Precipitation rate is the amount of water, measured in millimeters, that an irrigation system delivers per unit area per hour. It is the key parameter that determines watering uniformity, zone run times, and ultimately the health of your lawn.

What is precipitation rate?

Precipitation rate is measured in mm/h (or inches/h in the US). It tells you how quickly the system "fills up" an area. For example, if the precipitation rate is 15 mm/h, then after 20 minutes of operation the lawn receives 5 mm of water.

Precipitation rate depends on two factors:

  • Nozzle flow rate (Q) — how many liters per minute each sprinkler delivers
  • Sprinkler spacing (S) — the distance between heads, typically equal to the throw radius (head-to-head)

Square spacing pattern

In a square pattern, sprinklers are placed at the vertices of a square grid. Each sprinkler services an area of S × S.

Square pattern S S S x S

Formula for square spacing:

P = 96.3 x Q / (S x S)
P — precipitation rate (mm/h), Q — nozzle flow rate (L/min), S — spacing (m)

Example: Hunter MP1000 nozzle (Q = 0.16 L/min), spacing S = 2.5 m:

P = 96.3 x 0.16 / (2.5 x 2.5) = 15.4 / 6.25 = 2.46 mm/h

Triangular spacing pattern

In a triangular (staggered) pattern, every other row is offset by half the spacing. This provides better coverage uniformity because sprinklers are closer together diagonally.

Triangular pattern S S 0.866 x S

The row spacing in a triangular pattern is S x 0.866 (i.e., S x sin 60 degrees = S x sqrt(3)/2). The area each sprinkler covers is smaller than in a square layout, so the precipitation rate is higher.

Formula for triangular spacing:

P = 96.3 x Q / (S x S x 0.866)
P — precipitation rate (mm/h), Q — nozzle flow rate (L/min), S — spacing (m)

Example: same Hunter MP1000 nozzle (Q = 0.16 L/min), spacing S = 2.5 m:

P = 96.3 x 0.16 / (2.5 x 2.5 x 0.866) = 15.4 / 5.41 = 2.85 mm/h

Why matched precipitation matters

If you install nozzles with different precipitation rates in the same zone (e.g., rotors and fixed spray heads), part of the lawn will be overwatered while another part stays dry. This leads to:

  • Yellow patches and turf disease from overwatering
  • Dry spots where grass burns out
  • Wasted water and higher utility bills

This is exactly why Hunter developed the MP Rotator series — these nozzles have a very low precipitation rate (~10 mm/h), close to that of rotors. This makes it possible to achieve uniform watering even on complex layouts.

Square or triangular: when to use which?

Square pattern Rectangular areas Along buildings and fences Easier installation Fewer sprinklers needed Uniformity: ~83% Triangular pattern Open lawns Circular / complex shapes Best uniformity More efficient coverage Uniformity: ~90%

Practical tips

  1. Always use head-to-head spacing — distance between sprinklers = nozzle throw radius
  2. Do not mix nozzle types in the same zone — rotors, spray heads, and MP Rotators have different precipitation rates
  3. Check your pressure — low pressure reduces throw radius and increases precipitation rate
  4. Account for slopes — on hillsides, the precipitation rate should be lower so water has time to soak in
  5. The 96.3 constant — converts L/min and m² into mm/h (1 L/m² = 1 mm)

Calculate in SmartPluvia

Our SmartPluvia online planner automatically calculates precipitation rate for every zone and warns you when nozzles are mismatched. Try it for free!