In short: usable pressure from a gravity-fed tank is elevation head minus pipe friction loss — never the full tank height alone. A 60 ft tank feeding 10 GPM through 100 ft of 1-inch PVC delivers about 26.0 psi of elevation head, loses roughly 3.0 psi to pipe friction, and arrives at the tap with about 23.0 psi. Use the calculator above for your exact setup.
These figures are planning aids based on published hydraulic formulas. For a system you're relying on daily, confirm sizing with a licensed engineer or plumber.
How to calculate gravity feed pressure
A gravity-fed system has no pump — the only pressure available comes from the height of the water source. That gross pressure is reduced by friction as the water travels through the delivery pipe, so the pressure that actually reaches the tap is:
Worked example. A tank sits 60 ft above the tap (the calculator's default), feeding 10 GPM through 100 ft of 1-inch PVC pipe (C = 150). Elevation head: 60 × 0.43353 = 26.012 psi. Friction loss over that run: 6.929 ft, or 3.004 psi. Available pressure: 26.012 − 3.004 = 23.0 psi — enough to meet the common 20 psi minimum for fixtures.
Why gravity systems disappoint. It's easy to assume a tall tank guarantees strong pressure, but a long or undersized pipe can quietly consume a large share of that elevation head before it ever reaches the tap — especially once real household flow rates, not just a trickle, are pulled through the line.
Why height, pipe size and flow rate change the answer
Height sets the ceiling. Elevation head is the maximum pressure the system could ever deliver, at zero flow. Every foot of tank height is worth about 0.433 psi, so raising a tank is always the most direct way to add pressure.
Pipe diameter has an outsized effect on the loss. Friction loss falls steeply as diameter increases — roughly with diameter to the −4.87 power in the Hazen-Williams relationship — so a single pipe size upgrade can recover several psi with no other change.
Flow rate works against you. Friction loss rises with flow rate to about the 1.85 power, so pressure that looks comfortable for one slow tap can fall well below it once several fixtures draw at once. See the shower pressure calculator for that scenario applied to a single fixture.
Reference: available pressure by tank height & pipe length
Net psi at the tap, 10 GPM through 1-inch PVC pipe (C = 150). Every value matches what the calculator returns for the same inputs.
| Tank height | 50 ft run | 100 ft run | 150 ft run | 200 ft run |
|---|---|---|---|---|
| 20 ft | 7.2 | 5.7 | 4.2 | 2.7 |
| 30 ft | 11.5 | 10.0 | 8.5 | 7.0 |
| 40 ft | 15.8 | 14.3 | 12.8 | 11.3 |
| 60 ft (default) | 24.5 | 23.0 | 21.5 | 20.0 |
| 80 ft | 33.2 | 31.7 | 30.2 | 28.7 |
| 100 ft | 41.9 | 40.3 | 38.8 | 37.3 |
Worked examples
60 ft tank, 100 ft of 1″ PVC
23.0 psi
The calculator's default: 26.0 psi of elevation head minus 3.0 psi of friction loss at 10 GPM. Comfortably above the 20 psi minimum.
Same tank, longer & busier run
17.6 psi
Stretch the run to 200 ft and the flow to 12 GPM (still 1″ PVC) and friction loss jumps to 8.4 psi — pressure drops below the 20 psi guideline.
Fixed with a bigger pipe
23.2 psi
Keep the same 200 ft, 12 GPM run but upsize to 1.25″ PVC: friction loss falls to 2.8 psi, and pressure is back above 20 psi.
What this calculator doesn't include
Fittings and valves. Elbows, tees and shutoff valves add their own minor losses on top of straight-pipe friction — see the minor loss calculator to add those in for a more exact figure.
Multiple simultaneous fixtures. This tool assumes one steady flow rate. Running several fixtures at once raises total flow and therefore friction loss — rerun the calculator at your peak combined flow to check worst-case pressure.
Tank drawdown. As a tank empties, its water level — and therefore the available head — drops. This calculator uses a fixed height; for pressure over a full drawdown cycle, check the pressure at both a full and a low tank level.
Frequently asked questions
How does a gravity-fed water system create pressure?
A gravity-fed system relies entirely on the height of the water source above the point of use — a tank on a hill, a rooftop cistern, or an elevated water tower. That height converts directly to pressure at about 0.433 psi per foot of elevation. There is no pump; the water's own weight does the work.
Why is my gravity-fed pressure lower than the tank height suggests?
Friction loss along the delivery pipe eats into the elevation head before the water reaches the tap. A long, narrow, or rough pipe can consume a large share of the available pressure, especially at higher flow rates — which is why usable pressure is always elevation head minus friction loss, never the full elevation head alone.
How much pressure does a gravity-fed system need?
Most codes and fixture manufacturers call for a minimum of about 20 psi at the point of use, and many fixtures — especially low-flow showerheads — perform better at 30–45 psi. Below roughly 20 psi, flow becomes weak and multi-fixture demand can drop pressure further.
How do I increase pressure in a gravity-fed system?
Three practical options: raise the tank or source higher, use a larger-diameter pipe to cut friction loss, or shorten and straighten the delivery run. If none of those is feasible, a booster pump can add the pressure gravity alone can't provide.
Does flow rate affect gravity-fed pressure?
Yes, substantially. Friction loss rises fast with flow rate — roughly with flow to the 1.85 power in the Hazen-Williams relationship — so pressure that looks fine for a single slow-running tap can drop noticeably once two or three fixtures run at once.
What's the difference between this and a total dynamic head calculation?
They use the same underlying physics, but total dynamic head is normally used to size a pump that pushes water uphill against elevation and friction. A gravity feed calculation runs the same math for a system with no pump at all, asking how much of the elevation head survives the trip as usable pressure at the tap.
Sources & references
- Williams, G. S., & Hazen, A. (1933). Hydraulic Tables (3rd ed.). John Wiley & Sons.
- International Code Council. International Plumbing Code (IPC), Section 604 — Design of Building Water Distribution System.
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)