💧 Calculator

Flow rate & pressure estimator

Estimate what your head should be delivering, then compare it with what it actually delivers. A large gap means a restriction, not a weak supply.

gpm · L/minNo data sent

Your setup

Count one row and multiply, or estimate.
A pin is roughly 0.7 mm.
Typical residential: 40–60 psi. Below 20 psi, suspect the supply.

Estimate

Unrestricted estimate — Before the restrictor
After restrictor — What the head can pass
In litres per minute — 1 US gallon = 3.785 litres
Limiting factor — —

ResultAdjust the inputs to see how pressure, nozzle area and the restrictor interact.

Check next

  • Remove the head and run the open arm — is the flow strong there?
  • Rinse the inlet screen in both directions.
  • Soak the faceplate if the spray is uneven.
How this is calculated

Flow is estimated from nozzle area and the velocity implied by supply pressure, using an empirical discharge coefficient of 0.18 that keeps results in the range real shower heads measure. It is a sanity check, not a hydraulic model — pipe diameter, fittings and the valve all matter and none of them are inputs.

Reference points

US fixture ratings. A reading below 1.2 gpm at normal pressure usually means a restriction.
RatinggpmL/min
Low-flow (federal max in the US since 1992)2.59.5
Modern standard2.07.6
Water-saving1.86.8
High-efficiency1.55.7
Below this, suspect a blockage1.24.5
Estimates only. Measured flow depends on pipe bore, valve type, fittings and supply pressure under load. Check local water-use rules before removing or changing a flow restrictor.