How much tubing and how many emitters a drip irrigation system needs

How the drip system is worked out

The emitter tubing you need is the number of rows times the row length; add the header that runs along the end of the bed and the supply line from the faucet. Four rows of 20 ft take 80 ft of emitter tubing.

The header is as long as the distance from the first row to the last: (rows - 1) × row spacing. The supply line includes the runs between one bed and the next. With emitter tubing the emitters are built in; with plain 1/2" tubing the rows are the same tubing as the header and you punch in one emitter per plant.

Emitters per row are the row length divided by the emitter spacing, rounded down. The total flow is the number of emitters times the flow of one. Colorado State University Extension treats 75 percent of the faucet flow as the most you can use at once, and you measure that flow with a bucket: gallons in the bucket divided by the seconds to fill it, times 60 for gpm. The same guide puts the limit of 1/2" tubing at about 200 gph per zone (a conservative figure). When the rows need more, the calculator splits them into zones that run one after another, each with its own supply line.

Fittings: a tee for every row except the last one in each zone, an elbow on the last row, an end cap on every row and an adapter to connect the tubing to the timer. Each zone gets a filter and a pressure regulator.

Worked example: a vegetable bed with 4 rows

These are the values the calculator opens with in US units: one bed with 4 rows of 20 ft, 18" apart, emitter tubing with 0.9 gph emitters every 12" (like Rain Bird's 1/2" emitter tubing), 25 ft of tubing from the faucet, a 5 gpm faucet.

• Emitter tubing: 4 × 20 = 80 ft, one 100 ft roll
• Distribution tubing: 25 + 3 × 1' 6" = 29' 6", one 50 ft roll
• Emitters: 20 per row, 80 in all
• Flow: 80 × 0.9 = 72 gph
• Usable flow: 75% of 5 gpm is 225 gph, above the 200 gph the tubing carries, so the limit is 200 gph and 1 zone is enough
• Fittings: 3 tees, 1 elbow, 4 end caps, 1 faucet adapter

In the UK, with metric units: 4 rows of 5 m, 50 cm apart, dripline with 2 L/h emitters every 33 cm, 10 m from the tap and a 15 L/min tap give 20 m of dripline (a 25 m coil), 11.50 m of blank pipe, 60 emitters and 120 L/h, against 675 L/h of usable flow: one zone.

How many rows per zone: table by faucet flow

Zones for rows of 25 ft of emitter tubing with 0.9 gph emitters every 12" (25 emitters and 22.5 gph per row), using 75% of the faucet flow and no more than 200 gph per zone.

Rows of 25 ft	3 gpm	5 gpm	8 gpm
4	1 zone	1 zone	1 zone
8	2 zones	1 zone	1 zone
12	2 zones	2 zones	2 zones
20	4 zones	3 zones	3 zones
30	5 zones	4 zones	4 zones

From 5 gpm up the tubing is the limit, not the faucet: that is why 5 and 8 gpm give the same zones. Run the zones from a multi-zone hose timer or from separate timers.

Rolls, emitters and fittings you can buy

Rain Bird sells 1/2" emitter tubing with 0.9 gph emitters at 12" or 18", in 50 ft and 100 ft rolls, and 1/2" distribution tubing in 50, 100 and 500 ft. The calculator picks the mix of rolls that leaves the least tubing over. In Europe Claber sells 1/2" (13-16 mm) dripline with 2 L/h emitters about every 33 cm in 25 m and 50 m coils.

Colorado State University lists emitters of 1/2, 1, 2 and 4 gph and suggests spacing them 12" apart in sand, 18" in loam and 24" in clay. Pressure compensating emitters give the same flow over a wide range of pressure; buy all emitters from one maker, because the color codes for flow differ between brands.

The guide puts the head assembly in this order: backflow preventer, valve or timer, filter, then pressure regulator. A 150 to 200 mesh filter suits clean city water.

Common mistakes

• Putting too many rows on one zone. If the rows ask for more than the faucet can give, the last emitters barely drip: check the number of zones on the receipt.
• Counting the full faucet flow. The bucket test gives the flow with the faucet wide open; Colorado State University counts 75 percent of it as the most a zone can use.
• Leaving out the filter or pressure regulator. The same guide lists both among the most common setup mistakes.
• Taking the rated flow at any pressure. Claber's 2 L/h dripline gives 2.7 L/h at 1.5 bar and 3.1 L/h at 2 bar; pressure compensating emitters hold steady.
• Counting only the rows. The header and the supply line add up: in the example they are 29' 6" of the 109' 6" of tubing in all.