TryBuildCalcWater Pressure Booster Calculator (Weak 18 psi Incoming Supply Example)
Calculate the boost needed for a weak 18 psi incoming supply.
🕒 Last updated: August 1, 2026
Pump Scenario & Units
ℹ️Booster boosts an existing pressurized supply; Well/Transfer lifts from an unpressurized well, cistern, or storage tank.
ℹ️Metric converts your entries to/from US units at the boundary — the calculation and result always use US units, matching this site's Drain Pipe Slope calculator.
Required Flow Rate
ℹ️Already know your required flow rate? Enter it directly instead of building a fixture list.
ℹ️Total simultaneous flow rate the pump needs to deliver at peak demand.
Pipe & Head
ℹ️Roughness (Hazen-Williams C = 130, aged/design value) sets how much friction loss this material adds at your flow rate.
ℹ️The pipe size the pump discharges through on its way to the highest/most remote fixture.
ℹ️Full developed length from the pump to the highest/most remote fixture, including vertical runs.
ℹ️Additional elevation gain from where the booster ties in up to your highest fixture — can be small or zero for a single-story boost.
ℹ️Pressure you want left over at the highest/most remote fixture — IPC code minimums are 8-15 psi, but 20-30+ psi is a more comfortable design target.
ℹ️Measured pressure at the point the booster ties in — not a general impression of local water pressure.
Pump Efficiency
ℹ️Defaults to 65%, typical for a small centrifugal pump — adjust to a specific candidate pump's published efficiency at your duty point if known.
Optionally size a hydropneumatic pressure tank's drawdown alongside the pump.
Optionally estimate the pump's electricity cost from your own rate and daily run time.
🔧 Required: 12 GPM at 59.35 ft TDH
Scenario: Booster Pump · Flow rate entered directly
Flow Rate
Required Flow Rate: 12 GPM
Entered directly, not derived from a fixture list.
Total Dynamic Head Breakdown
Elevation: 10 ft
Friction Loss: 33.18 ft
41.48 ft per 100 ft of 3/4" pipe
Required Pressure Head: 57.75 ft (25 psi)
Available Incoming Head: 41.58 ft (18 psi)
Total Dynamic Head: 59.35 ft
Pump Power & Motor Class
Water HP: 0.18 HP
Brake HP (Efficiency 65%): 0.277 HP
Motor-Size Class: 1/3 HP class
Pump configuration: Single unit, or duplex for standby redundancy
Design Notes
Friction loss is computed directly from your pipe material, size, and run length via the Hazen-Williams equation — using conservative, aged-condition roughness values rather than optimistic new-pipe numbers.
A booster is needed because your available incoming pressure, once elevation and friction losses are subtracted, falls short of your required residual pressure at the highest fixture.
Always confirm a shortlisted pump's actual published pump curve at this required GPM/TDH duty point — a horsepower label alone doesn't guarantee a specific pump meets this exact operating point.
Approximate results for planning only. Verify with a professional.
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Water Pump / Pressure Booster Sizing Calculator.
Weak 18 psi incoming supply booster example
At 12 GPM through 80 ft of 3/4" copper (33.18 ft of friction loss — meaningfully higher per foot than a larger pipe would produce at this flow), a 10 ft elevation gain, and a 25 psi target against only 18 psi already arriving, Total Dynamic Head comes to 59.35 ft.
That's approximately 0.277 BHP, rounding up to a 1/3 HP motor class with a comfortable 20.3% margin — even a fairly weak incoming supply only needs a modest booster pump once the actual numbers (not just a general "my pressure feels low" impression) are run.
- 18 psi incoming vs. 25 psi target — a 7 psi (16.2 ft) shortfall before friction/elevation.
- 59.35 ft TDH once elevation and friction are added.
- ≈0.277 BHP → 1/3 HP motor class.
How Water Pump Sizing Is Calculated
The calculation runs in six parts — friction loss, Total Dynamic Head, pump power, motor-size class, and (optionally) pressure tank sizing and operating cost.
Step 1 — Friction Loss (Hazen-Williams)
C is the Hazen-Williams roughness coefficient for your selected pipe material (Copper 130, Galvanized Steel 100, CPVC/PEX 150 — conservative "aged pipe" design values), Q is your required flow rate in GPM, and d is the selected nominal size's actual inner diameter in inches. The result scales directly with your pipe run length.
