
  A: If the drops are frequent or increasing, arrange service within a day or two; urgent if you find active leaking or no water at all.
• Q: Will replacing a shower cartridge fix this problem?
  A: Only if the issue is isolated to that shower. Confirm other fixtures before replacing the cartridge.