TryBuildCalcPressure Tank Sizing Calculator (44 Gallon Tank, 30/50 Switch Example)
Size a pressure tank's drawdown for a 30/50 pressure switch.
🕒 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 = 150, 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.
ℹ️Full vertical lift from your well/cistern/tank water level up to your highest fixture.
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.
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.
ℹ️Check a Tank tells you the drawdown a tank you already have (or are considering) delivers. Size a Tank tells you the minimum tank size for a target pump run time.
ℹ️Total tank volume (not usable drawdown) — printed on the tank's nameplate.
ℹ️Measure with the tank drained of water, using a tire-type gauge — commonly set 2 psi below cut-in pressure.
ℹ️Pressure at which the pressure switch turns the pump on.
ℹ️Pressure at which the pressure switch turns the pump off — must be higher than cut-in.
Optionally estimate the pump's electricity cost from your own rate and daily run time.
🔧 Required: 10 GPM at 157.87 ft TDH
Scenario: Well / Transfer Pump · Flow rate entered directly
Flow Rate
Required Flow Rate: 10 GPM
Entered directly, not derived from a fixture list.
Total Dynamic Head Breakdown
Elevation: 80 ft
Friction Loss: 8.57 ft
8.57 ft per 100 ft of 1" pipe
Required Pressure Head: 69.3 ft (30 psi)
Total Dynamic Head: 157.87 ft
Pump Power & Motor Class
Water HP: 0.399 HP
Brake HP (Efficiency 60%): 0.664 HP
Motor-Size Class: 3/4 HP class
Pump configuration: Single unit, or duplex for standby redundancy
Pressure Tank Drawdown
Usable Drawdown: 13 gal
Gallons delivered per pump cycle between cut-in and cut-out — check against your pump's minimum run time to avoid short-cycling.
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 Well/Transfer Pump always needs to create its full head from scratch — there's no existing pressurized supply to credit against, unlike a Booster scenario.
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.
44 gallon tank, 30/50 switch drawdown example
Using Boyle's Law with all pressures converted to absolute (gauge + 14.7 psi): a 44 gallon tank at a 28 psi precharge, cutting in at 30 psi and cutting out at 50 psi, delivers about 13 gallons of usable drawdown per pump cycle.
That 13 gallon figure is what actually matters for pump cycling — check it against your shortlisted pump's minimum recommended run time (often listed in its installation manual) to avoid short-cycling, not the tank's full 44 gallon nameplate volume.
- Precharge 28 psi, cut-in 30 psi, cut-out 50 psi (all converted to absolute before the calculation).
- Drawdown ≈ 13 gallons from a 44 gallon tank.
- The well pump sized alongside it: 10 GPM at 157.87 ft TDH, ≈0.75 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 | Well / Transfer Pump (From Well, Cistern, or Tank) |
| Required Flow Rate | Entered directly (10 GPM) |
| Pipe | PEX (CTS), 1", 100 ft run |
| Elevation / Required Pressure | 80 ft / 30 psi |
Step 1 — Friction Loss
| Calculation | Substitution | Result |
|---|---|---|
| Friction Loss (per 100 ft) | C=150, Q=10 GPM, d=0.959 in | 8.57 ft/100ft |
| Friction Loss (full run) | 8.57 × (100 ÷ 100) | 8.57 ft |
Step 2 — Total Dynamic Head
| Calculation | Substitution | Result |
|---|---|---|
| Required Pressure Head | 30 psi × 2.31 | 69.3 ft |
| Total Dynamic Head | 80 + 8.57 + 69.3 | 157.87 ft |
Step 3 — Pump Power & Motor Class
| Calculation | Substitution | Result |
|---|---|---|
| Water HP | (10 × 157.87) ÷ 3960 | 0.399 HP |
| Brake HP | 0.399 ÷ 0.6 | 0.664 HP |
| Motor-Size Class | Nearest standard class ≥ 0.664 HP | 3/4 HP (borderline) |
Therefore, this scenario needs a pump rated for at least 10 GPM at 157.87 ft of TDH — approximately 0.664 HP brake horsepower, closest to a 3/4 HP standard motor class (borderline).
Step 4 — Pressure Tank Drawdown
This tank provides approximately 13 gallons of usable drawdown between cut-in and cut-out.
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.