Enter your flow rate, total head, and pump efficiency to get the exact horsepower and motor size your irrigation system needs — for any field, farm, or drip setup.
Enter flow demand, total head, and efficiency to get required horsepower
Flow & Pressure Requirements
Flow Rate
Total Head
Pump Efficiency
%
Don't know your total head? Build it from its parts
Static Lift
From the pumping water level (not the top of the well) to the highest outlet.
Pressure Needed at Outlets
Emitter or sprinkler operating pressure from the product chart (drip is often around 1 bar / 15 psi).
Pipe Friction & Fittings
m
From a pipe friction calculator for your pipe size, length and flow.
Filter, Valves & Injector
bar
From the manufacturer's loss charts at your flow; use the dirty-filter figure.
Fill in the parts — the total is copied into Total Head above.
Optional — Motor Derating
Motor Efficiency
%
Safety Factor
×
Power Source
Electricity Price (optional)
$/kWh
⚠ Please fill in Flow Rate, Total Head, and Efficiency with values greater than zero.
Pump Shaft Power (Brake HP)
—HP
Shaft Power (kW)
— kW
Electrical Input (kW)
— kW
Electrical Input (HP)
— HP
Recommended Motor
— HP
Flow Rate (L/s)
— L/s
Head (m)
— m
Design Flow (GPM)
— GPM
Head (ft)
— ft
Water Horsepower
— WHP
Running Cost
— $/hr
System Overview
📖 How To Use
How to Use This Irrigation Pump Size Calculator
Sizing an irrigation pump comes down to three numbers. Get those right and you'll never buy an undersized pump or waste money on an oversized one.
Enter your required flow rate
This is the total water demand of your irrigation system — add up all emitters, sprinklers, or drip lines running simultaneously. Choose your preferred unit (L/min, L/s, m³/h, or GPM).
Enter the Total Dynamic Head (TDH)
TDH is the total pressure the pump must overcome: static lift from the pumping water level to the highest outlet, friction in pipes and fittings, losses through filters and valves, and the operating pressure your emitters or sprinklers need. Open "Build it from its parts" and the calculator adds them up for you. Don't guess from static lift alone — on a drip system the emitter pressure and filter losses alone can be 13 m (about 1.3 bar) or more.
Enter pump efficiency
Read it from the pump curve at your design flow and head — that is the only reliable figure. Larger centrifugal pumps near their best efficiency point often reach 70–80%, but small pumps (under about 2 HP) are often well below that. If you don't have a curve yet, a lower figure such as 55–60% for a small pump is the safer assumption, because it sizes the motor up rather than down.
Optionally add motor efficiency and safety factor
Motor efficiency (typically 85–95% for modern motors) converts the pump's shaft power into the electrical power you pay for. The safety factor (default 1.15 = 15% margin) is applied to the pump's shaft power when picking the motor size, so the motor isn't running at its limit as the pump wears or the head changes. Choose an engine instead if the pump is diesel or petrol driven.
Read your results
The calculator outputs water horsepower, pump shaft (brake) horsepower, the standard motor size to buy (shaft power × safety factor, rounded up), the electrical input power the motor draws, and running cost per hour if you enter your electricity price.
Tip: A motor's nameplate horsepower is its rated output at the shaft, so compare it with the pump's brake horsepower — not with the electrical input. The recommended size here already includes your safety factor. Then check the pump curve: the pump must deliver your flow at your head, and the motor size alone can't tell you that.
📐 The Formula
Irrigation Pump Sizing Formula Explained
Pump sizing uses two steps: first calculate hydraulic (shaft) power, then account for losses through the pump and motor.
Step 1: Hydraulic (Water) Power
P_hydraulic (kW) = (Q × H × ρ × g) ÷ 1,000
Where:
Q = Flow rate (m³/s)
H = Total Dynamic Head (m)
ρ = Water density = 1,000 kg/m³
g = 9.81 m/s²
Step 2: Shaft Power (accounting for pump efficiency)
Step 3: Motor Input Power (accounting for motor efficiency + safety factor)
Motor size (HP) ≥ P_shaft (HP) × SF, rounded up to a standard size P_input (kW) = P_shaft ÷ η_motor (what the motor draws from the supply)
η_motor = motor efficiency (e.g., 0.90 for 90%)
SF = safety factor (e.g., 1.15 for 15% margin)
HP = kW × 1.341
Engine: rated HP ≥ P_shaft ÷ 0.80 (continuous-duty derating)
In US units the same calculation is the classic extension formula: water horsepower = GPM × TDH (ft) ÷ 3,960, and brake horsepower = WHP ÷ pump efficiency (K-State MF-1040). One psi equals 2.31 ft of water head. The safety factor is applied to the shaft power because a motor's nameplate rating is its output; applying it to the electrical input as well would double-count the motor's own losses. K-State's rule of thumb for electric units — nameplate HP ≈ BHP ÷ 0.9 — gives a similar margin. For engines, K-State also derates for right-angle drives (×0.95), altitude (×0.91 at 3,000 ft) and heat (×0.96 at 100 °F); apply those if they fit your setup.
