Pressure Loss: What Size Supply Line Should I Use?
Product FAQ · Supply Lines
Pressure Loss: What Size Supply Line Should I Use?
If your supply line is too small, you lose too much pressure before the water reaches your sprinklers or drip tape, and you get weak spray, uneven watering and dry spots. Use our free line size calculator to find the right size and see how much pressure you'll have left at the field, or follow the four steps below.
What is pressure loss?Why a line that's a little too small causes a big problem
As water moves through a pipe or hose, it rubs against the inside walls and against every fitting and bend along the way. That resistance is called friction loss, and it makes the pressure drop as the water travels.
A smaller line means faster water. To push the same gallons per minute through a narrower pipe, the water has to move faster, and faster water creates much more friction.
Friction loss isn't linear. Double the flow through the same hose and the pressure loss goes up by more than double. For example:
Double the flow2×
Pressure loss goes up3.6×
Same 40 GPM, 500 ft of PVC, different sizes
Each step up in size cuts water speed and pressure loss sharply.
1-1/4"
−41 psi
1-1/2"
−19 psi
2"
−5.7 psi
3"
−0.8 psi
Bar length = pressure lost. Stripe speed = water speed.
An undersized line gives you low pressure at the sprinklers or drip lines, uneven coverage, dry spots, and extra strain on pumps. An oversized line works fine but costs more than it needs to. The goal is the smallest line that still delivers enough pressure.
Five things that drive pressure loss
Flow rate (GPM)
More water through the same line means more friction and more loss.
Line size
Smaller diameter means faster water and more friction.
Length of the line
The longer the run, the more pressure you lose along it.
Fittings & valves
Elbows, tees, filters and valves each add resistance.
Elevation
Pumping uphill costs about 0.43 psi for every foot of rise.
Not enough pressure at the field?You have two fixes, and often the best answer is a bit of both
Fix 1 · Go up a line size
- A larger diameter slows the water down and cuts friction loss.
- One size up often cuts the loss by more than half.
- Best for long runs out to the field.
Fix 2 · Split the field into zones
- Use valves to divide the field into sections.
- Run fewer sprinklers, or shorter lengths of drip tape, at one time.
- Less flow per zone means less loss, and a smaller pump can do the job.
Rules of thumb (when you don't have a calculator)
| Total flow | Consider at least |
|---|---|
| Over 10 GPM | 1" pipe |
| Over 50 GPM | 2" pipe |
| Over 100 GPM | 3" pipe |
For long runs (over 100 feet) or high flows, use our pressure loss calculator. Rules of thumb don't account for distance, so they can easily undersize a long line.
Example: 10 sprinklers × 4 GPM each = 40 GPM. The rule of thumb says at least 1" pipe. But at 40 GPM, 1" PVC moves water at about 15 ft/s and loses about 31 psi every 100 ft. Over a 600 ft run, a pressure loss calculator shows you need 2".
Most flow each size should carry · PVC Schedule 40
| Size | Inside dia. | Max at 5 ft/s | Loss / 100 ft at max |
|---|---|---|---|
| 1/2" | 0.622" | 4.7 GPM | 7.57 psi |
| 3/4" | 0.824" | 8.3 GPM | 5.46 psi |
| 1" | 1.049" | 13 GPM | 4.13 psi |
| 1-1/4" | 1.380" | 23 GPM | 3.00 psi |
| 1-1/2" | 1.610" | 32 GPM | 2.51 psi |
| 2" | 2.067" | 52 GPM | 1.88 psi |
| 2-1/2" | 2.469" | 75 GPM | 1.53 psi |
| 3" | 3.068" | 115 GPM | 1.19 psi |
| 4" | 4.026" | 198 GPM | 0.87 psi |
| 6" | 6.065" | 450 GPM | 0.54 psi |
Keeping water at or under 5 ft/s limits friction and protects against water hammer.
Big guns, big flow
Rain guns and large impact sprinklers can each use 20 to 200+ GPM. At those flows, line size matters more than anything else in your system. Size the mainline before you choose the pump.
Shop supply lines
Lay flat discharge hoseSun-Flow PVC lay flat in 1-1/2" to 6". Rolls out fast for seasonal mainlines.Shop lay flat →
Toro Blue Stripe® poly1/2" to 1" poly tubing for small zones, gardens and drip headers.Shop poly tubing →
Aluminum mainlineRing lock and grooved couplers for 2" to 8" portable mainline.Shop mainline →
PVC fittings & flangesFittings and flanges for buried Schedule 40 PVC mainlines.Shop PVC fittings →
Common questions
Where do I find the flow for my sprinklers or drip tape?
It's on the product spec sheet. Sprinklers list GPM at a given pressure and nozzle size. Drip tape lists flow per 100 ft, so multiply by the total feet of tape running at once and divide by 100.
What pressure should I enter for my source?
Use the pressure your pump or valve delivers while running at the flow you need, not the static pressure with everything shut off. A pressure gauge at the pump outlet is the easiest way to check.
Why not just buy the biggest line?
A bigger line always works, but it costs more and is heavier to handle, and above a point the extra pressure you save is tiny. Size for the pressure you need plus a few psi of margin, and spend the savings elsewhere in the system.
Does going downhill help?
Yes. Every foot of drop adds about 0.43 psi. Check that the low end doesn't exceed your sprinklers' or drip tape's maximum pressure.
Where can I run the numbers?
Use our pressure loss calculators. Enter your flow, the line's inside diameter, length and material, and it shows the pressure lost along the line.
How to size your supply line
- Find your total flowAdd up the GPM of everything running at once.
- Measure the distanceFrom the source to the farthest connection.
- Check pressure at the endSource pressure minus losses must meet your minimum.
- Fix it if it's lowGo up a line size, split into zones, or both.
Still not sure? Ask us.
Send us your flow, distance and pressure and we'll help you size the line. Call (844) 259-0640 or email Support@IrrigationKing.com, Monday to Friday, 9:00 AM to 5:00 PM PST.