Pump Sizing Calculator

How this calculator sizes a pump

A pump is defined by its duty point: the flow it must deliver and the head it must deliver it against. Every other choice — the motor, the pipe, the pump type — follows from those two numbers.

Flow when filling a tank

For a transfer pump, the flow is the day’s water divided by the hours you are willing to run the pump:

Flow (m³/h) = daily water use (L) ÷ 1,000 ÷ run hours

We default to 4 hours. A pump sized to fill the tank in 4 hours can catch up after a power cut or a dry sump, and it isn’t so oversized that it hammers the pipework. You can change it.

If you don’t know the daily use, the calculator estimates it from the National Building Code of India (NBC 2016, Part 9) per-head norms:

Building	Norm
Homes and flats	135 L per person a day (150–200 L for premium homes), 5 people per flat
Offices	45 L per person
Hotels	180 L per bed (320 L for 4-star and above)
Hospitals	340 L per bed (450 L above 100 beds)
Hostels and boarding schools	135 L per resident
Day schools	45 L per person
Restaurants	70 L per seat
Factories	45 L per worker (30 L without bathrooms)
Gardens	about 5 L per m² a day

Example: 50 flats × 5 people × 135 L = 33,750 L a day. Over 4 hours, that is 8.44 m³/h (141 L/min).

Flow when holding pressure

A booster pump has no tank to fill. It runs when taps open, so it must keep up with the busiest minute, not the average hour. The calculator gives you three ways to find that peak:

1. Count fixtures. Each fixture carries a load in water supply fixture units (WSFU): a cistern WC is 2.2, a wash basin 0.7, a shower or kitchen sink 1.4, a flush-valve WC 10. Hunter’s curve converts the building’s total into a probable peak flow. Fifty flats with two bathrooms each add up to about 570 WSFU, or around 520 L/min.
2. Count users. Daily use ÷ hours of heavy use × a peak factor (default 4 hours and 1.5×).
3. Enter a measured flow from a flow meter, a data logger or the existing system.

Hunter’s method dates from the 1940s and runs high with today’s low-flow fittings. The calculator shows both estimates side by side, so you can see when they disagree.

Head: lift, friction and pressure

Total head = static lift + friction + pressure (or discharge) at the outlet

• Static lift is the vertical distance from the lowest water level in the sump or borewell to the outlet. As a rule of thumb, allow about 3 m per floor plus 2 m for the tank stand.
• Pressure at the outlet — 1 bar is 10.2 m of head. A tank inlet needs about 2 m just to discharge. A booster serving showers needs about 2 bar at the highest outlet, and a rain shower about 2.5 bar.
• Friction uses the Hazen-Williams equation: h = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87), with Q in m³/s and d in metres. C is 140 for uPVC and CPVC and 120 for GI. Bends, valves and fittings add 25% by default.

The calculator picks the smallest standard pipe that keeps water at or under 1.8 m/s. Faster than about 2 m/s, pipes get noisy and friction climbs steeply.

From head to motor

Hydraulic power (kW) = 9.81 × flow (m³/s) × head (m)

Divide by pump efficiency — 55% is typical for small and mid-size centrifugal pumps — to get shaft power. Add 10% margin, then round up to the next standard motor: 0.37, 0.55, 0.75, 1.1, 1.5, 2.2, 3.0, 3.7 kW and so on.

Worked example

Fifty flats, an overhead tank 30 m above the pump, and the sump water level 2 m below it:

Step	Value
Flow	33,750 L ÷ 4 h = 8.44 m³/h
Static lift	30 + 2 = 32 m
Discharge at tank	2 m
Friction, 50 m of 40 NB at 1.7 m/s	3.9 m + 25% = 4.9 m
Total head	38.9 m
Hydraulic power	9.81 × 0.00234 × 38.9 = 0.89 kW
Shaft power at 55%	1.62 kW
Motor	2.2 kW (3 HP)

Running and standby pumps

With more than one pump running, each one carries its share of the flow at the same head. Two running pumps on the example above would each be 4.2 m³/h at 38.9 m. Standby pumps don’t change the duty point — they’re identical spares. An automatic controller should alternate duty and standby, so both pumps wear evenly and the standby is proven to work when it’s needed.

Sizing the pressure tank

A fixed-speed booster switches on at the cut-in pressure and off at cut-out. Every motor start heats the winding, so each pump has a limit on starts per hour — commonly up to 60 for small surface pumps, fewer for larger motors and submersibles. The tank supplies water between cut-out and cut-in, so the pump doesn’t start for every glass of water.

Cycling is fastest when demand is exactly half the pump’s flow. One on/off cycle then takes 4 × drawdown ÷ pump flow, which sets the drawdown the tank must hold:

Drawdown (L) = pump flow (L/s) × (3,600 ÷ starts per hour) ÷ 4

Only part of a diaphragm tank is usable. With the air precharge set 0.2 bar below cut-in, Boyle’s law gives the usable fraction from absolute pressures:

Usable fraction = (P_off − P_on) ÷ P_off × P_pre ÷ P_on

Example: a 120 L/min pump limited to 60 starts an hour needs 2 L/s × 60 s ÷ 4 = 30 L of drawdown. On a 3.0 to 4.0 bar switch, about 19% of the tank is usable, so it needs 30 ÷ 0.19 = 158 L — a 200 L tank. Widening the band to 1.5 bar, or a pump that allows more starts, shrinks the tank. The calculator shows this trade-off as a table.

With several pumps in cascade, size the tank for one pump: the last pump to switch is the one that cycles. A variable-speed (VFD) booster slows down instead of switching off, so it needs only a small tank.