In short: water pressure drops about 0.43 psi for every foot of elevation, or roughly 4.3 psi per typical 10-ft story. At a common 60 psi street supply, a building will read about 38 psi on the 5th floor and can run out of usable pressure by the 13th or 14th floor without a booster pump.
This is a planning estimate based on elevation (hydrostatic) head alone. Actual pressure also depends on pipe friction, fixture demand, and code-required minimums — confirm final design with a licensed plumber or engineer.
How to calculate water pressure by floor
Every foot you climb, the water in the pipe has to work against that much more elevation. The relationship is a simple, fixed conversion: one foot of water column is worth about 0.43353 psi, whichever direction you measure it — head becomes pressure, or pressure becomes head. Climbing a building simply subtracts that elevation pressure from whatever pressure arrived at street level.
Worked example. Take a building supplied at 60 psi at the street main, with 10-ft floor-to-floor heights. At the 5th floor, the elevation is 5 × 10 = 50 ft. The pressure lost is 50 × 0.43353 = 21.68 psi. Subtract that from the supply: 60 − 21.68 = 38.32 psi at the 5th floor — exactly what the calculator above shows.
This is static pressure. It assumes no water is flowing anywhere in the building. Once fixtures are running, an additional loss from pipe friction stacks on top of the elevation loss calculated here — see the friction head loss calculator for that piece.
Why floor height changes the answer
Water pressure at any point comes from the weight of water sitting above it. At street level, a building's pressure comes from the municipal main or a rooftop/ground tank pushing water through the system. As a pipe rises through the building, it has to overcome the weight of the taller column of water beneath it just to reach that height — this is called elevation head, and it is described by the basic hydrostatic relation P = ρ·g·h. Taller floor-to-floor heights, or more floors, mean more elevation head to overcome, so the delivered pressure keeps falling the higher you go, regardless of how much water is being used.
This effect is independent of pipe diameter, material, or flow rate. A 4-inch main and a ½-inch branch line lose exactly the same elevation pressure over the same rise — pipe size only changes the separate, flow-dependent friction loss.
Pressure lost climbing floors (10 ft per floor)
Elevation loss at a common 10-ft floor height, and what remains of a 60 psi street supply at each point. These match the calculator above with its default settings.
| Floor | Elevation loss | Pressure remaining |
|---|---|---|
| 1 | 4.34 psi | 55.66 psi |
| 2 | 8.67 psi | 51.33 psi |
| 3 | 13.01 psi | 46.99 psi |
| 4 | 17.34 psi | 42.66 psi |
| 5 | 21.68 psi | 38.32 psi |
| 10 | 43.35 psi | 16.65 psi |
| 13 | 56.36 psi | 3.64 psi |
| 14 (runs out) | 60.69 psi | −0.69 psi |
Worked examples
Three-story townhouse
48.3 psi
60 psi city supply, 9-ft floors. The 3rd floor sits 27 ft up, a loss of 11.7 psi, leaving 48.3 psi — plenty for good flow.
10-story apartment building
21.6 psi
65 psi supply, 10-ft floors. The top (10th) floor sits 100 ft up, a loss of 43.4 psi, leaving only about 21.6 psi — borderline low and a good candidate for a booster pump.
30-story high-rise
−76.1 psi
80 psi supply, 12-ft commercial floors. The 30th floor sits 360 ft up, a loss of about 156.1 psi — far more than the supply alone provides, which is why tall buildings use staged booster pumps and pressure zones.
What else affects the result
Friction loss. Once water is actually flowing, pipe length, diameter, material and fittings add a further pressure drop on top of elevation. This tool reports elevation loss only; add a friction allowance in the advanced field above, or use the friction head loss calculator for a full estimate.
Minimum usable pressure. Plumbing codes typically set a low bound of roughly 8–20 psi at a fixture for it to work at all, but most people notice weak flow well before that, once pressure drops under about 30–40 psi. Treat any floor reading near or below that range as a candidate for a booster pump.
Pressure-reducing valves. On the lower floors of a tall building, the same math works in reverse — street pressure plus a rooftop tank or pump can push pressure on the ground floor uncomfortably high, which is why lower floors are often fitted with pressure-reducing valves.
Gravity-fed systems. Buildings supplied from a rooftop or elevated tank instead of street pressure follow the same formula, just measured downward from the tank. See the gravity feed pressure calculator for that case.
Frequently asked questions
How much does water pressure drop per floor?
Water pressure drops about 0.43 psi for every foot of elevation. For a typical 10-foot story, that works out to roughly 4.3 psi lost per floor you climb, regardless of pipe size.
Why does water pressure get weaker on higher floors?
Pressure in a water column depends on the height of water above the point you are measuring. As you go up a building, there is less column pressing down from above and more elevation to fight against, so the pressure the supply has to push against grows and the delivered pressure falls.
How many floors can 60 psi of water pressure supply?
At 60 psi and 10-foot floors, pressure falls by about 4.3 psi per floor, so it reaches roughly 0 psi around the 14th floor. In practice you would want a booster pump well before that, once pressure drops below about 30 to 40 psi.
What can I do if upper-floor water pressure is too low?
The standard fix is a booster pump system that adds pressure back before the water reaches the upper floors, sometimes combined with pressure zones so lower floors are not over-pressurized. A pressure-reducing valve may also be needed on the lower floors of a tall building.
Does pipe size affect how much pressure is lost per floor?
No. The elevation loss calculated here comes only from the height of the water column and is the same regardless of pipe diameter. Pipe size does affect a separate loss, friction loss, which only occurs while water is flowing.
Is the water pressure the same on every floor when no one is using water?
Static pressure, meaning no flow anywhere in the building, still varies by floor for exactly this elevation reason. What changes with usage is the additional friction loss inside the pipes, which only appears once water is moving.
Sources & references
- International Code Council (ICC). International Plumbing Code (IPC), water distribution provisions.
- Standard hydrostatic pressure relation, P = ρgh, as used throughout fluid statics and civil/plumbing engineering.
- American Water Works Association (AWWA). Water distribution system design references.
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)