Drip Irrigation Zone Flow Calculator

Drip zones come in two flavors, and the math differs from spray/rotor heads (covered by the Sprinkler Head Spacing Calculator, which handles spray and rotor zone GPM).

Point emitters
  zone GPH = emitter count × GPH per emitter
  zone GPM = zone GPH / 60

Dripline / tubing
  emitters per 100 ft = 100 / (spacing in ft)
  flow per 100 ft (GPH) = emitters per 100 ft × GPH per emitter
  zone GPH = (total length / 100) × flow per 100 ft
  zone GPM = zone GPH / 60

Supply check
  usable GPM = available supply GPM × (usable % / 100)
  over supply when zone GPM > usable GPM

Header pipe sizing  (Irrigation Tutorials)
  V (ft/s) = 0.408 × GPM / ID²
  minimum ID = √(0.408 × GPM / velocity ceiling)

Worked example (dripline): 650 ft of tubing, 18" emitter spacing, 0.9 GPH inline emitters. 18" = 1.5 ft, so 100 / 1.5 = 66.7 emitters per 100 ft, each at 0.9 GPH = 60 GPH per 100 ft (1.0 GPM per 100 ft). 650 ft produces 390 GPH = 6.5 GPM. At a 5 ft/s velocity ceiling, the minimum header ID is 0.73", so 3/4" Schedule 40 PVC (0.824" ID) clears it.

The velocity constant 0.408 and the 5 ft/s ceiling come from Irrigation Tutorials (formulas page and lateral pipe sizing chart). Pipe inner diameters are Schedule 40 PVC per Engineering Toolbox. The 75% default is a planning starting point, not a published figure; set it from your own supply test.

Once you know the zone GPM, run it through the Irrigation Run Time Calculator to set the controller minutes. For plant layout upstream of the drip plan, the Plant & Ground Cover Spacing Calculator gives the grid. Contractors pricing an install will want the Irrigation System Installation Pricing Calculator.