Why an Inch of Water Is 0.62 Gallons a Square Foot

This page contains affiliate links. If you buy through one we may earn a commission, at no cost to you.

Probably not pressure, and that is worth knowing before anyone buys a booster pump. Pressure enters a sprinkler's flow as a square root, so losing half of it only costs you 29 percent of the water. And it has almost nothing to do with reach, because air drag is already eating 93.5 percent of the distance a garden jet could theoretically travel.

What is actually missing from the whole conversation is a unit. An inch of water is 0.62338 gallons for every square foot it lands on. Once that number is in hand, every question in this category becomes arithmetic, and the striking thing is that not one watering product in this catalogue publishes the other half of it, which is a flow rate.

The conversion, and the one instrument that is calibrated

A column of water one inch deep over one square foot is 144 cubic inches. A US gallon is 231 cubic inches. So one inch of water is 144 divided by 231, which is 0.62338 gallons per square foot. The metric version of the same sentence is exact and far tidier: one millimeter of water is one liter per square meter.

A Toiclebor outdoor rain gauge with a dual scale glass tube is the only product here with a published resolution, and it is a good one: a dual inch and centimeter scale running to 7 inches or 17 centimeters, with ten graduations per inch and per centimeter, a red float that marks the level, a 16.53 inch overall height and a metal stand.

Ten graduations to the inch means it resolves 0.1 inch, which is 0.06234 gallons per square foot. Across a 1,000 square foot lawn that is 62.3 gallons. So one mark on that glass tube is sixty-two gallons of hose time, and the difference between the quarter inch and the inch that our own record uses as its example is 468 gallons.

That is what makes a rain gauge an instrument rather than an ornament, and it is the same point our guide to tracking rainfall in your garden makes qualitatively. The record for the gauge is also careful about the two things it cannot tell you, and both are right: "No glass thickness is published", and on frost, that the listing "never mentions frost or freezing at all". A narrow glass tube full of water is the classic thing a hard night splits.

A rate is a flow divided by an area

A FANHAO 2-in-1 rotating lawn sprinkler hose nozzle publishes four watering patterns from mist to jet, a head that rotates between hand spraying and sprinkler mode, an ABS and PP body and a 3/4 inch GHT thread that fits standard US hoses without an adapter. What it does not publish, and what nothing in this catalogue publishes, is a flow rate.

So take a representative 3 gallons a minute, which is ordinary for a hose-end sprinkler, and a wetted circle 30 feet across, which is 706.86 square feet. Three divided by 706.86 is 0.004244 gallons per minute per square foot. Divide by 0.62338 and that is 0.006808 inches a minute, so 0.408 inches an hour, and two hours and twenty-seven minutes for a full inch over the circle.

Now the part that is genuinely counterintuitive and follows from nothing more than the area of a circle. The rate is a flow divided by an area, and the area goes as the square of the diameter, so the rate goes as the inverse square of the diameter.

A 20 foot circle is 314.16 square feet and receives 0.918 inches an hour. A 30 foot circle is 706.86 square feet and receives 0.408. A 40 foot circle is 1,256.6 square feet and receives 0.2295.

Wider coverage from the same hose is slower watering, in exact proportion. Setting the dial to throw further does not water more of the lawn in the same time; it waters more of the lawn more thinly for the same number of gallons. The record's own drawback, "Coverage depends on water pressure", is true and incomplete in a specific way: coverage also trades directly against rate, and the listing publishes neither a coverage nor a pressure, so it names a dependency and supplies no side of it.

And the total, for the lawn rather than the circle. One inch over 1,000 square feet is 623.4 gallons, which at 3 gallons a minute is 207.8 minutes, so three hours and twenty-eight minutes including moving the sprinkler. That is the honest answer to how long an evening's watering takes, and it is why the record is right that "large lawns may need repositioning".

Why pressure is the wrong lever, twice over

A DRAGOW heavy duty 10-pattern garden hose nozzle publishes ten spray patterns from mist to jet, a TPR and ABS body measuring 6.7 by 5.5 by 2.4 inches at 0.34 pounds, thumb-adjusted flow control, and a drawback that puts the folk theory plainly: "Spray force limited by your water pressure."