Step 2 — Total Dynamic Head (TDH)
Well/Transfer: TDH = Elevation + Friction Loss + Required Pressure × 2.31
2.31 converts psi to feet of head (1 psi = 2.31 ft, from water's density). For a Booster scenario, if the available incoming pressure already covers the requirement once elevation and friction are subtracted, TDH comes out to zero or less — this calculator reports "pump not needed" rather than sizing one anyway. A Well/Transfer scenario always needs a pump, since there's no existing pressurized supply to credit against.
Step 3 — Pump Power
Brake HP = Water HP ÷ Efficiency
Water Horsepower is the theoretical minimum power for a perfect, lossless pump; Brake Horsepower divides that by your pump's actual efficiency (default 65%, typical for a small centrifugal pump) to get the real shaft power the motor must supply.
Step 4 — Motor-Size Class & Feasibility
Margin % = (Motor Class − Brake HP) ÷ Brake HP × 100
Brake Horsepower is matched against standard NEMA fractional/whole HP steps (1/3 through 20 HP) to show what to actually shop for, classified as comfortable (15-50% headroom), borderline (under 15%), or oversized(over 50%). A requirement beyond 20 HP triggers a feasibility warning and a dual-unit (parallel pumps) check rather than an extrapolated guess. A discharge-pressure check also flags if the pump would push a nearby fixture above IPC 604.8's 80 psi maximum.
Step 5 — Pressure Tank Sizing (Optional)
Size a Tank: Required Tank Volume (gal) = (Flow Rate × Run Time) ÷ (P1 ÷ P2 − P1 ÷ P3)
Both directions use Boyle's Law, where P1 = precharge, P2 = cut-in, P3 = cut-out — all three converted to absolute pressure(gauge psi + 14.7) before the ratio. "Check a Tank" tells you the drawdown a candidate tank volume delivers; "Size a Tank" solves the same equation in reverse, turning a target minimum pump run time into the minimum tank volume needed, rounded up to the nearest common standard size. Skip this step by leaving Pressure Tank Sizing on "Skip" if you only need the pump sized.
Step 6 — Operating Cost (Optional)
Daily Cost = Electrical kW × Hours per Day × Electricity Rate
Brake HP is shaft power, not electrical input power — a motor's own electrical-to-mechanical conversion is never 100%, so dividing by Motor Efficiency (default 85%, adjustable) converts shaft power to the actual power drawn at the wall. Hours per day is always a value you enter directly, never guessed from other inputs — actual daily pump run time varies too much by household and system to safely infer. Monthly and annual figures simply multiply the daily cost by 30 and 365 respectively, at the same constant daily usage.
Worked Example
This example uses the active inputs above and follows the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Pump Scenario | Booster Pump (Boost Existing Supply) |
| Required Flow Rate | Entered directly (12 GPM) |
| Pipe | Copper (Type L), 3/4", 80 ft run |
| Elevation / Required Pressure | 10 ft / 25 psi |
| Available Incoming Pressure | 18 psi |
Step 1 — Friction Loss
| Calculation | Substitution | Result |
|---|---|---|
| Friction Loss (per 100 ft) | C=130, Q=12 GPM, d=0.785 in | 41.48 ft/100ft |
| Friction Loss (full run) | 41.48 × (80 ÷ 100) | 33.18 ft |
Step 2 — Total Dynamic Head
| Calculation | Substitution | Result |
|---|---|---|
| Required Pressure Head | 25 psi × 2.31 | 57.75 ft |
| Available Incoming Head | 18 psi × 2.31 | 41.58 ft |
| Total Dynamic Head | 10 + 33.18 + 57.75 − 41.58 | 59.35 ft |
Step 3 — Pump Power & Motor Class
| Calculation | Substitution | Result |
|---|---|---|
| Water HP | (12 × 59.35) ÷ 3960 | 0.18 HP |
| Brake HP | 0.18 ÷ 0.65 | 0.277 HP |
| Motor-Size Class | Nearest standard class ≥ 0.277 HP | 1/3 HP (comfortable) |
Therefore, this scenario needs a pump rated for at least 12 GPM at 59.35 ft of TDH — approximately 0.277 HP brake horsepower, closest to a 1/3 HP standard motor class (comfortable).
This does not cover suction-side sizing, NPSH, or electrical service. Cross-check against the Reference Tables in the complete Water Pump / Pressure Booster Sizing Calculator.
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Water Pump / Pressure Booster Sizing Calculator.