Gallons per hour is the whole difference
Sprinklers are rated in gallons per minute and drip emitters in gallons per hour, and the factor of sixty between them reorganises everything. A single rotor might move 2.5 GPM. A hundred and fifty half-gallon-per-hour emitters move 75 GPH, which is 1.25 GPM — half of one rotor. That is why a drip zone can be enormous in plant count and still fit comfortably on a supply that would only carry three or four sprinkler heads.
The same factor works against you on the clock. Delivering two gallons to a plant through two 1 GPH emitters takes an hour, and there is no way to hurry it that does not involve more emitters. This is the most common cause of disappointment with a new drip system: it is set to run for the twenty minutes the sprinklers used to run, delivers two thirds of a gallon per plant, and everything wilts anyway.
Once the emitter count is fixed, the arithmetic is unusually trustworthy. There is no throw pattern, no wind, no overlap. Each emitter delivers its rating to one spot, so the zone flow is the count times the rating and the volume delivered is the flow times the time. That directness is why well-built drip systems audit at high uniformity while sprinklers struggle.
Emitters per plant and where they go
One emitter wets a column of soil beneath itself, and the shape of that column depends entirely on the soil. In sand it goes almost straight down, spreading perhaps six inches. In clay it spreads sideways much further before it goes deep. A single emitter on sandy soil next to a shrub can water a spot the plant is barely using while the rest of the root zone stays dry.
So multiple emitters spread around the plant beat one large emitter of the same total output, and the difference grows with the size of the plant. Small annuals and containers manage on one. Shrubs commonly get two, placed on opposite sides. Trees get four or more arranged around the drip line of the canopy, moved outwards every year or two as the roots extend, because a tree's absorbing roots are near the edge of the canopy rather than against the trunk.
How much water any of them actually needs is not a question this page can answer, and anyone who gives you a single number for it is guessing. It depends on the species, the size of the plant, the soil, the season, the exposure and how established the planting is. Your local extension service deals in exactly this and will have figures for your area and your plants.
The zone flow limit and what sets it
Three separate things cap a drip zone, and the lowest one wins. The supply itself, measured at the point of connection rather than assumed. The valve, filter and pressure regulator, each of which has a flow range in which it works properly — a regulator run below its minimum flow will not regulate, and an oversized filter passes water too slowly to self-clean. And the tubing, which has a practical maximum flow before friction loss down the line starves the far emitters.
That last one is why long single runs of half-inch tubing disappoint. Manufacturers publish a maximum run length for each tubing and emitter spacing, and it is not generous. Beyond it, pressure at the end of the line falls far enough that non-compensating emitters flow visibly less than the ones near the valve, and no run time makes the two ends match. The answer is to feed the line in the middle rather than the end, split into branches from a manifold, or use pressure compensating emitters, which hold their rating across a range of pressure and cost a little more each.
Velocity, and why five feet per second
Velocity in a round pipe is flow divided by cross-sectional area, and in the units this trade uses it reduces to a single expression: feet per second equals 0.4085 times GPM divided by the inside diameter in inches squared. The constant is not arbitrary — it is one gallon per minute converted to cubic feet per second, divided by the area of a one inch circle in square feet. At 0.4085, a one inch bore carries 12.2 GPM at five feet per second.
Five feet per second is a working design ceiling that irrigation designers hold themselves to on plastic supply lines. It is not a code figure and nothing about it is mandatory. It exists because of two effects that both get worse with speed. Friction loss climbs faster than flow does, so pressure disappears into the pipe wall instead of arriving at the emitters. And when a valve closes on a fast-moving column of water, that momentum has to be absorbed somewhere, which produces the pressure spike known as water hammer. A single hammer event rarely breaks anything; thousands of them over years work threaded fittings loose, fatigue solvent-welded joints and wear valve seats. Designing under five feet per second is how the trade buys itself out of that problem cheaply, by using one pipe size larger than the flow strictly demands.
Before you connect and before you dig
An irrigation system lies in soil, fertiliser, mulch and whatever else is on the ground, and a drip system is submerged in it by design. If a pressure drop occurs on the supply main, unprotected pipework can draw that back towards the drinking water, which is why backflow prevention on a potable connection is a public health requirement rather than a refinement. Which device is required, how it is installed and how often it must be tested are decided by your water authority, and this page gives no guidance on any of it. Contact them before you connect anything to a drinking water supply.
Anything that involves burying a supply line means digging, and buried services are not always where anyone remembers them being. Have utilities located before the first spade goes in — the call-before-you-dig service is free and exists precisely for this.
Watering restrictions, permitted days and hours, and whether a system needs registering or a permit are all local and change with conditions and season, sometimes at short notice. Your water utility is the source for all of it. For the arithmetic either side of this page: total flow against your measured supply is on the irrigation zone calculator, converting drip output to the depth-per-hour language sprinklers use is on the precipitation rate calculator, and the pressure a long supply run costs you is on the water flow pressure drop calculator. If the water comes off a roof rather than a meter, the rainwater harvesting calculator sizes the supply, and the greywater reuse calculator covers the other alternative source.
Questions people ask
How many drip emitters can one zone handle?
Divide the zone flow limit in GPM by the emitter rating in GPH, then multiply by sixty. A 6 GPM limit is 360 GPH, which is 360 emitters at 1 GPH, 720 at half a gallon, or 180 at 2 GPH. Those counts sound enormous next to a sprinkler zone and they are, which is the point. In practice the binding limit is usually the tubing rather than the flow: the manufacturer maximum run length for the tubing and emitter spacing will cap a single line long before the valve or the supply runs out of capacity, so large emitter counts get split across branches from a manifold.
How long should a drip zone run?
Long enough to deliver the volume you decided on, which is the volume per plant divided by the GPH reaching that plant. Two 1 GPH emitters deliver 2 GPH, so two gallons takes an hour. Four half-gallon emitters also deliver 2 GPH and take the same hour while wetting a much better shape of soil. What that volume should be is not something this page decides — it depends on species, plant size, soil and season, and the local extension service is where to get a figure that means anything for your plants.
What is the difference between pressure compensating emitters and ordinary ones?
An ordinary emitter is a fixed restriction, so its output rises and falls with pressure. Near the valve it flows more than rated, at the end of a long line it flows less, and on a slope the downhill emitters flow more than the uphill ones. A pressure compensating emitter has a flexible element that closes down as pressure rises, holding output roughly constant across a stated range. They cost more each and are worth it on long runs, on sloping ground, on any zone with several branches of different lengths, and anywhere you want the schedule to mean the same thing at both ends of the line.
Why does drip need a filter and a pressure regulator?
The filter is because emitter passages are very small, and a particle that would pass a sprinkler nozzle without being noticed will block an emitter completely. A blocked emitter is silent — nothing sprays, nothing sounds wrong, and the plant it served quietly declines. The regulator is because most drip is designed for a low pressure, often in the region of 20 to 30 psi depending on the product, and household supply pressure is commonly double that or more. Feeding unregulated pressure into drip tubing blows fittings off, splits the tubing at barbs and pushes non-compensating emitters far above their rating. Both are inexpensive and both are ahead of everything else in the zone.
Can I mix drip and sprinklers on the same valve?
No, and it is worth being clear about why. It is not primarily about pressure, though the two want very different pressures. It is that they apply water at rates that differ by an order of magnitude and there is no run time that suits both. Whatever duration waters the drip zone properly will have flooded the sprinkler zone hours earlier, and whatever suits the sprinklers delivers a small fraction of what the drip plants needed. They also want different filtration and different scheduling frequencies. Separate valves, always, even if it means an extra station on the controller.