Limited, yes, but by a square root. Flow through an orifice goes as the square root of the pressure drop across it, so halving your supply pressure leaves 70.7 percent of the flow and adds 41.4 percent to the clock. Real, and not the dramatic effect people expect. Our page on why a longer hose loses pressure and a thinner one loses far more works out where the pressure goes along the hose, and this page takes its conclusions rather than repeating them.

Reach is a different story entirely, and here the folk theory is not merely weak but pointing at the wrong physics. Water leaving an orifice at 50 pounds per square inch, which is 344.7 kilopascals, has a velocity of the square root of twice that over the density of water, which is 26.26 meters a second. A projectile launched at that speed at 45 degrees, with no air in the way, travels its velocity squared divided by gravity, which is 70.28 meters. Two hundred and thirty feet.

A hose sprinkler reaches about fifteen. So air drag is consuming 93.5 percent of the available range before pressure is discussed at all, and anything that changes the droplets changes the reach far more than anything that changes the supply.

Which is exactly what a pattern dial does. Drag force scales with a droplet's cross-sectional area, so with the square of its diameter, while its mass scales with its volume, so with the cube. Deceleration is force over mass, so it goes as one over the diameter. Halve a droplet and it slows down twice as fast.

So the ten patterns are a range selector wearing a gentleness label, and both records get the direction right without the mechanism. The sprinkler's says "Gentle misting suits seedlings, a high-pressure jet tackles cleaning, and the sprinkler pattern spreads water across a yard." All three statements are correct and all three are consequences of one over the diameter. A mist lands at your feet because its droplets are small, not because the nozzle is weak.

Drip is a different unit, and the kit publishes neither

An XHIKOWAT 100FT smart drip irrigation system publishes a great deal: a programmable tap timer with auto, manual and weather-delay modes, a 1 to 15 day rain delay, adjustable brass heads with drip, mist and jet patterns, a 360 degree rotating aluminium stake, a universal fit for 1/2 and 3/4 inch faucets, quick-connect fittings and 100 feet of coverage.

Drip emitters are rated in gallons per hour, not per minute, and that change of unit is the whole point of the technology. A 1 gallon per hour emitter running for 30 minutes delivers half a gallon. Over one square foot that is 0.802 inches, which is a week's rainfall. Over a wetted bulb of one foot radius, which is 3.1416 square feet, it is 0.255 inches.

That factor of three is the honest limit on drip arithmetic and it is a soil property rather than a product one. How far the bulb spreads depends on texture and structure, and no emitter can publish it. What an emitter can publish is its flow rate, and this kit publishes 100 feet of tubing instead, which is a length of plastic rather than a quantity of water.

The consequence shows up in the kit's best feature. A 1 to 15 day rain delay is genuinely useful and it cannot be set correctly from anything the kit tells you. If the system applies about 0.1 inch a day, the delay range spans 0.1 to 1.5 inches of equivalent application, and the right setting after a storm is the depth the storm delivered divided by the daily rate. The first of those numbers comes from the glass tube above. The second is not published. The two products are sold separately and they are halves of one instrument.

Our guide to automatic garden watering covers the timer and zoning side, and our roundup of smart irrigation controllers covers the controllers. The figure to carry into both is gallons per hour per emitter, because without it a schedule is a guess with a battery in it.

What "several points" means for a tree

A Wodesid deep root watering tool for trees is a stainless probe that connects to a hose and delivers water below the surface, doubling as a root feeder and an aerator, with a T-handle. Its drawbacks include a sentence worth quantifying: "Big trees need watering at several points."

Several is a count and the count is large. Our guide to watering trees and shrubs properly gets the geometry right and deserves the credit, because it is the half of the advice people most often skip: water "at the drip line, not the trunk", and "water that wider zone, not just the stem".

So take a tree with a 12 foot canopy. The area inside its drip line is pi times 6 squared, which is 113.1 square feet. One inch over that is 70.5 gallons, and at 3 gallons a minute through the probe the whole job is 23.5 minutes.

A single probe insertion wets a bulb of roughly 1 to 1.5 feet in radius in ordinary soil, so 3.14 to 7.07 square feet per point. Dividing 113.1 by those gives 16 to 36 insertions. That is what several means, and at sixteen points the 23.5 minutes works out at about a minute and a half at each one, which is a very different instruction from the one the word carries.

The record is also right that the tool is "Manual push-in, hard work in dry or clay soil", and sixteen to thirty-six insertions is the reason that caveat matters.

