Gallons per Square Foot Calculator
One conversion runs rain gauges, flood reports and aquarium math alike: an inch of water over a square foot is 0.623 gallons. Enter an area and a water depth, and this calculator returns the gallons — for the roof harvest, the basement flood, and every flat-bottomed thing in between.
The 0.623 Constant, Derived Honestly
Some calculator constants are empirical; this one is pure definition, and its derivation fits in a breath. One inch of water over one square foot occupies 12 × 12 × 1 = 144 cubic inches. A US gallon is 231 cubic inches by law. Divide: 144 ÷ 231 = 0.6234 gallons — the whole mystery, with no measurement anywhere in it. Everything this page does is that fraction multiplied by an area and a depth, and its trustworthiness is exactly the trustworthiness of legal definitions. (The imperial gallon of older British measure is a larger unit; the constant would be 0.519, and the calculator's silence about it is a declared US-gallons choice, as the hub's American audience expects.) Keep 0.623 in your pocket and an entire genre of water questions — roof yields, tank footprints, flood depths — becomes mental arithmetic with a calculator's blessing.
Worked Example: An Inch of Rain on a Roof
The defaults price the rain harvester's canonical event: 1 inch of rain over 1,000 square feet of roof. Gallons: 1,000 × 1 × 0.6234 = 623 gallons (about 2,361 litres) — from a single modest storm, off a modest roof: two rain barrels' worth many times over, which is why serious harvesting graduates quickly from barrels to cisterns. The weight line earns its place beside the volume: those 623 gallons weigh about 5,200 lb — trivial distributed over a roof doing its job, and a serious design load when the same water stands trapped in a ponding flat roof, which is the drainage engineer's version of this exact multiplication. Change the depth to a season's rainfall (say 30 inches) and the same roof yields about 18,700 gallons a year — the cistern-sizing figure, produced by one multiplication and one honest rainfall record.
Depths, Areas and the Dreaded Flood Line
The calculator's second career is in water where it is not wanted. An inch of water across a 1,200 sq ft basement floor is about 748 gallons to pump out (and the inches stack linearly — three inches is 2,244 gallons and most of a day for a small pump), a translation that turns a remediation quote from poetry into arithmetic. Fire services run the same figure at city scale for runoff and reservoir planning; aquarium keepers run it at glass scale (the gallons in a tank are its footprint in square feet times its water depth in feet times 7.48 — the same constant's other dress, since 0.6234 × 12 = 7.48). And the weight conversion in the answer line — 8.345 lb per gallon — is the one that matters when water stands on structures never designed as tanks: floors, roofs and decks all have load limits that a few inches of water approach with surprising speed, which is this humble conversion's most serious professional use.
The Acre-Inch: This Constant at Landscape Scale
Hydrologists scale this page's fraction up to the unit of watersheds: the acre-inch — one inch of water over one acre — which is 0.6234 × 43,560 ≈ 27,154 gallons. Irrigation districts bill in it, reservoir rules are written in its big sibling the acre-foot (325,851 gallons), and stormwater engineers translate a city's rainfall into treatment volumes through exactly the multiplication on this page, one catchment at a time. The continuity is worth savouring: the number that sizes a rain barrel also sizes a reservoir, because volume's arithmetic has no scale at which it changes its mind — only areas that grow from gardens to counties, multiplied by the same humble 0.623, one decimal of legal definition carrying the whole edifice.
Swimming Pools: The Backyard's Big Multiplication
The backyard pool is this constant's most popular stage: a 16 × 32 ft pool is 512 sq ft of surface, and its average depth decides the rest — at 5 feet of average depth (a shelf-and-deep-end profile doing its averaging honestly), the water is 512 × 60 inches × 0.6234 ≈ 19,150 gallons, the figure the pump, filter, heater and chemical shelf all take their orders from. The pool arithmetic teaches the depth lesson at domestic scale: a one-foot error in the average (shallow-end optimism is universal) misstates the gallons by nearly 3,900, and every chemical dose inherits the error in proportion — pool shops that demand measured dimensions before dosing are running this calculator's discipline with better branding. Cover losses ride along too: an uncovered pool in a hot week can evaporate half an inch a day, 160 gallons through the sky, which the water bill notices before the owner does.