Standard Motor Sizes (HP)
Motor HP
Nearest IEC kW
Max Pump Shaft HP (with 1.15 SF)
Typical Application
0.5 HP
0.37 kW
0.43 HP
Small garden drip
0.75 HP
0.55 kW
0.65 HP
Small garden drip
1 HP
0.75 kW
0.87 HP
Small farm drip
1.5 HP
1.1 kW
1.3 HP
Small farm drip
2 HP
1.5 kW
1.74 HP
Orchard / small field
3 HP
2.2 kW
2.61 HP
Orchard / small field
5 HP
3.7 kW
4.35 HP
Medium field
7.5 HP
5.5 kW
6.52 HP
Medium–large field
10 HP
7.5 kW
8.7 HP
Large field sprinkler
15 HP
11 kW
13 HP
Farm main pump
20 HP
15 kW
17.4 HP
Large farm
25 HP
18.5 kW
21.7 HP
Large farm
30 HP
22 kW
26.1 HP
Commercial irrigation
40 HP
30 kW
34.8 HP
Commercial irrigation
50 HP
37 kW
43.5 HP
Large commercial
75 HP
55 kW
65.2 HP
Industrial / canal
100 HP
75 kW
87 HP
Industrial / canal
Always select the next standard size above your pump shaft power × safety factor, not below. Available sizes vary by market and motor type — check your supplier's range.
Pump Efficiency Benchmarks
Pump Type
Typical Efficiency
Best Use
Centrifugal (end suction)
60–80%
Surface irrigation, open channels
Submersible (borehole)
65–75%
Groundwater, deep wells
Turbine (vertical)
70–85%
High-flow, moderate head
Small pumps (under ~2 HP)
often 40–60%
Gardens, small drip zones
Axial flow
75–90%
Very high flow, low head
Typical best-efficiency ranges; a pump running far from its best efficiency point, or a worn pump, can be much lower. Use the efficiency from your pump's own curve at your duty point.
🌾 Use Cases
When to Use an Irrigation Pump Size Calculator
💧
Drip Irrigation Systems
Drip systems run at low pressure (1–3 bar) but often serve large areas with many emitters. Calculate total flow demand before selecting a pump — undersizing causes uneven distribution across zones.
🌀
Sprinkler Systems
Sprinklers require higher pressure (2–5 bar) and moderate to high flow. Pump sizing must account for the highest-demand zone running simultaneously, not average demand.
🏗️
Borehole / Submersible Pumps
Deep well pumps face high static head (sometimes 40–100+ m). Every extra metre of lift demands more power. Accurate head measurement is critical — never guess on borehole depth.
🌿
Greenhouse & Nursery
Controlled environments need precise pressure for misting and fertigation systems. Small centrifugal pumps common in greenhouses are often less efficient than large field pumps, so use the efficiency from the pump curve rather than a general figure.
🌱
Crop Field Replacement
When upgrading from gravity-fed to pressurized irrigation, or expanding a field, recalculate pump size from scratch — don't assume the old pump will handle the new load.
🔋
Solar Pump Systems
Solar pumps only run while the sun is up, so the daily volume has to be pumped in a few strong-sun hours — that means a higher flow rate than a pump running all day. Calculate the shaft and input power at that flow and use the pump maker's solar sizing tool for the array.
🧮 Worked Example
Worked Example: Borehole Pump for 1 Acre of Drip
One acre of drip tomatoes needs about 32,400 L/day at peak (see our Drip Irrigation Tank Size Calculator). It runs as 2 zones, so the pump delivers 90 L/min (1.5 L/s, about 23.8 GPM) for 6 hours. The pumping water level is 30 m down, drippers need 1 bar, the disc filter loses up to 0.3 bar when dirty, and pipe friction is 4 m.