Ten cans an inch

A PMU 2 gallon watering can with a detachable sprinkler head publishes two gallons, an envelope of 17 by 7.25 by 13 inches, an open top and a head that comes off to give a direct stream.

Those two figures are worth a quick consistency check, because they are the only pair in the category that can be checked against each other. The envelope is 1,602 cubic inches and two US gallons is 462, so the can holds 28.8 percent of its bounding box. For a shape with a long spout and a handle arch that is exactly right, and the figures agree.

Now put the capacity through the conversion. Two gallons over a 4 by 8 foot raised bed, which is 32 square feet, is 2 divided by 0.62338 times 32, which is 0.1003 inches. One canful is a tenth of an inch, and an inch is ten cans.

That is the most useful number on this page for anyone with beds rather than a lawn, because it converts a vague intention into a count of trips. It also explains why the hose wins on area and the can wins on precision: at 3 gallons a minute the hose delivers that same canful in 40 seconds, and it delivers it wherever the nozzle happens to be pointing.

The gap that runs across the whole category

Worth stating plainly, because it is the most reportable thing here. Not one watering product in this catalogue publishes a flow rate.

The sprinkler publishes four patterns and a thread size. The nozzle publishes ten patterns, a body size and a weight to two decimal places. The drip kit publishes a hundred feet of tubing, a timer, three head patterns and a rain delay in days. The deep root tool publishes a material and a handle. The watering can publishes a volume, which is the closest anything comes, and a volume is not a rate.

Every one of those five products exists to move water, and none of them says how fast. Which means the central arithmetic of the category, gallons per minute divided by square feet, cannot be performed from any listing in it. That is not an obscure specification: our own roundup of electric pressure washers defines it correctly and in plain words, that "gpm is gallons per minute, the water flow rate", so the unit is familiar to the site. It simply stops at the boundary between cleaning and growing.

Two of the five flag the dependency without supplying either side of it. "Coverage depends on water pressure" on the sprinkler and "Spray force limited by your water pressure" on the nozzle are both honest and both unactionable, since neither a coverage nor a pressure appears anywhere on either page.

The honest limit, and the three numbers to keep

What is exact here is the unit and everything that is pure geometry. One inch over one square foot is 144 cubic inches and a gallon is 231, so 0.62338 is arithmetic. One millimeter is one liter per square meter by definition. The precipitation rate goes as the inverse square of the sprinkler's diameter because the area of a circle does. Orifice velocity is the square root of twice the pressure over the density, and a drag-free 45 degree range is the velocity squared over gravity, so the 230.6 feet is exact for a particle. Droplet deceleration goes as one over the diameter because drag goes as the square and mass goes as the cube.

What is representative is every quantity of water, and the reason is the gap above. The 3 gallons a minute is an ordinary figure for a hose-end device and nothing publishes one, so every time and every gallon total on this page scales directly with it: at 2 gallons a minute the three and a half hours becomes five and a quarter, and at 5 it becomes two and a quarter. Every ratio and every inverse square relation is untouched. The 50 pounds per square inch, the 30 foot circle, the fifteen foot real reach, the 1 gallon an hour emitter, the 1 to 1.5 foot wetted bulb and the 12 foot canopy are all in the same category.

What this page deliberately does not compute is how much water a plant or a lawn needs, and the omission is not laziness. That depends on the species, the rooting depth, the soil's water holding capacity, the temperature and the wind, and none of those is a product specification. The familiar "an inch a week" is a rule of thumb for cool-season turf and this page has used it only as a unit to convert, never as a target. Nor has it computed distribution uniformity, which is what the question was really about when it mentioned grass blocking the spray: that needs a catch-can test across the pattern, and the honest statement is that a rotating hose sprinkler puts more water near the middle of its circle than at the edge and no listing publishes the profile.

And the three numbers to keep are small. An inch is 0.62 gallons a square foot, so any area times 0.62 is the gallons it takes. A rate is that flow divided by the area it falls on, so throwing wider is watering slower by the same factor squared. And reach is a droplet size, so if the far corner is dry the dial is more likely to be the problem than the tap. On the question that started this, grass blocking the spray is a height problem that the same arithmetic touches from one side only: a sprinkler set low enough to be shaded by the turf is also one whose effective circle has shrunk, and a smaller circle at the same flow is a faster rate over less ground, which is why one patch turns to mud while another stays brown.