How long should drip run?
Long enough to deliver the gallons each plant needs this session, at the rate its emitters put out. Drip doesn't have a "precipitation rate" the way sprinklers do, because it waters plants rather than area — so the calculator works per plant: the circle under the canopy is roughly the root zone, a target depth over that circle is a number of gallons, and gallons divided by the plant's emitters' GPH is hours.
A tomato with a 2 ft canopy at an inch a week needs 2.0 gallons a week — over three sessions, 0.65 gal each. On two 1 GPH emitters that's 20 minutes. Twelve of them on one line draw 24 GPH, which is nothing: ½ in tubing carries 200.
Gallons per plant, by canopy
Root zone ≈ canopy circle; 0.623 gallons puts an inch on a square foot. The whole calculator is these two facts.
| Canopy diameter | Root zone | 1 in / week | ½ in / week |
|---|---|---|---|
| 1 ft | 0.8 sq ft | 0.5 gal | 0.2 gal |
| 2 ft | 3.1 sq ft | 2.0 gal | 1.0 gal |
| 3 ft | 7.1 sq ft | 4.4 gal | 2.2 gal |
| 5 ft | 19.6 sq ft | 12.2 gal | 6.1 gal |
| 8 ft | 50.3 sq ft | 31.3 gal | 15.7 gal |
| 15 ft | 176.7 sq ft | 110.1 gal | 55.0 gal |
| 25 ft | 490.9 sq ft | 305.8 gal | 152.9 gal |
Per plant per week, before dividing into sessions. A full inch is a vegetable-garden or new-planting figure; established shrubs and trees in most climates do well on half that, and drought-adapted plants on less.
Emitters by plant size — and how long each runs
The standard layouts from the drip design guides, with what they work out to at an inch a week over 3 sessions:
| Plant | Canopy | Emitters | Gal / week | Per session |
|---|---|---|---|---|
| Vegetable / annual | 1.5 ft | 1 × 1 GPH | 1.1 | 22 min |
| Perennial / ground cover | 2 ft | 1 × 1 GPH | 2.0 | 39 min |
| Small shrub (to 3 ft) | 3 ft | 2 × 1 GPH | 4.4 | 44 min |
| Large shrub (3–6 ft) | 5 ft | 2 × 2 GPH | 12.2 | 61 min |
| Small tree (to 10 ft) | 8 ft | 3 × 2 GPH | 31.3 | 1.7 h |
| Medium tree (10–20 ft) | 15 ft | 5 × 2 GPH | 110.1 | 3.7 h |
| Large tree (20 ft +) | 25 ft | 8 × 2 GPH | 305.8 | 6.4 h |
Rain Bird and Colorado State Extension layouts; run times computed by this calculator. Emitters multiply as a plant grows — the ring moves out with the dripline.
People used to sprinklers see "6.4 h" for a large tree and assume the calculator is broken. It isn't. A 25 ft canopy covers 491 sq ft and an inch over it is 306 gallons a week; eight 2 GPH emitters deliver 16 gallons an hour. The arithmetic is the arithmetic. Drip is slow by design — it's applying water at the rate soil takes it, which is the entire point — and the honest fix for a run that's too long is more or bigger emitters, not a shorter timer. (Runs over 4 hours get flagged for exactly that.)
The two limits every zone must fit
A drip zone can be too big in two independent ways, and the calculator checks both. The tubing: ½ in poly carries about 200 GPH and runs 200 ft before the last emitters are starved — the "200/200" rule. The tap: whatever the bucket test says, less the pressure regulator and filter. Exceed either and the far end of the line gets nothing while the near end looks fine.
| Tubing | Max flow | Max run | 0.5 GPH emitters | 1 GPH | 2 GPH |
|---|---|---|---|---|---|
| ¼ in micro tubing | 30 GPH | 30 ft | 60 | 30 | 15 |
| ½ in poly mainline | 200 GPH | 200 ft | 400 | 200 | 100 |
| ¾ in poly mainline | 480 GPH | 400 ft | 960 | 480 | 240 |
Published supplier rules of thumb (DripWorks, DIG, Rain Bird); the ½ in figure is the conservative end of a 200–240 GPH range. Maximum emitters = max flow ÷ emitter flow.
When a zone overruns the tubing, the answer is a second valve or a step up to ¾ in mainline — not "run it longer", which only makes the far end wait longer for water it isn't getting. The calculator says how many lines it takes.
The bucket test
Put a 5 gallon bucket under the tap the zone will run from, open it fully, and time the fill. Gallons ÷ seconds × 60 is your supply in GPM: 5 gallons in 30 seconds is 10 GPM, or 600 GPH — which is why a single ½ in drip line, capped at 200 GPH, never comes close to exhausting a normal tap. The tap matters when you gang several lines on one manifold, or when the supply is a well pump or a long run of ½ in hose.
Inline drip line in vegetable beds
Emitter tubing laid in rows — 0.6 or 0.9 GPH every 12 in is the common spec — waters a bed as an area rather than plant by plant, so it does have a precipitation rate: 231 × GPH per emitter ÷ (emitter spacing × row spacing), in inches. That number goes straight into the run-time calculator like a sprinkler rate.
| Line | Emitter spacing | Row spacing | Rate | Minutes per inch |
|---|---|---|---|---|
| 0.6 GPH | 12 in | 12 in | 0.96 in/hr | 62 |
| 0.9 GPH | 12 in | 12 in | 1.44 in/hr | 42 |
| 0.6 GPH | 12 in | 18 in | 0.64 in/hr | 94 |
| 0.9 GPH | 12 in | 18 in | 0.96 in/hr | 62 |
| 0.9 GPH | 18 in | 24 in | 0.48 in/hr | 125 |
| 1 GPH | 24 in | 24 in | 0.40 in/hr | 150 |
231 cubic inches per gallon. A 4 × 8 bed with four rows of 0.9 GPH line at 12 in spacing has 32 emitters — 29 GPH — and puts down an inch in about 42 minutes.
Pressure, filter, and the two parts people skip
Household pressure is 40–80 psi; drip wants 25–30. Without a pressure regulator, emitters blow off the line and non-compensating ones flow far above their rating, which quietly wrecks every number above. Without a filter — 150 mesh is standard — the emitters clog one by one and the plants at the end of the line die first. Backflow preventer, filter, regulator, then tubing: that's the order at the tap, and the parts cost less than one replaced shrub.
Check the system by walking it while it runs. The far emitter on each line should be dripping at the same rate as the first; if it isn't, the zone is over one of the two limits above.
Sources: Rain Bird, Landscape Drip Irrigation Design Guide (emitter layouts, tubing limits, pressure); Colorado State Extension Fact Sheet 4.702, Drip Irrigation for Home Gardens; DripWorks and DIG Corporation tubing flow charts; Irrigation Association, Landscape Irrigation Best Management Practices.