The formula
Total head = well depth + tank height + friction loss (auto 5%)
Calculated HP = flow × head ÷ (3960 × efficiency), with flow in gallons per minute, assuming the day’s water is pumped in 1 hour. Efficiency is 0.58.
Recommended HP = the next band up (up to 200 ft → 1 HP; 200–350 ft → 1.5 HP; above 350 ft → 2 HP).
Hourly cost = HP × 0.746 ÷ 0.58 × tariff.
Calculated HP and recommended HP are shown separately — the widget keeps both side by side.
Worked example
120 ft well, two-storey tank (20 ft), 5 people, tariff Tk 8.50:
Total head = 120 + 20 + 7 (5%) = 147 ft. Daily water = 5 × 135 = 675 litres → flow ≈ 2.97 GPM. Calculated HP = 2.97 × 147 ÷ (3960 × 0.58) ≈ 0.19. Head is within 200 ft, so the recommended motor is 1 HP submersible (approx. Tk 11,800). Hourly cost = 1 × 0.746 ÷ 0.58 × 8.50 ≈ Tk 10.93.
A 320 ft well with a third-floor tank (30 ft): head = 320 + 30 + 17.5 = 367.5 ft → past 350, so even though the calculated HP is only 0.48, the recommended motor is 2 HP (approx. Tk 15,800, about Tk 21.87 an hour).
What to know in Bangladesh
- The 7-metre limit: centrifugal/jet pumps cannot suck from deeper than about 7 m below the surface. Deeper wells need a submersible — the calculator refuses the wrong choice itself.
- Tariff: the Tk 8.50 default is the June-2026 slab rate for 76–200 units. Set your own slab from your bill; the electricity-bill calculator can confirm it.
- Prices are approx.: motor prices are Sep-2026 shop listings (a 1 HP submersible runs Tk 11,400–12,300 across Gazi/RFL). Confirm at a shop before buying.
- Safety: motor selection, wire size, breaker and earthing belong to a licensed electrician; never run the motor dry.
Motor by head
| Total head | Recommended motor | Approx. price (submersible) |
|---|---|---|
| Up to 200 ft | 1 HP | Tk 11,800 |
| 200–350 ft | 1.5 HP | Tk 13,199 |
| Above 350 ft | 2 HP | Tk 15,800 |
Prices are Sep-2026 shop listings and being verified — confirm at a shop.
More: why small motors burn out, and how to cut running cost
Why a bigger motor than calculated: the formula gives the ideal figure. In real life voltage swings, pipes scale up, and the water table drops in summer. So the rule is the next size up by head band — 0.19 HP still means a 1 HP home motor, and 0.48 HP at 367 ft of head means 2 HP.
Jet vs submersible: jet pumps are cheaper for shallow wells (1 HP ≈ Tk 7,200–9,100) and easy to repair, but the suction limit rules them out for deep borings. Submersibles sit in the water, run quieter and more efficiently.
Cutting running cost: fill the tank in one go and switch off — repeated starts use more power. Fix leaks: a dripping tap adds hundreds of litres a day. Correct wire size cuts voltage drop, so the motor runs cooler.