Hot Tubs, Tanks and the Fill-Time Corollary
One corollary of the constant earns its own section because it schedules weekends: fill time. A standard garden hose delivers roughly 5–10 gallons a minute, so the calculator's volumes translate directly into hours at the tap — the worked example's 623 storm gallons is about 1–2 hours of hose; a hot tub's typical 400 gallons, about an hour; the pool of the previous section, 19,150 gallons, is a day and a half of continuous flow, which is why pool fills are planned around meter readings, municipal notifications in some towns (a sudden 20,000-gallon draw can look, to a water department, exactly like a main break), and the humility to let the hose run overnight instead of the evening. Water pressure and hose diameter move the tap's rate by factors of two, so measure your own hose once — a bucket and a stopwatch — and every volume this page ever gives you becomes a schedule as well as a quantity.
Related Tools
More from the Construction Calculators hub: the Pond Calculator — the same water, welcomed and lined — and the Tank Volume Calculator for vessels with walls.
Frequently Asked Questions
How many gallons is 2 inches of water over 500 square feet, exactly?
500 × 2 × 0.6234 = 623 gallons — the same as the worked example, because area and depth trade evenly in this multiplication (it is volume, and volume does not care which dimension supplied it). That interchangeability is the quick mental check for any figure this page returns: halve the area and double the depth, and the gallons must stand still; if a quoted number flinches under the trade, it was never a volume to begin with.
How much rain can I harvest from my roof in a year?
Run the worked example's method on your own record: roof plan area (the footprint, not the sloped surface — rain falls vertically and the roof collects its shadow's worth) × annual rainfall in inches × 0.6234, then take about 80–90% of the result for the losses (first-flush diverters, splashes, gutter overflows in the big storms, evaporation). That realistic fraction, divided by your garden's or household's appetite, sizes the cistern — and the calculator's constant is the spine of the whole estimate, dressed here in the season's totals instead of the single storm's.
Why do people also quote 7.48 gallons, though?
Because depth in feet wants its own constant: a cubic foot holds 7.4805 gallons, and a square foot one foot deep is a cubic foot — so area × depth-in-feet × 7.4805 is the same multiplication as area × depth-in-inches × 0.6234 (0.6234 × 12 = 7.481, the rounding being the only difference). Tanks and ponds conventionally speak feet; rain and floods speak inches. The two constants are one fact in two dialects, and knowing both lets you audit any water figure you are handed in either.
How heavy is the water in a flooded room or a tank?
At 8.345 lb per gallon, the answer line computes it automatically, and the magnitudes discipline the imagination: the 748-gallon basement inch weighs about 6,240 lb (spread mercifully over the slab), a 500-gallon aquarium weighs over two tons concentrated on its stand's footprint, and a rooftop's two inches of ponded water over 2,000 sq ft is about 20,800 lb of unplanned load. Structural engineers take water this seriously because water takes itself this seriously — the calculator simply refuses to let gallons sound weightless.
Does the shape of the area matter here?
Not to the multiplication: volume is area times depth for any footprint — circle, kidney or L-shape — provided the area is the true one and the depth is level (water insists on the level part itself). Irregular areas get their honesty from the area estimate, not from a shape factor like the pond page's; measure the footprint as a surveyor would (break it into rectangles and triangles, or count grid squares on a plan) and the gallons follow exactly. Sloped bottoms are the depth analogue of the pond problem — average the soundings, as that page teaches.
Litres and imperial gallons?
The answer line prints litres alongside (gallons × 3.785), which covers most metric needs. Imperial gallons (the British unit, 4.546 litres) appear in older Commonwealth specifications: divide the litre figure by 4.546, or multiply this page's US gallons by 0.8327. The constant 0.623 belongs to the US gallon alone — a fact worth stating on a page whose readers arrive from every gallon-using nation, since the inch and the square foot are shared but the gallon, awkwardly, is not.