Step
Working
Result
Total head
30 + 10.2 (1 bar) + 3.1 (0.3 bar) + 4
47.3 m (155 ft)
Water power
1.5 L/s × 47.3 m × 9.81
0.70 kW (0.93 WHP)
Shaft power at 60%
0.70 ÷ 0.60
1.16 kW (1.56 HP)
Motor size
1.56 HP × 1.15 = 1.79 HP
2 HP (1.5 kW)
Electrical input at 90%
1.16 ÷ 0.90
1.29 kW
Energy per day
1.29 kW × 6 h
7.7 kWh
In US units: 23.8 GPM × 155 ft ÷ 3,960 = 0.93 WHP — the same answer. Notice that the emitter pressure and filter loss make up 13.3 m, more than a quarter of the head; sizing from the 30 m lift alone would have picked a pump that cannot open the drippers properly. Before buying, check the pump curve passes through 1.5 L/s at 47 m.
❓ FAQ
Frequently Asked Questions
How do I calculate irrigation pump horsepower?
In metric: shaft HP = (flow in L/s × head in m × 9.81) ÷ (pump efficiency × 746). In US units: water horsepower = GPM × head in feet ÷ 3,960, and brake (shaft) horsepower = WHP ÷ pump efficiency. Pick a motor whose nameplate HP is at least the shaft HP × your safety factor (typically 1.15), rounded up to a standard size. This calculator does all of that automatically.
What is Total Dynamic Head (TDH) in irrigation?
Total Dynamic Head is the total equivalent height of water the pump must push against. It includes: (1) Static head — the vertical distance from water source to delivery point; (2) Friction head — pressure loss due to pipe friction, bends, and fittings; (3) Pressure head — the operating pressure required at sprinklers or drip emitters, converted to head (1 bar ≈ 10.2 m; 1 psi = 2.31 ft). Losses through filters, valves and fertiliser injectors are part of the friction head and are easy to forget. Add them all for your TDH.
What pump efficiency should I use if I don't know mine?
Use the efficiency from the manufacturer's pump curve at your design flow and head whenever you can. If you don't have a curve yet, choose a lower figure rather than a higher one — about 60% is a reasonable planning value for a small submersible or surface pump, and 70% for a larger pump running near its best efficiency point. A lower efficiency gives a bigger motor, which is the safe direction.
What safety factor should I apply to irrigation pump sizing?
A margin of about 15% (1.15) on the pump's shaft power is common. K-State Extension's rule of thumb for electric units, nameplate HP ≈ BHP ÷ 0.9, gives a similar margin of about 11%. Use more if the head is uncertain, the pump will run at varying duty points, or you plan to expand. A motor running continuously at or above its nameplate rating runs hot and wears out early.
What is the difference between shaft horsepower and motor horsepower?
Water horsepower is the power actually given to the water (flow × head). Shaft or brake horsepower is what the pump needs at its shaft, which is higher because the pump has losses. A motor's nameplate HP is its rated output at the shaft, so it is compared with the pump's brake HP (plus a margin). The electrical power the motor draws is higher again, because the motor is typically 85–95% efficient — that is the figure for your power bill, generator or solar array.
How do I convert GPM and feet of head to metric for this calculator?
You don't need to — this calculator accepts GPM directly for flow rate and feet for head. Just select the matching unit from the dropdown next to each input. All conversions happen internally: 1 GPM = 0.0631 L/s; 1 ft head = 0.3048 m head.
What pump size is needed for 1 acre of drip irrigation?
Work from the crop's water need, not a rule of thumb. One acre of drip tomatoes at peak needs about 32,400 L a day. Pumped in 6 hours that is 90 L/min (about 24 GPM). With 47 m of total head (30 m lift, 1 bar at the drippers, filter and pipe losses) and a 60% efficient pump, the shaft power is 1.56 HP, so a 2 HP motor — see the worked example above. With a shallow source and short pipes, the same flow might need only a 1 HP pump.
Can I use this calculator for a solar-powered irrigation pump?
Yes, for the power side. Because a solar pump only runs in daylight, first set the flow so the daily volume is pumped in your strong-sun hours, then read the electrical input (kW). Panels rarely deliver their full rated output in real conditions, so the array's rated watts need to be noticeably larger than that figure; use the pump maker's solar sizing tool, which accounts for your location and controller.
✅ Sources & Review
Sources and review
The formulas and conversion factors on this page come from these references:
The pump efficiency ranges and typical applications are general guidance; the pump manufacturer's curve for your duty point always takes priority.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
